On 9/9/21 10:10 PM, Simon Glass wrote:

At present some of the ideas and techniques behind devicetree in U-Boot are assumed, implied or unsaid. Add some documentation to cover how devicetree is build, how it can be modified and the rules about using the various CONFIG_OF_... options.

Signed-off-by: Simon Glass sjg@chromium.org Reviewed-by: Marcel Ziswiler marcel.ziswiler@toradex.com

---

Changes in v3:

• Fix typos linst suppled receive EFL
• Drop 'and' before 'self-defeating'
• Reword mention of control of QEMU's devicetree generation
• Add mention of dropping CONFIG_OF_BOARD
• Clarify the 'Once this bug is fixed' paragraph a bit
• Expand ways that CONFIG_OF_PRIOR_STAGE can support the U-Boot devicetree
• Add a note at the top explaining that his patch covers 'now', not 'future'
• Add note 'Note: Some boards use a devicetree in U-Boot which does not match'

Changes in v2:

• Fix typos per Sean (thank you!) and a few others

• Add a 'Use of U-Boot /config node' section

• Drop mention of dm-verity since that actually uses the kernel cmdline

• Explain that OF_BOARD will still work after these changes (in 'Once this bug is fixed...' paragraph)

• Expand a bit on the reason why the 'Current situation' is bad

• Clarify in a second place that Linux and U-Boot use the same devicetree in 'To be clear, while U-Boot...'

• Expand on why we should have rules for other projects in 'Devicetree in another project'

• Add a comment as to why devicetree in U-Boot is not 'bad design'

• Reword 'in-tree U-Boot devicetree' to 'devicetree source in U-Boot'

• Rewrite 'Devicetree generated on-the-fly in another project' to cover points raised on v1

• Add 'Why does U-Boot have its nodes and properties?'

• Add 'Why not have two devicetrees?'

  doc/develop/index.rst | 1 + doc/develop/package/devicetree.rst | 583 +++++++++++++++++++++++++++++ doc/develop/package/index.rst | 1 + 3 files changed, 585 insertions(+) create mode 100644 doc/develop/package/devicetree.rst

diff --git a/doc/develop/index.rst b/doc/develop/index.rst index 83c929babda..d5ad8f9fe53 100644 --- a/doc/develop/index.rst +++ b/doc/develop/index.rst @@ -36,6 +36,7 @@ Packaging :maxdepth: 1

 package/index

• package/devicetree

  Testing

diff --git a/doc/develop/package/devicetree.rst b/doc/develop/package/devicetree.rst new file mode 100644 index 00000000000..b1bd310d906 --- /dev/null +++ b/doc/develop/package/devicetree.rst @@ -0,0 +1,583 @@ +.. SPDX-License-Identifier: GPL-2.0+

+Updating the devicetree +=======================

+Note: This documentation describes how things are today, mostly, with some +mention of things that need to be fixed. It is not intended to point the way to +what might be done in the future. That should be the subject of discussions on +the mailing list.

+U-Boot uses devicetree for runtime configuration and storing required blobs or +any other information it needs to operate. It is possible to update the +devicetree separately from actually building U-Boot. This provides a good degree +of control and flexibility for firmware that uses U-Boot in conjunction with +other project.

+There are many reasons why it is useful to modify the devicetree after building +it:

+- Configuration can be changed, e.g. which UART to use +- A serial number can be added +- Public keys can be added to allow image verification +- Console output can be changed (e.g. to select serial or vidconsole)

+This section describes how to work with devicetree to accomplish your goals.

+See also :doc:`../devicetree/control` for a basic summary of the available +features.

+Devicetree source +-----------------

+Every board in U-Boot must include a devicetree sufficient to build and boot +that board on suitable hardware (or emulation). This is specified using the +`CONFIG DEFAULT_DEVICE_TREE` option.

+Current situation (August 2021) +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

+As an aside, at present U-Boot allows `CONFIG_DEFAULT_DEVICE_TREE` to be empty, +e.g. if `CONFIG_OF_BOARD` or `CONFIG_OF_PRIOR_STAGE` are used. This has +unfortunately created an enormous amount of confusion and some wasted effort. +This was not intended and this bug will be fixed soon.

+Some of the problems created are:

+- It is not obvious that the devicetree is coming from another project

+- There is no way to see even a sample devicetree for these platform in U-Boot,

• so it is hard to know what is going on, e.g. which devices are typically
• present

+- The other project may not provide a way to support U-Boot's requirements for

• devicetree, such as the /config node. Note: On the U-Boot mailing list, this
• was only discovered after weeks of discussion and confusion

+- For QEMU specifically, consulting two QEMU source files is required, for which

• there are no references in U-Boot documentation. The code is generating a
• devicetree, with some control from command-line args, but it is not clear
• how to add properties required by U-Boot.

+Specifically on the changes in U-Boot:

+- `CONFIG_OF_BOARD` was added in rpi_patch_ for Raspberry Pi, which does have

• an in-tree devicetree, but this feature has since been used for boards that
• don't

+- `CONFIG_OF_PRIOR_STAGE` was added in bcm_patch_ as part of a larger Broadcom

• change with a tag indicating it only affected one board, so the change in
• behaviour was not noticed at the time. It has since been used by RISC-V qemu
• boards.

+Note: It is not clear that we actually need both of these. Possibly +`CONFIG_OF_BOARD` can be dropped.

+Once this bug is fixed, CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE will override

What does "bug" refer to? Above you describe the current design not a bug.

+(at runtime) the devicetree supplied with U-Boot, but will otherwise use +CONFIG_OF_SEPARATE for the in-tree build. So these two will become options, +moving out of the 'choice' in `dts/Kconfig`. To be clear, the devicetree in the +U-Boot tree may be largely for documentation and build-testing purposes, if at +runtime the devicetree if provided by another project. But the in-tree +devicetree is packaged with U-Boot as a fallback if it does not get one from a +prior stage at runtime. This does not create two devicetrees that need to be +merged, or anything like that. If the prior stage provides one, it is used as +is, with the one provided by U-Boot being ignored.

+This means that there is a basic devicetree build in the U-Boot tree, for +build-testing, consistency and documentation purposes, but at runtime U-Boot can +accept its devicetree from another source.

The incoming devicetree may not contain any U-Boot specific stuff. So don't you need the buildtime devicetree for all of this information at runtime? E.g. you were requesting to move certificate blobs into the build-time devicetree.

Best regards

Heinrich

+To be clear, while U-Boot has its own copy of the devicetree source for each +board, this must match the Linux source, perhaps with some u-boot.dtsi +additions. The intent here is not to create a separate binding, just to provide +a representative devicetree in U-Boot.

+Note: Some boards use a devicetree in U-Boot which does not match that in Linux. +This is a significant problem which needs to be fixed.

+Offending boards are:

+- bcm7260 +- bcm7445 +- qemu_arm64 +- qemu_arm +- qemu-ppce500 +- qemu-riscv32 +- qemu-riscv32_smode +- qemu-riscv64 +- qemu-riscv64_smode

+All of these need to have a devicetree added in-tree. This is targeted to be +fixed in the 2022.01 release.

+Building the devicetree +-----------------------

+U-Boot automatically builds the devicetree for a board, from the +`arch/<arch>/dts` directory. The Makefile in those directories has rules for +building devicetree files. It is preferable to avoid target-specific rules in +those files: i.e. all boards for a particular SoC should be built at once, +where practical. Apart from simplifying the Makefile, this helps to efficiently +(and immediately) ensure that changes in one board's DT do not break others that +are related. Building devicetrees is fast, so performance is seldom a concern +here.

+Overriding the default devicetree +---------------------------------

+When building U-Boot, the `DEVICE_TREE` environment variable allows the +default devicetree file to be overridden at build time. This can be useful if +modifications have to be made to the in-tree devicetree file, for the benefit +of a downstream build system. Note that the in-tree devicetree must be +sufficient to build and boot, so this is not a way to bypass that requirement.

+Modifying the devicetree after building +---------------------------------------

+While it is generally painful and hacky to modify the code or rodata of a +program after it is built, in many cases it is useful to do so, e.g. to add +configuration information like serial numbers, enabling/disabling features, etc.

+Devicetree provides a very nice solution to these problems since it is +structured data and it is relatively easy to change it, even in binary form +(see fdtput).

+U-Boot takes care that the devicetree is easily accessible after the build +process. In fact it is placed in a separate file called `u-boot.dtb`. If the +build system wants to modify or replace that file, it can do so. Then all that +is needed is to run `binman update` to update the file inside the image. If +binman is not used, then `u-boot-nodtb.bin` and the new `u-boot.dtb` can simply +be concatenated to achieve the desired result. U-Boot happily copes with the +devicetree growing or shrinking.

+The `u-boot.bin` image contains both pieces. While it is possible to locate the +devicetree within the image using the signature at the start of the file, this +is a bit messy.

+This is why `CONFIG_OF_SEPARATE` should always be used when building U-Boot. +The `CONFIG_OF_EMBED` option embeds the devicetree somewhere in the U-Boot ELF +image as rodata, meaning that it is hard to find it and it cannot increase in +size.

+When modifying the devicetree, the different cases to consider are as follows:

+- CONFIG_OF_SEPARATE

• This is easy, described above. Just change, replace or rebuild the
• devicetree so it suits your needs, then rerun binman or redo the `cat`
• operation to join `u-boot-nodtb.bin` and the new `u-boot.dtb`

+- CONFIG_OF_EMBED

• This is tricky, since the devicetree cannot easily be located. If the ELF
• file is available, then the _dtb_dt_begin and __dtb_dt_end symbols can be
• examined to find it. While it is possible to contract the file, it is not
• possible to expand the file since that would involve re-linking

+- CONFIG_OF_PRIOR_STAGE

• In this case the devicetree must be modified in the project which provides
• it, as described below. This can be achieved by copying the devicetree from
• the U-Boot tree, for example, or by providing an option to (at build-time)
• merge U-Boot's version with the one provided by the project.

+- CONFIG_OF_BOARD

• This is a board-specific situation, so needs to be considered on a
• case-by-case base. The devicetree must be modified so that the correct
• one is provided to U-Boot. How this is done depends entirely on the
• implementation of this option for the board. It might require injecting the
• changes into a different project somehow using tooling available there, or
• it might involve merging an overlay file at runtime to obtain the desired
• result.

+Use of U-Boot /config node +--------------------------

+A common problem with firmware is that many builds are needed to deal with the +slight variations between different, related models. For example, one model may +have a TPM and another may not. Devicetree provides an excellent solution to +this problem, in that the devicetree to actually use on a platform can be +injected in the factory based on which model is being manufactured at the time.

+A related problem causing build proliferation is dealing with the differences +between development firmware, developer-friendly firmware (e.g. with all +security features present but with the ability to access the command line), +test firmware (which runs tests used in the factory), final production firmware +(before signing), signed firmware (where the signatures have been inserted) and +the like. Ideally all or most of these should use the same U-Boot build, with +just some options to determine the features available. For example, being able +to control whether the UART console or JTAG are available, on any image, is a +great debugging aid.

+When the firmware consists of multiple parts, it is helpful that all operate +the same way at runtime, regardless of how they were built. This can be achieved +by passing the runtime configuration (e.g. 'enable UART console) along the chain +through each firmware stage. It is frustrating to have to replicate a bug on +production firmware which does happen on developer firmware, because they are +completely different builds.

+The /config node provides useful functionality for this. It allows the different +controls to be 'factored out' of the U-Boot binary, so they can be controlled +separately from the initial source-code build. The node can be easily updated by +a build or factory tool and can control various features in U-Boot. It is +similar in concept to a Kconfig option, except that it can be changed after +U-Boot is built.

+The /config node is similar in concept to the `/chosen node`_ except that it is +for passing information *into* firmware instead of from firmware to the +Operating System. Also, while Linux has a (sometimes extremely long) command +line, U-Boot does not support this. The devicetree provides a more structured +approach in any case.

+Devicetree in another project +-----------------------------

+In some cases U-Boot receives its devicetree at runtime from a program that +calls it. For example ARM's Trusted Firmware A (`TF-A`_) may have a devicetree +that it passes to U-Boot. This overrides any devicetree build by U-Boot. When +packaging the firmware, the U-Boot devicetree may in fact be left out if it can +be guaranteed that it will receive one from another project.

+In this case, the devicetree in the other project must track U-Boot's use of +device tree, for the following reasons:

+- U-Boot only has one devicetree. See `Why not have two devicetrees?`_. +- For a consistent firmware build, decisions made in early stages should be

• communicated to later ones at runtime. For example, if the serial console is
• enabled in an early stage, it should be enabled in U-Boot too.

+- U-Boot is quite capable of managing its own copy of the devicetree. If

• another project wants to bypass this (often for good reason), it is reasonable
• that it should take on the (fairly small) requirements that U-Boot features
• that rely on devicetree are still available

+- The point here is not that *U-Boot needs this extra node*, or *U-Boot needs

• to have this public key*. These features are present in U-Boot in service of
• the entire firmware system. If the U-Boot features are used, but cannot be
• supported in the normal way, then there is pressure to implement these
• features in other ways. In the end, we would have a different mechanism for
• every other project that uses U-Boot. This introduces duplicate ways of doing
• the same thing, needlessly increases the complexity of the U-Boot source code,
• forces authors to consider parallel implementations when writing new features,
• makes U-Boot harder to test, complicates documentation and confuses the
• runtime flow of U-Boot. If every board did things its own way rather than
• contributing to the common code, U-Boot would lose a lot of its cross-platform
• value.

+The above does not indicate *bad design* within U-Boot. Devicetree is a core +component of U-Boot and U-Boot makes use of it to the full. It solves a myriad +of problems that would otherwise need their own special C struct, binary format, +special property, tooling for viewing and updating, etc.

+Specifically, the other project must provide a way to add configuration and +other information to the devicetree for use by U-Boot, such as the /config node. +Note that the U-Boot in-tree devicetree source must be sufficient to build and +boot, so this is not a way to bypass that requirement.

+If binman is used, the devicetree source in U-Boot must contain the binman +definition so that a valid image can be build. This helps people discover what +other firmware components are needed and seek out appropriate documentation.

+If verified boot is used, the project must provide a way to inject a public key, +certificate or other material into the U-Boot devicetree so that it is available +to U-Boot at runtime. See `Signing with U-Boot devicetree`_. This may be +through tooling in the project itself or by making use of U-Boot's tooling.

+Devicetree generated on-the-fly in another project +--------------------------------------------------

+In some rare cases, another project may wish to create a devicetree for U-Boot +entirely on-the-fly, then pass it to U-Boot at runtime. The only known example +of this at the time of writing (2021) is qemu, for ARM (`QEMU ARM`_) and +RISC-V (`QEMU RISC-V`_).

+In effect, when the board boots, U-Boot is *downstream* of the other project. +It is entirely reliant on that project for its correct operation.

+This does not mean to imply that the other project is creating its own, +incompatible devicetree. In fact QEMU generates a valid devicetree which is +suitable for both U-Boot and Linux. It is quite normal for a devicetree to be +present in flash and be made available to U-Boot at runtime. What matters is +where the devicetree comes from. If the other project builds a devicetree for +U-Boot then it needs to support adding the things needed by U-Boot features. +Without them, for example:

+- U-Boot may not boot because too many devices are enabled before relocation +- U-Boot may not have access to the developer or production public keys used for

• signing

+- U-Boot may not support controlling whether the console is enabled +- U-Boot may not be know which MMC device to boot from +- U-Boot may not be able to find other firmware components that it needs to load

+Normally, supporting U-Boot's features is trivial, since the devicetree compiler +(dtc) can compile the source, including any U-Boot pieces. So the burden is +extremely low.

+In this case, the devicetree in the other project must track U-Boot's use of +device tree, so that it remains compatible. See `Devicetree in another project`_ +for reasons why.

+If a particular version of the project is needed for a particular version of +U-Boot, that must be documented in both projects.

+Further, it must provide a way to add configuration and other information to +the devicetree for use by U-Boot, such as the `/config` node and the tags used +by driver model. Note that the U-Boot in-tree devicetree must be sufficient to +build and boot, so this is not a way to bypass that requirement.

+More specifically, tooling or command-line arguments must provide a way to +add a `/config` node or items within that node, so that U-Boot can receive a +suitable configuration. It must provide a way of adding `u-boot,dm-...` tags for +correct operation of driver model. These options can then be used as part of the +build process, which puts the firmware image together. For binman, a way must be +provided to add the binman definition into the devicetree in the same way.

+One way to do this is to allow a .dtsi file to be merged in with the generated +devicetree.

+Note that the burden goes both ways. If a new feature is added to U-Boot which +needs support in another project, then the author of the U-Boot patch must add +any required support to the other project.

+Passing the devicetree through to Linux +---------------------------------------

+Ideally U-Boot and Linux use the same devicetree source, even though it is +hosted in separate projects. U-Boot adds some extra pieces, such as the +`config/` node and tags like `u-boot,dm-spl`. Linux adds some extra pieces, such +as `linux,default-trigger` and `linux,code`. This should not interfere with +each other.

+In principle it is possible for U-Boot's control devicetree to be passed to +Linux. This is, after all, one of the goals of devicetree and the original +Open Firmware project, to have the firmware provide the hardware description to +the Operating System.

+For boards where this approach is used, care must be taken. U-Boot typically +needs to 'fix up' the devicetree before passing it to Linux, e.g. to add +information about the memory map, about which serial console is used, provide +the kernel address space layout randomization (KASLR) seed or select whether the +console should be silenced for a faster boot.

+Fix-ups involve modifying the devicetree. If the control devicetree is used, +that means the control devicetree could be modified, while U-Boot is using it. +Removing a device and reinserting it can cause problems if the devicetree offset +has changed, for example, since the device will be unable to locates its +devicetree properties at the expected devicetree offset, which is a fixed +integer.

+To deal with this, it is recommended to employ one or more of the following +approaches:

+- Make a copy of the devicetree and 'fix up' the copy, leaving the control

• devicetree alone

+- Enable `CONFIG_OF_LIVE` so that U-Boot makes its own copy of the devicetree

• during relocation; fixups then happen on the original flat tree

+- Ensure that fix-ups happen after all loading has happened and U-Boot has

• completed image verification

+In practice,the last point is typically observed, since boot_prep_linux() is +called just before jumping to Linux, long after signature verification, for +example. But it is important to make sure that this line is not blurred, +particularly if untrusted user data is involved.

+Devicetree use cases that must be supported +-------------------------------------------

+Regardless of how the devicetree is provided to U-Boot at runtime, various +U-Boot features must be fully supported. This section describes some of these +features and the implications for other projects.

+If U-Boot uses its own in-tree devicetree these features are supported +automatically.

+Signing with U-Boot devicetree +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

+U-Boot supports signing a payload so that it can be verified to have been +created by a party owning a private key. This is called verified boot in U-Boot +(see doc/uImage.FIT/verified-boot.txt).

+Typically this works by creating a FIT and then running the `mkimage` tool to +add signatures for particular images. As part of this process, `mkimage` writes +a public key to the U-Boot devicetree, although this can be done separately. +See fdt_add_pubkey_ for patches for a suitable tool, for example.

+As with all configuration information, if another project is providing the +devicetree to U-Boot, it must provide a way to add this public key into the +devicetree it passes to U-Boot. This could be via a tooling option, making use +of `mkimage`, or allowing a .dtsi file to be merged in with what is generated in +the other project.

+Providing the binman image definition +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

+In complex systems U-Boot must locate and make use of other firmware components, +such as images for the user interface, files containing peripheral firmware, +multiple copies of U-Boot for use with A/B boot, etc. U-Boot uses +:doc:`Binman <binman>` as a standard way of putting an image together.

+Typically this works by running binman with the devicetree as an input, to +create the file image. Binman then outputs an updated devicetree which is +packed in the firmware image, so U-Boot can access the binman definition and +locate all the components.

+As with all configuration information, if another project is providing the +devicetree to U-Boot, it must provide a way to add this binman definition into +the devicetree it passes to U-Boot. This could be via a tooling option, making +use of `binman`, or alowing a .dtsi file to be merged in with what is generated +in the other project.

+Protecting the devicetree +-------------------------

+U-Boot relies heavily on devicetree for correct operation. A corrupt or invalid +device can cause U-Boot to fail to start, behave incorrectly, crash (e.g. if +`CONFIG_OF_LIBFDT_ASSUME_MASK` is adjusted, or fail to boot an Operating System. +Within U-Boot, the devicetree is as important as any other part of the source +code. At ruuntime, the devicetree can be considered to be structured rodata.

+With secure systems, care must be taken that the devicetree is valid:

+- If the code / rodata has a hash or signature, the devicetree should also, if

• they are packaged separately.

+- If the code / rodata is write-protected when running, the devicetree should be

• also. Note that U-Boot relocates its code and devicetree, so this is not as
• simple as it sounds. U-Boot must write-protect these items after relocating.

+Why does U-Boot have its nodes and properties? +----------------------------------------------

+See also :doc:`../devicetree/intro`.

+There has been pushback at the concept that U-Boot dares have its own nodes and +properties in the devicetree.

+Apart from these nodes and properties, U-Boot uses the same bindings as Linux. +A `u-boot.dtsi` file helps to keep U-Boot-specific changes in separate files, +making it easier to keep devicetree source files in U-Boot in sync with Linux.

+As a counter-example, the Zephyr OS project takes a different approach. It uses +entirely different bindings, in general, making no effort to sync devicetree +source files with Linux. U-Boot strives to be compatible with Linux in a number +of ways, such as source code style and common APIs, to aid porting of code +between the projects. Devicetree is another way where U-Boot and Linux follow a +similar approach.

+Fundamentally, the idea that U-Boot cannot have its own tags flies in the face +of the devicetree specification (see dtspec_), which says:

• Nonstandard property names should specify a **unique string prefix**, such as
• a stock ticker symbol, identifying the name of the company **or organization**
• that defined the property. Examples:
• • fsl,channel-fifo-len
• • ibm,ppc-interrupt-server#s
• • **linux**,network-index

+It is also fundamentally unbalanced. Linux has many tags of its own (some 36 in +version 5.13) and at least one Linux-specific node, even if you ignore things +like flash partitions which clearly provide configuration information to Linux.

+Practically speaking there are many reasons why U-Boot has its own nodes and +properties. Some examples:

+- Binding every device before relocation even if it won't be used, consumes time

• and memory: tags on each node can specify which are needed in SPL or before
• relocation. Linux has no such constraints.

+- Requiring the full clock tree to be up and running just to get the debug UART

• running is inefficient. It is also and self-defeating, since if that much
• code is working properly, you probably don't need the debug UART. A devicetree
• property to provide the UART input-clock frequency is a simple solution.

+- U-Boot does not have a user space to provide policy and configuration. It

• cannot do what Linux does and run programs and look up filesystems to figure
• out how to boot.

+Why not have two devicetrees? +-----------------------------

+Setting aside the argument for restricting U-Boot from having its own nodes and +properties, another idea proposed is to have two devicetrees, one for the +U-Boot-specific bits (here called `special`) and one for everything else (here +called `linux`).

+On the positive side, it might quieten the discussion alluded to in the section +above. But there are many negatives to consider and many open questions to +resolve.

+- **Bindings** - Presumably the special devicetree would have its own bindings.

• It would not be necessary to put a `u-boot,` prefix on anything. People coming
• across the devicetree source would wonder how it fits in with the Linux
• devicetree.

+- **Access** - U-Boot has a nice `ofnode` API for accessing the devicetree. This

• would need to be expanded to support two trees. Features which need to access
• both (such as a device driver which reads the special devicetree to get some
• configuration info) could become quite confusing to read and write.

+- **Merging** - Can the two devicetree be merged if a platform desires it? If

• so, how is this managed in tooling? Does it happen during the build, in which
• case they are not really separate at all. Or does U-Boot merge them at
• runtime, in which case this adds time and memory?

+- **Efficiency** - A second device tree adds more code and more code paths. It

• requires that both be made available to the code in U-Boot, e.g. via a
• separate pointer or argument or API. Overall the separation would certainly
• not speed up U-Boot, nor decrease its size.

+- **Source code** - At present `u-boot.dtsi` files provide the pieces needed for

• U-Boot for a particular board. Would we use these same files for the special
• devicetree?

+- **Complexity** - Two devicetrees complicates the build system since it must

• build and package them both. Errors must be reported in such a way that it
• is obvious which one is failing.

+- **Referencing each other** - The `u-boot,dm-xxx` tags used by driver model

• are currently placed in the nodes they relate to. How would these tags
• reference a node that is in a separate devicetree? What extra validation would
• be needed?

+- **Storage** - How would the two devicetrees be stored in the image? At present

• we simply concatenate the U-Boot binary and the devicetree. We could add the
• special devicetree before the Linux one, so two are concatenated, but it is
• not pretty. We could use binman to support more complex arrangements, but only
• some boards use this at present, so it would be a big change.

+- **API** - How would another project provide two devicetree files to U-Boot at

• runtime? Presumably this would just be too painful. But if it doesn't, it
• would be unable to configure run-time features of U-Boot during the boot.

+- **Confusion** - No other project has two devicetrees. U-Boot would be in the

• unfortunate position of having to describe this fact to new users, along with
• the (arguably contrived) reason for the arrangement.

+- **Signing flow** - The current signing flow is simple as it involves running

• `mkimage` with the U-Boot devicetree. This would have to be updated to use the
• special devicetree. Some way of telling the user that they have done it wrong
• would have to be invented.

+Overall, adding a second devicetree would create enormous confusion and +complexity. It seems a lot cheaper to solve this by a change of attitude.

+.. _rpi_patch: https://patchwork.ozlabs.org/project/uboot/patch/20170402082520.32546-1-deym... +.. _bcm_patch: https://patchwork.ozlabs.org/project/uboot/patch/16fc0901f4521d3c399eac950c5... +.. _`TF-A`: https://www.trustedfirmware.org/projects/tf-a +.. _`QEMU ARM`: https://github.com/qemu/qemu/blob/master/hw/arm/virt.c +.. _`QEMU RISC-V`: https://github.com/qemu/qemu/blob/master/hw/riscv/virt.c +.. _`/chosen node`: https://www.kernel.org/doc/Documentation/devicetree/bindings/chosen.txt +.. _fdt_add_pubkey: https://patchwork.ozlabs.org/project/uboot/list/?series=157843&state=* +.. _dtspec: https://www.devicetree.org/specifications/ diff --git a/doc/develop/package/index.rst b/doc/develop/package/index.rst index 9374be2e62c..188c376950e 100644 --- a/doc/develop/package/index.rst +++ b/doc/develop/package/index.rst @@ -17,3 +17,4 @@ SPI flash. :maxdepth: 2

 binman

• devicetree

On 9/10/21 2:34 PM, Tom Rini wrote:

On Fri, Sep 10, 2021 at 10:38:17AM +0200, Heinrich Schuchardt wrote:

...

On 9/9/21 10:10 PM, Simon Glass wrote:

...

At present some of the ideas and techniques behind devicetree in U-Boot are assumed, implied or unsaid. Add some documentation to cover how devicetree is build, how it can be modified and the rules about using the various CONFIG_OF_... options.

Signed-off-by: Simon Glass sjg@chromium.org Reviewed-by: Marcel Ziswiler marcel.ziswiler@toradex.com

---

Changes in v3:

• Fix typos linst suppled receive EFL
• Drop 'and' before 'self-defeating'
• Reword mention of control of QEMU's devicetree generation
• Add mention of dropping CONFIG_OF_BOARD
• Clarify the 'Once this bug is fixed' paragraph a bit
• Expand ways that CONFIG_OF_PRIOR_STAGE can support the U-Boot devicetree
• Add a note at the top explaining that his patch covers 'now', not 'future'
• Add note 'Note: Some boards use a devicetree in U-Boot which does not match'

Changes in v2:

• Fix typos per Sean (thank you!) and a few others

• Add a 'Use of U-Boot /config node' section

• Drop mention of dm-verity since that actually uses the kernel cmdline

• Explain that OF_BOARD will still work after these changes (in 'Once this bug is fixed...' paragraph)

• Expand a bit on the reason why the 'Current situation' is bad

• Clarify in a second place that Linux and U-Boot use the same devicetree in 'To be clear, while U-Boot...'

• Expand on why we should have rules for other projects in 'Devicetree in another project'

• Add a comment as to why devicetree in U-Boot is not 'bad design'

• Reword 'in-tree U-Boot devicetree' to 'devicetree source in U-Boot'

• Rewrite 'Devicetree generated on-the-fly in another project' to cover points raised on v1

• Add 'Why does U-Boot have its nodes and properties?'

• Add 'Why not have two devicetrees?'

  doc/develop/index.rst | 1 + doc/develop/package/devicetree.rst | 583 +++++++++++++++++++++++++++++ doc/develop/package/index.rst | 1 + 3 files changed, 585 insertions(+) create mode 100644 doc/develop/package/devicetree.rst

diff --git a/doc/develop/index.rst b/doc/develop/index.rst index 83c929babda..d5ad8f9fe53 100644 --- a/doc/develop/index.rst +++ b/doc/develop/index.rst @@ -36,6 +36,7 @@ Packaging :maxdepth: 1

  package/index

• package/devicetree

  Testing

diff --git a/doc/develop/package/devicetree.rst b/doc/develop/package/devicetree.rst new file mode 100644 index 00000000000..b1bd310d906 --- /dev/null +++ b/doc/develop/package/devicetree.rst @@ -0,0 +1,583 @@ +.. SPDX-License-Identifier: GPL-2.0+

+Updating the devicetree +=======================

+Note: This documentation describes how things are today, mostly, with some +mention of things that need to be fixed. It is not intended to point the way to +what might be done in the future. That should be the subject of discussions on +the mailing list.

+U-Boot uses devicetree for runtime configuration and storing required blobs or +any other information it needs to operate. It is possible to update the +devicetree separately from actually building U-Boot. This provides a good degree +of control and flexibility for firmware that uses U-Boot in conjunction with +other project.

+There are many reasons why it is useful to modify the devicetree after building +it:

+- Configuration can be changed, e.g. which UART to use +- A serial number can be added +- Public keys can be added to allow image verification +- Console output can be changed (e.g. to select serial or vidconsole)

+This section describes how to work with devicetree to accomplish your goals.

+See also :doc:`../devicetree/control` for a basic summary of the available +features.

+Devicetree source +-----------------

+Every board in U-Boot must include a devicetree sufficient to build and boot +that board on suitable hardware (or emulation). This is specified using the +`CONFIG DEFAULT_DEVICE_TREE` option.

+Current situation (August 2021) +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

+As an aside, at present U-Boot allows `CONFIG_DEFAULT_DEVICE_TREE` to be empty, +e.g. if `CONFIG_OF_BOARD` or `CONFIG_OF_PRIOR_STAGE` are used. This has +unfortunately created an enormous amount of confusion and some wasted effort. +This was not intended and this bug will be fixed soon.

+Some of the problems created are:

+- It is not obvious that the devicetree is coming from another project

+- There is no way to see even a sample devicetree for these platform in U-Boot,

• so it is hard to know what is going on, e.g. which devices are typically
• present

+- The other project may not provide a way to support U-Boot's requirements for

• devicetree, such as the /config node. Note: On the U-Boot mailing list, this
• was only discovered after weeks of discussion and confusion

+- For QEMU specifically, consulting two QEMU source files is required, for which

• there are no references in U-Boot documentation. The code is generating a
• devicetree, with some control from command-line args, but it is not clear
• how to add properties required by U-Boot.

+Specifically on the changes in U-Boot:

+- `CONFIG_OF_BOARD` was added in rpi_patch_ for Raspberry Pi, which does have

• an in-tree devicetree, but this feature has since been used for boards that
• don't

+- `CONFIG_OF_PRIOR_STAGE` was added in bcm_patch_ as part of a larger Broadcom

• change with a tag indicating it only affected one board, so the change in
• behaviour was not noticed at the time. It has since been used by RISC-V qemu
• boards.

+Note: It is not clear that we actually need both of these. Possibly +`CONFIG_OF_BOARD` can be dropped.

+Once this bug is fixed, CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE will override

What does "bug" refer to? Above you describe the current design not a bug.

The bug is that we have two options to provide seemingly the same functionality. Is there a functional difference between CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE ?

...

...

+(at runtime) the devicetree supplied with U-Boot, but will otherwise use +CONFIG_OF_SEPARATE for the in-tree build. So these two will become options, +moving out of the 'choice' in `dts/Kconfig`. To be clear, the devicetree in the +U-Boot tree may be largely for documentation and build-testing purposes, if at +runtime the devicetree if provided by another project. But the in-tree +devicetree is packaged with U-Boot as a fallback if it does not get one from a +prior stage at runtime. This does not create two devicetrees that need to be +merged, or anything like that. If the prior stage provides one, it is used as +is, with the one provided by U-Boot being ignored.

+This means that there is a basic devicetree build in the U-Boot tree, for +build-testing, consistency and documentation purposes, but at runtime U-Boot can +accept its devicetree from another source.

The incoming devicetree may not contain any U-Boot specific stuff. So don't you need the buildtime devicetree for all of this information at runtime? E.g. you were requesting to move certificate blobs into the build-time devicetree.

This is wrong because (a) no, there's no functional reason the prior stage cannot populate / be pre-populated with what we need and (b) we're documenting what we have today.

The problem is not functional but organizational. The prior boot stage may be burnt into PROM while U-Boot is on an SD-card.

Don't expect that on a board where you could install EDK II or U-Boot or anything else the prior boot stage cares about U-Boot.

How could a prior boot stage possibly know years ahead what is not even yet supported in U-Boot when the prior boot stage is created?

Best regards

Heinrich

On 9/10/21 6:37 PM, Tom Rini wrote:

On Fri, Sep 10, 2021 at 06:28:36PM +0200, Heinrich Schuchardt wrote:

...

On 9/10/21 2:34 PM, Tom Rini wrote:

...

On Fri, Sep 10, 2021 at 10:38:17AM +0200, Heinrich Schuchardt wrote:

...

On 9/9/21 10:10 PM, Simon Glass wrote:

...

At present some of the ideas and techniques behind devicetree in U-Boot are assumed, implied or unsaid. Add some documentation to cover how devicetree is build, how it can be modified and the rules about using the various CONFIG_OF_... options.

Signed-off-by: Simon Glass sjg@chromium.org Reviewed-by: Marcel Ziswiler marcel.ziswiler@toradex.com

---

Changes in v3:

• Fix typos linst suppled receive EFL
• Drop 'and' before 'self-defeating'
• Reword mention of control of QEMU's devicetree generation
• Add mention of dropping CONFIG_OF_BOARD
• Clarify the 'Once this bug is fixed' paragraph a bit
• Expand ways that CONFIG_OF_PRIOR_STAGE can support the U-Boot devicetree
• Add a note at the top explaining that his patch covers 'now', not 'future'
• Add note 'Note: Some boards use a devicetree in U-Boot which does not match'

Changes in v2:

• Fix typos per Sean (thank you!) and a few others

• Add a 'Use of U-Boot /config node' section

• Drop mention of dm-verity since that actually uses the kernel cmdline

• Explain that OF_BOARD will still work after these changes (in 'Once this bug is fixed...' paragraph)

• Expand a bit on the reason why the 'Current situation' is bad

• Clarify in a second place that Linux and U-Boot use the same devicetree in 'To be clear, while U-Boot...'

• Expand on why we should have rules for other projects in 'Devicetree in another project'

• Add a comment as to why devicetree in U-Boot is not 'bad design'

• Reword 'in-tree U-Boot devicetree' to 'devicetree source in U-Boot'

• Rewrite 'Devicetree generated on-the-fly in another project' to cover points raised on v1

• Add 'Why does U-Boot have its nodes and properties?'

• Add 'Why not have two devicetrees?'

  doc/develop/index.rst | 1 + doc/develop/package/devicetree.rst | 583 +++++++++++++++++++++++++++++ doc/develop/package/index.rst | 1 + 3 files changed, 585 insertions(+) create mode 100644 doc/develop/package/devicetree.rst

diff --git a/doc/develop/index.rst b/doc/develop/index.rst index 83c929babda..d5ad8f9fe53 100644 --- a/doc/develop/index.rst +++ b/doc/develop/index.rst @@ -36,6 +36,7 @@ Packaging :maxdepth: 1

   package/index

• package/devicetree

  Testing

diff --git a/doc/develop/package/devicetree.rst b/doc/develop/package/devicetree.rst new file mode 100644 index 00000000000..b1bd310d906 --- /dev/null +++ b/doc/develop/package/devicetree.rst @@ -0,0 +1,583 @@ +.. SPDX-License-Identifier: GPL-2.0+

+Updating the devicetree +=======================

+Note: This documentation describes how things are today, mostly, with some +mention of things that need to be fixed. It is not intended to point the way to +what might be done in the future. That should be the subject of discussions on +the mailing list.

+U-Boot uses devicetree for runtime configuration and storing required blobs or +any other information it needs to operate. It is possible to update the +devicetree separately from actually building U-Boot. This provides a good degree +of control and flexibility for firmware that uses U-Boot in conjunction with +other project.

+There are many reasons why it is useful to modify the devicetree after building +it:

+- Configuration can be changed, e.g. which UART to use +- A serial number can be added +- Public keys can be added to allow image verification +- Console output can be changed (e.g. to select serial or vidconsole)

+This section describes how to work with devicetree to accomplish your goals.

+See also :doc:`../devicetree/control` for a basic summary of the available +features.

+Devicetree source +-----------------

+Every board in U-Boot must include a devicetree sufficient to build and boot +that board on suitable hardware (or emulation). This is specified using the +`CONFIG DEFAULT_DEVICE_TREE` option.

+Current situation (August 2021) +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

+As an aside, at present U-Boot allows `CONFIG_DEFAULT_DEVICE_TREE` to be empty, +e.g. if `CONFIG_OF_BOARD` or `CONFIG_OF_PRIOR_STAGE` are used. This has +unfortunately created an enormous amount of confusion and some wasted effort. +This was not intended and this bug will be fixed soon.

+Some of the problems created are:

+- It is not obvious that the devicetree is coming from another project

+- There is no way to see even a sample devicetree for these platform in U-Boot,

• so it is hard to know what is going on, e.g. which devices are typically
• present

+- The other project may not provide a way to support U-Boot's requirements for

• devicetree, such as the /config node. Note: On the U-Boot mailing list, this
• was only discovered after weeks of discussion and confusion

+- For QEMU specifically, consulting two QEMU source files is required, for which

• there are no references in U-Boot documentation. The code is generating a
• devicetree, with some control from command-line args, but it is not clear
• how to add properties required by U-Boot.

+Specifically on the changes in U-Boot:

+- `CONFIG_OF_BOARD` was added in rpi_patch_ for Raspberry Pi, which does have

• an in-tree devicetree, but this feature has since been used for boards that
• don't

+- `CONFIG_OF_PRIOR_STAGE` was added in bcm_patch_ as part of a larger Broadcom

• change with a tag indicating it only affected one board, so the change in
• behaviour was not noticed at the time. It has since been used by RISC-V qemu
• boards.

+Note: It is not clear that we actually need both of these. Possibly +`CONFIG_OF_BOARD` can be dropped.

+Once this bug is fixed, CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE will override

What does "bug" refer to? Above you describe the current design not a bug.

The bug is that we have two options to provide seemingly the same functionality. Is there a functional difference between CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE ?

Does this clarify your question?

...

...

...

...

+(at runtime) the devicetree supplied with U-Boot, but will otherwise use +CONFIG_OF_SEPARATE for the in-tree build. So these two will become options, +moving out of the 'choice' in `dts/Kconfig`. To be clear, the devicetree in the +U-Boot tree may be largely for documentation and build-testing purposes, if at +runtime the devicetree if provided by another project. But the in-tree +devicetree is packaged with U-Boot as a fallback if it does not get one from a +prior stage at runtime. This does not create two devicetrees that need to be +merged, or anything like that. If the prior stage provides one, it is used as +is, with the one provided by U-Boot being ignored.

+This means that there is a basic devicetree build in the U-Boot tree, for +build-testing, consistency and documentation purposes, but at runtime U-Boot can +accept its devicetree from another source.

The incoming devicetree may not contain any U-Boot specific stuff. So don't you need the buildtime devicetree for all of this information at runtime? E.g. you were requesting to move certificate blobs into the build-time devicetree.

This is wrong because (a) no, there's no functional reason the prior stage cannot populate / be pre-populated with what we need and (b) we're documenting what we have today.

The problem is not functional but organizational. The prior boot stage may be burnt into PROM while U-Boot is on an SD-card.

Don't expect that on a board where you could install EDK II or U-Boot or anything else the prior boot stage cares about U-Boot.

How could a prior boot stage possibly know years ahead what is not even yet supported in U-Boot when the prior boot stage is created?

I don't follow you, sorry. Or perhaps, if you %s/U-Boot/Linux/ the above, what's your answer then?

The boot ROM is burnt into the SoC at production. You don't want to unsolder it if you don't like the devicetree in the boot ROM.

Equally there should be no need to replace TF-A when moving from U-Boot v2020.10 to v2040.10 during the lifetime of a car.

This is why we cannot expect anything from a prior boot stage but an accurate description of the hardware and the prior boot stage itself.

Best regards

Heinrich

On 9/10/21 6:57 PM, Tom Rini wrote:

On Fri, Sep 10, 2021 at 06:50:16PM +0200, Heinrich Schuchardt wrote:

...

On 9/10/21 6:37 PM, Tom Rini wrote:

...

On Fri, Sep 10, 2021 at 06:28:36PM +0200, Heinrich Schuchardt wrote:

...

On 9/10/21 2:34 PM, Tom Rini wrote:

...

On Fri, Sep 10, 2021 at 10:38:17AM +0200, Heinrich Schuchardt wrote:

...

On 9/9/21 10:10 PM, Simon Glass wrote: > At present some of the ideas and techniques behind devicetree in U-Boot > are assumed, implied or unsaid. Add some documentation to cover how > devicetree is build, how it can be modified and the rules about using > the various CONFIG_OF_... options. > > Signed-off-by: Simon Glass sjg@chromium.org > Reviewed-by: Marcel Ziswiler marcel.ziswiler@toradex.com > --- > > Changes in v3: > - Fix typos linst suppled receive EFL > - Drop 'and' before 'self-defeating' > - Reword mention of control of QEMU's devicetree generation > - Add mention of dropping CONFIG_OF_BOARD > - Clarify the 'Once this bug is fixed' paragraph a bit > - Expand ways that CONFIG_OF_PRIOR_STAGE can support the U-Boot devicetree > - Add a note at the top explaining that his patch covers 'now', not 'future' > - Add note 'Note: Some boards use a devicetree in U-Boot which does not match' > > Changes in v2: > - Fix typos per Sean (thank you!) and a few others > - Add a 'Use of U-Boot /config node' section > - Drop mention of dm-verity since that actually uses the kernel cmdline > - Explain that OF_BOARD will still work after these changes (in > 'Once this bug is fixed...' paragraph) > - Expand a bit on the reason why the 'Current situation' is bad > - Clarify in a second place that Linux and U-Boot use the same devicetree > in 'To be clear, while U-Boot...' > - Expand on why we should have rules for other projects in > 'Devicetree in another project' > - Add a comment as to why devicetree in U-Boot is not 'bad design' > - Reword 'in-tree U-Boot devicetree' to 'devicetree source in U-Boot' > - Rewrite 'Devicetree generated on-the-fly in another project' to cover > points raised on v1 > - Add 'Why does U-Boot have its nodes and properties?' > - Add 'Why not have two devicetrees?' > > doc/develop/index.rst | 1 + > doc/develop/package/devicetree.rst | 583 +++++++++++++++++++++++++++++ > doc/develop/package/index.rst | 1 + > 3 files changed, 585 insertions(+) > create mode 100644 doc/develop/package/devicetree.rst > > diff --git a/doc/develop/index.rst b/doc/develop/index.rst > index 83c929babda..d5ad8f9fe53 100644 > --- a/doc/develop/index.rst > +++ b/doc/develop/index.rst > @@ -36,6 +36,7 @@ Packaging > :maxdepth: 1 > > package/index > + package/devicetree > > Testing > ------- > diff --git a/doc/develop/package/devicetree.rst b/doc/develop/package/devicetree.rst > new file mode 100644 > index 00000000000..b1bd310d906 > --- /dev/null > +++ b/doc/develop/package/devicetree.rst > @@ -0,0 +1,583 @@ > +.. SPDX-License-Identifier: GPL-2.0+ > + > +Updating the devicetree > +======================= > + > +Note: This documentation describes how things are today, mostly, with some > +mention of things that need to be fixed. It is not intended to point the way to > +what might be done in the future. That should be the subject of discussions on > +the mailing list. > + > +U-Boot uses devicetree for runtime configuration and storing required blobs or > +any other information it needs to operate. It is possible to update the > +devicetree separately from actually building U-Boot. This provides a good degree > +of control and flexibility for firmware that uses U-Boot in conjunction with > +other project. > + > +There are many reasons why it is useful to modify the devicetree after building > +it: > + > +- Configuration can be changed, e.g. which UART to use > +- A serial number can be added > +- Public keys can be added to allow image verification > +- Console output can be changed (e.g. to select serial or vidconsole) > + > +This section describes how to work with devicetree to accomplish your goals. > + > +See also :doc:`../devicetree/control` for a basic summary of the available > +features. > + > + > +Devicetree source > +----------------- > + > +Every board in U-Boot must include a devicetree sufficient to build and boot > +that board on suitable hardware (or emulation). This is specified using the > +`CONFIG DEFAULT_DEVICE_TREE` option. > + > + > +Current situation (August 2021) > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +As an aside, at present U-Boot allows `CONFIG_DEFAULT_DEVICE_TREE` to be empty, > +e.g. if `CONFIG_OF_BOARD` or `CONFIG_OF_PRIOR_STAGE` are used. This has > +unfortunately created an enormous amount of confusion and some wasted effort. > +This was not intended and this bug will be fixed soon. > + > +Some of the problems created are: > + > +- It is not obvious that the devicetree is coming from another project > + > +- There is no way to see even a sample devicetree for these platform in U-Boot, > + so it is hard to know what is going on, e.g. which devices are typically > + present > + > +- The other project may not provide a way to support U-Boot's requirements for > + devicetree, such as the /config node. Note: On the U-Boot mailing list, this > + was only discovered after weeks of discussion and confusion > + > +- For QEMU specifically, consulting two QEMU source files is required, for which > + there are no references in U-Boot documentation. The code is generating a > + devicetree, with some control from command-line args, but it is not clear > + how to add properties required by U-Boot. > + > +Specifically on the changes in U-Boot: > + > +- `CONFIG_OF_BOARD` was added in rpi_patch_ for Raspberry Pi, which does have > + an in-tree devicetree, but this feature has since been used for boards that > + don't > +- `CONFIG_OF_PRIOR_STAGE` was added in bcm_patch_ as part of a larger Broadcom > + change with a tag indicating it only affected one board, so the change in > + behaviour was not noticed at the time. It has since been used by RISC-V qemu > + boards. > + > +Note: It is not clear that we actually need both of these. Possibly > +`CONFIG_OF_BOARD` can be dropped. > + > +Once this bug is fixed, CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE will override

What does "bug" refer to? Above you describe the current design not a bug.

The bug is that we have two options to provide seemingly the same functionality. Is there a functional difference between CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE ?

Does this clarify your question?

Again, does this clarify or answer your question?

...

...

...

...

...

> +(at runtime) the devicetree supplied with U-Boot, but will otherwise use > +CONFIG_OF_SEPARATE for the in-tree build. So these two will become options, > +moving out of the 'choice' in `dts/Kconfig`. To be clear, the devicetree in the > +U-Boot tree may be largely for documentation and build-testing purposes, if at > +runtime the devicetree if provided by another project. But the in-tree > +devicetree is packaged with U-Boot as a fallback if it does not get one from a > +prior stage at runtime. This does not create two devicetrees that need to be > +merged, or anything like that. If the prior stage provides one, it is used as > +is, with the one provided by U-Boot being ignored. > + > +This means that there is a basic devicetree build in the U-Boot tree, for > +build-testing, consistency and documentation purposes, but at runtime U-Boot can > +accept its devicetree from another source.

The incoming devicetree may not contain any U-Boot specific stuff. So don't you need the buildtime devicetree for all of this information at runtime? E.g. you were requesting to move certificate blobs into the build-time devicetree.

This is wrong because (a) no, there's no functional reason the prior stage cannot populate / be pre-populated with what we need and (b) we're documenting what we have today.

The problem is not functional but organizational. The prior boot stage may be burnt into PROM while U-Boot is on an SD-card.

Don't expect that on a board where you could install EDK II or U-Boot or anything else the prior boot stage cares about U-Boot.

How could a prior boot stage possibly know years ahead what is not even yet supported in U-Boot when the prior boot stage is created?

I don't follow you, sorry. Or perhaps, if you %s/U-Boot/Linux/ the above, what's your answer then?

The boot ROM is burnt into the SoC at production. You don't want to unsolder it if you don't like the devicetree in the boot ROM.

Equally there should be no need to replace TF-A when moving from U-Boot v2020.10 to v2040.10 during the lifetime of a car.

This is why we cannot expect anything from a prior boot stage but an accurate description of the hardware and the prior boot stage itself.

OK. But again, %s/U-Boot/Linux/ and then what? Or are you saying the device tree that's burned in shouldn't be used for Linux either?

Linux is *not* requiring that depending on config options of Linux the prior boot stage adds properties to the devicetree.

Linux does not require to add build specific objects like Linux module signing keys to the devicetree.

In theory any newer Linux should boot with any device-tree that a prior Linux version booted with.

If U-Boot does the same I am fine.

Best regards

Heinrich

Date: Fri, 10 Sep 2021 08:34:20 -0400 From: Tom Rini trini@konsulko.com

On Fri, Sep 10, 2021 at 10:38:17AM +0200, Heinrich Schuchardt wrote:

...

On 9/9/21 10:10 PM, Simon Glass wrote:

...

At present some of the ideas and techniques behind devicetree in U-Boot are assumed, implied or unsaid. Add some documentation to cover how devicetree is build, how it can be modified and the rules about using the various CONFIG_OF_... options.

Signed-off-by: Simon Glass sjg@chromium.org Reviewed-by: Marcel Ziswiler marcel.ziswiler@toradex.com

---

Changes in v3:

• Fix typos linst suppled receive EFL
• Drop 'and' before 'self-defeating'
• Reword mention of control of QEMU's devicetree generation
• Add mention of dropping CONFIG_OF_BOARD
• Clarify the 'Once this bug is fixed' paragraph a bit
• Expand ways that CONFIG_OF_PRIOR_STAGE can support the U-Boot devicetree
• Add a note at the top explaining that his patch covers 'now', not 'future'
• Add note 'Note: Some boards use a devicetree in U-Boot which does not match'

Changes in v2:

• Fix typos per Sean (thank you!) and a few others

• Add a 'Use of U-Boot /config node' section

• Drop mention of dm-verity since that actually uses the kernel cmdline

• Explain that OF_BOARD will still work after these changes (in 'Once this bug is fixed...' paragraph)

• Expand a bit on the reason why the 'Current situation' is bad

• Clarify in a second place that Linux and U-Boot use the same devicetree in 'To be clear, while U-Boot...'

• Expand on why we should have rules for other projects in 'Devicetree in another project'

• Add a comment as to why devicetree in U-Boot is not 'bad design'

• Reword 'in-tree U-Boot devicetree' to 'devicetree source in U-Boot'

• Rewrite 'Devicetree generated on-the-fly in another project' to cover points raised on v1

• Add 'Why does U-Boot have its nodes and properties?'

• Add 'Why not have two devicetrees?'

  doc/develop/index.rst | 1 + doc/develop/package/devicetree.rst | 583 +++++++++++++++++++++++++++++ doc/develop/package/index.rst | 1 + 3 files changed, 585 insertions(+) create mode 100644 doc/develop/package/devicetree.rst

diff --git a/doc/develop/index.rst b/doc/develop/index.rst index 83c929babda..d5ad8f9fe53 100644 --- a/doc/develop/index.rst +++ b/doc/develop/index.rst @@ -36,6 +36,7 @@ Packaging :maxdepth: 1

 package/index

• package/devicetree

  Testing

diff --git a/doc/develop/package/devicetree.rst b/doc/develop/package/devicetree.rst new file mode 100644 index 00000000000..b1bd310d906 --- /dev/null +++ b/doc/develop/package/devicetree.rst @@ -0,0 +1,583 @@ +.. SPDX-License-Identifier: GPL-2.0+

+Updating the devicetree +=======================

+Note: This documentation describes how things are today, mostly, with some +mention of things that need to be fixed. It is not intended to point the way to +what might be done in the future. That should be the subject of discussions on +the mailing list.

+U-Boot uses devicetree for runtime configuration and storing required blobs or +any other information it needs to operate. It is possible to update the +devicetree separately from actually building U-Boot. This provides a good degree +of control and flexibility for firmware that uses U-Boot in conjunction with +other project.

+There are many reasons why it is useful to modify the devicetree after building +it:

+- Configuration can be changed, e.g. which UART to use +- A serial number can be added +- Public keys can be added to allow image verification +- Console output can be changed (e.g. to select serial or vidconsole)

+This section describes how to work with devicetree to accomplish your goals.

+See also :doc:`../devicetree/control` for a basic summary of the available +features.

+Devicetree source +-----------------

+Every board in U-Boot must include a devicetree sufficient to build and boot +that board on suitable hardware (or emulation). This is specified using the +`CONFIG DEFAULT_DEVICE_TREE` option.

+Current situation (August 2021) +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

+As an aside, at present U-Boot allows `CONFIG_DEFAULT_DEVICE_TREE` to be empty, +e.g. if `CONFIG_OF_BOARD` or `CONFIG_OF_PRIOR_STAGE` are used. This has +unfortunately created an enormous amount of confusion and some wasted effort. +This was not intended and this bug will be fixed soon.

+Some of the problems created are:

+- It is not obvious that the devicetree is coming from another project

+- There is no way to see even a sample devicetree for these platform in U-Boot,

• so it is hard to know what is going on, e.g. which devices are typically
• present

+- The other project may not provide a way to support U-Boot's requirements for

• devicetree, such as the /config node. Note: On the U-Boot mailing list, this
• was only discovered after weeks of discussion and confusion

+- For QEMU specifically, consulting two QEMU source files is required, for which

• there are no references in U-Boot documentation. The code is generating a
• devicetree, with some control from command-line args, but it is not clear
• how to add properties required by U-Boot.

+Specifically on the changes in U-Boot:

+- `CONFIG_OF_BOARD` was added in rpi_patch_ for Raspberry Pi, which does have

• an in-tree devicetree, but this feature has since been used for boards that
• don't

+- `CONFIG_OF_PRIOR_STAGE` was added in bcm_patch_ as part of a larger Broadcom

• change with a tag indicating it only affected one board, so the change in
• behaviour was not noticed at the time. It has since been used by RISC-V qemu
• boards.

+Note: It is not clear that we actually need both of these. Possibly +`CONFIG_OF_BOARD` can be dropped.

+Once this bug is fixed, CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE will override

What does "bug" refer to? Above you describe the current design not a bug.

The bug is that we have two options to provide seemingly the same functionality. Is there a functional difference between CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE ?

With CONFIG_OF_BOARD there is a function that returns the pointer to the DTB, so you can do all sort of things with it.

With CONFIG_OF_PRIOR_STAGE there is a variable that you need to set in low-level code to point at the DTB and there is a pre-defined function that returns that pointer.

CONFIG_OF_BOARD is more flexible than CONFIG_OF_PRIOR_STAGE, but if the only thing you want to do is to pass on a DTB that is passed in a CPU register to U-Boot then CONFIG_OF_PRIOR_STAGE is probably easier to use.

I'm not convinced there is a bug here.

Date: Fri, 10 Sep 2021 17:17:37 -0400 From: Tom Rini trini@konsulko.com

On Fri, Sep 10, 2021 at 11:12:20PM +0200, Mark Kettenis wrote:

...

...

Date: Fri, 10 Sep 2021 08:34:20 -0400 From: Tom Rini trini@konsulko.com

On Fri, Sep 10, 2021 at 10:38:17AM +0200, Heinrich Schuchardt wrote:

...

On 9/9/21 10:10 PM, Simon Glass wrote:

...

At present some of the ideas and techniques behind devicetree in U-Boot are assumed, implied or unsaid. Add some documentation to cover how devicetree is build, how it can be modified and the rules about using the various CONFIG_OF_... options.

Signed-off-by: Simon Glass sjg@chromium.org Reviewed-by: Marcel Ziswiler marcel.ziswiler@toradex.com

---

Changes in v3:

• Fix typos linst suppled receive EFL
• Drop 'and' before 'self-defeating'
• Reword mention of control of QEMU's devicetree generation
• Add mention of dropping CONFIG_OF_BOARD
• Clarify the 'Once this bug is fixed' paragraph a bit
• Expand ways that CONFIG_OF_PRIOR_STAGE can support the U-Boot devicetree
• Add a note at the top explaining that his patch covers 'now', not 'future'
• Add note 'Note: Some boards use a devicetree in U-Boot which does not match'

Changes in v2:

• Fix typos per Sean (thank you!) and a few others

• Add a 'Use of U-Boot /config node' section

• Drop mention of dm-verity since that actually uses the kernel cmdline

• Explain that OF_BOARD will still work after these changes (in 'Once this bug is fixed...' paragraph)

• Expand a bit on the reason why the 'Current situation' is bad

• Clarify in a second place that Linux and U-Boot use the same devicetree in 'To be clear, while U-Boot...'

• Expand on why we should have rules for other projects in 'Devicetree in another project'

• Add a comment as to why devicetree in U-Boot is not 'bad design'

• Reword 'in-tree U-Boot devicetree' to 'devicetree source in U-Boot'

• Rewrite 'Devicetree generated on-the-fly in another project' to cover points raised on v1

• Add 'Why does U-Boot have its nodes and properties?'

• Add 'Why not have two devicetrees?'

  doc/develop/index.rst | 1 + doc/develop/package/devicetree.rst | 583 +++++++++++++++++++++++++++++ doc/develop/package/index.rst | 1 + 3 files changed, 585 insertions(+) create mode 100644 doc/develop/package/devicetree.rst

diff --git a/doc/develop/index.rst b/doc/develop/index.rst index 83c929babda..d5ad8f9fe53 100644 --- a/doc/develop/index.rst +++ b/doc/develop/index.rst @@ -36,6 +36,7 @@ Packaging :maxdepth: 1

 package/index

• package/devicetree

  Testing

diff --git a/doc/develop/package/devicetree.rst b/doc/develop/package/devicetree.rst new file mode 100644 index 00000000000..b1bd310d906 --- /dev/null +++ b/doc/develop/package/devicetree.rst @@ -0,0 +1,583 @@ +.. SPDX-License-Identifier: GPL-2.0+

+Updating the devicetree +=======================

+Note: This documentation describes how things are today, mostly, with some +mention of things that need to be fixed. It is not intended to point the way to +what might be done in the future. That should be the subject of discussions on +the mailing list.

+U-Boot uses devicetree for runtime configuration and storing required blobs or +any other information it needs to operate. It is possible to update the +devicetree separately from actually building U-Boot. This provides a good degree +of control and flexibility for firmware that uses U-Boot in conjunction with +other project.

+There are many reasons why it is useful to modify the devicetree after building +it:

+- Configuration can be changed, e.g. which UART to use +- A serial number can be added +- Public keys can be added to allow image verification +- Console output can be changed (e.g. to select serial or vidconsole)

+This section describes how to work with devicetree to accomplish your goals.

+See also :doc:`../devicetree/control` for a basic summary of the available +features.

+Devicetree source +-----------------

+Every board in U-Boot must include a devicetree sufficient to build and boot +that board on suitable hardware (or emulation). This is specified using the +`CONFIG DEFAULT_DEVICE_TREE` option.

+Current situation (August 2021) +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

+As an aside, at present U-Boot allows `CONFIG_DEFAULT_DEVICE_TREE` to be empty, +e.g. if `CONFIG_OF_BOARD` or `CONFIG_OF_PRIOR_STAGE` are used. This has +unfortunately created an enormous amount of confusion and some wasted effort. +This was not intended and this bug will be fixed soon.

+Some of the problems created are:

+- It is not obvious that the devicetree is coming from another project

+- There is no way to see even a sample devicetree for these platform in U-Boot,

• so it is hard to know what is going on, e.g. which devices are typically
• present

+- The other project may not provide a way to support U-Boot's requirements for

• devicetree, such as the /config node. Note: On the U-Boot mailing list, this
• was only discovered after weeks of discussion and confusion

+- For QEMU specifically, consulting two QEMU source files is required, for which

• there are no references in U-Boot documentation. The code is generating a
• devicetree, with some control from command-line args, but it is not clear
• how to add properties required by U-Boot.

+Specifically on the changes in U-Boot:

+- `CONFIG_OF_BOARD` was added in rpi_patch_ for Raspberry Pi, which does have

• an in-tree devicetree, but this feature has since been used for boards that
• don't

+- `CONFIG_OF_PRIOR_STAGE` was added in bcm_patch_ as part of a larger Broadcom

• change with a tag indicating it only affected one board, so the change in
• behaviour was not noticed at the time. It has since been used by RISC-V qemu
• boards.

+Note: It is not clear that we actually need both of these. Possibly +`CONFIG_OF_BOARD` can be dropped.

+Once this bug is fixed, CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE will override

What does "bug" refer to? Above you describe the current design not a bug.

The bug is that we have two options to provide seemingly the same functionality. Is there a functional difference between CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE ?

With CONFIG_OF_BOARD there is a function that returns the pointer to the DTB, so you can do all sort of things with it.

With CONFIG_OF_PRIOR_STAGE there is a variable that you need to set in low-level code to point at the DTB and there is a pre-defined function that returns that pointer.

CONFIG_OF_BOARD is more flexible than CONFIG_OF_PRIOR_STAGE, but if the only thing you want to do is to pass on a DTB that is passed in a CPU register to U-Boot then CONFIG_OF_PRIOR_STAGE is probably easier to use.

I'm not convinced there is a bug here.

Thanks for explaining. Couldn't CONFIG_OF_PRIOR_STAGE be rewritten as an implementation of CONFIG_OF_BOARD, possibly at the same or less overall code size? That I think is the potential bug.

Probably a little bit more code:

void * board_fdt_blob_setup(void) { return (void *)(uintptr_t)prior_stage_fdt_address; }

Hi,

On Fri, 10 Sept 2021 at 16:44, Tom Rini trini@konsulko.com wrote:

On Sat, Sep 11, 2021 at 12:09:40AM +0200, Mark Kettenis wrote:

...

...

Date: Fri, 10 Sep 2021 17:17:37 -0400 From: Tom Rini trini@konsulko.com

On Fri, Sep 10, 2021 at 11:12:20PM +0200, Mark Kettenis wrote:

...

...

Date: Fri, 10 Sep 2021 08:34:20 -0400 From: Tom Rini trini@konsulko.com

On Fri, Sep 10, 2021 at 10:38:17AM +0200, Heinrich Schuchardt wrote:

...

On 9/9/21 10:10 PM, Simon Glass wrote: > At present some of the ideas and techniques behind devicetree in U-Boot > are assumed, implied or unsaid. Add some documentation to cover how > devicetree is build, how it can be modified and the rules about using > the various CONFIG_OF_... options. > > Signed-off-by: Simon Glass sjg@chromium.org > Reviewed-by: Marcel Ziswiler marcel.ziswiler@toradex.com > --- > > Changes in v3: > - Fix typos linst suppled receive EFL > - Drop 'and' before 'self-defeating' > - Reword mention of control of QEMU's devicetree generation > - Add mention of dropping CONFIG_OF_BOARD > - Clarify the 'Once this bug is fixed' paragraph a bit > - Expand ways that CONFIG_OF_PRIOR_STAGE can support the U-Boot devicetree > - Add a note at the top explaining that his patch covers 'now', not 'future' > - Add note 'Note: Some boards use a devicetree in U-Boot which does not match' > > Changes in v2: > - Fix typos per Sean (thank you!) and a few others > - Add a 'Use of U-Boot /config node' section > - Drop mention of dm-verity since that actually uses the kernel cmdline > - Explain that OF_BOARD will still work after these changes (in > 'Once this bug is fixed...' paragraph) > - Expand a bit on the reason why the 'Current situation' is bad > - Clarify in a second place that Linux and U-Boot use the same devicetree > in 'To be clear, while U-Boot...' > - Expand on why we should have rules for other projects in > 'Devicetree in another project' > - Add a comment as to why devicetree in U-Boot is not 'bad design' > - Reword 'in-tree U-Boot devicetree' to 'devicetree source in U-Boot' > - Rewrite 'Devicetree generated on-the-fly in another project' to cover > points raised on v1 > - Add 'Why does U-Boot have its nodes and properties?' > - Add 'Why not have two devicetrees?' > > doc/develop/index.rst | 1 + > doc/develop/package/devicetree.rst | 583 +++++++++++++++++++++++++++++ > doc/develop/package/index.rst | 1 + > 3 files changed, 585 insertions(+) > create mode 100644 doc/develop/package/devicetree.rst > > diff --git a/doc/develop/index.rst b/doc/develop/index.rst > index 83c929babda..d5ad8f9fe53 100644 > --- a/doc/develop/index.rst > +++ b/doc/develop/index.rst > @@ -36,6 +36,7 @@ Packaging > :maxdepth: 1 > > package/index > + package/devicetree > > Testing > ------- > diff --git a/doc/develop/package/devicetree.rst b/doc/develop/package/devicetree.rst > new file mode 100644 > index 00000000000..b1bd310d906 > --- /dev/null > +++ b/doc/develop/package/devicetree.rst > @@ -0,0 +1,583 @@ > +.. SPDX-License-Identifier: GPL-2.0+ > + > +Updating the devicetree > +======================= > + > +Note: This documentation describes how things are today, mostly, with some > +mention of things that need to be fixed. It is not intended to point the way to > +what might be done in the future. That should be the subject of discussions on > +the mailing list. > + > +U-Boot uses devicetree for runtime configuration and storing required blobs or > +any other information it needs to operate. It is possible to update the > +devicetree separately from actually building U-Boot. This provides a good degree > +of control and flexibility for firmware that uses U-Boot in conjunction with > +other project. > + > +There are many reasons why it is useful to modify the devicetree after building > +it: > + > +- Configuration can be changed, e.g. which UART to use > +- A serial number can be added > +- Public keys can be added to allow image verification > +- Console output can be changed (e.g. to select serial or vidconsole) > + > +This section describes how to work with devicetree to accomplish your goals. > + > +See also :doc:`../devicetree/control` for a basic summary of the available > +features. > + > + > +Devicetree source > +----------------- > + > +Every board in U-Boot must include a devicetree sufficient to build and boot > +that board on suitable hardware (or emulation). This is specified using the > +`CONFIG DEFAULT_DEVICE_TREE` option. > + > + > +Current situation (August 2021) > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > + > +As an aside, at present U-Boot allows `CONFIG_DEFAULT_DEVICE_TREE` to be empty, > +e.g. if `CONFIG_OF_BOARD` or `CONFIG_OF_PRIOR_STAGE` are used. This has > +unfortunately created an enormous amount of confusion and some wasted effort. > +This was not intended and this bug will be fixed soon. > + > +Some of the problems created are: > + > +- It is not obvious that the devicetree is coming from another project > + > +- There is no way to see even a sample devicetree for these platform in U-Boot, > + so it is hard to know what is going on, e.g. which devices are typically > + present > + > +- The other project may not provide a way to support U-Boot's requirements for > + devicetree, such as the /config node. Note: On the U-Boot mailing list, this > + was only discovered after weeks of discussion and confusion > + > +- For QEMU specifically, consulting two QEMU source files is required, for which > + there are no references in U-Boot documentation. The code is generating a > + devicetree, with some control from command-line args, but it is not clear > + how to add properties required by U-Boot. > + > +Specifically on the changes in U-Boot: > + > +- `CONFIG_OF_BOARD` was added in rpi_patch_ for Raspberry Pi, which does have > + an in-tree devicetree, but this feature has since been used for boards that > + don't > +- `CONFIG_OF_PRIOR_STAGE` was added in bcm_patch_ as part of a larger Broadcom > + change with a tag indicating it only affected one board, so the change in > + behaviour was not noticed at the time. It has since been used by RISC-V qemu > + boards. > + > +Note: It is not clear that we actually need both of these. Possibly > +`CONFIG_OF_BOARD` can be dropped. > + > +Once this bug is fixed, CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE will override

What does "bug" refer to? Above you describe the current design not a bug.

The bug is that we have two options to provide seemingly the same functionality. Is there a functional difference between CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE ?

With CONFIG_OF_BOARD there is a function that returns the pointer to the DTB, so you can do all sort of things with it.

With CONFIG_OF_PRIOR_STAGE there is a variable that you need to set in low-level code to point at the DTB and there is a pre-defined function that returns that pointer.

CONFIG_OF_BOARD is more flexible than CONFIG_OF_PRIOR_STAGE, but if the only thing you want to do is to pass on a DTB that is passed in a CPU register to U-Boot then CONFIG_OF_PRIOR_STAGE is probably easier to use.

I'm not convinced there is a bug here.

Thanks for explaining. Couldn't CONFIG_OF_PRIOR_STAGE be rewritten as an implementation of CONFIG_OF_BOARD, possibly at the same or less overall code size? That I think is the potential bug.

Probably a little bit more code:

void * board_fdt_blob_setup(void) { return (void *)(uintptr_t)prior_stage_fdt_address; }

Tiny bit more. Probably worth doing to make the choices clearer on which to select when? Bin, Rick, thoughts on this since riscv is the main user of CONFIG_OF_PRIOR_STAGE at this point?

Bin, Rick?

What is the prior stage in the RISC-V stage? Could we get it to set up a bloblist? Then we can add a devicetree in there, with the option to add more things in future.

Regards, Simon

Le sam. 18 sept. 2021 à 15:18, Tom Rini trini@konsulko.com a écrit :

On Sat, Sep 18, 2021 at 03:38:45AM -0600, Simon Glass wrote:

...

Hi,

On Fri, 10 Sept 2021 at 16:44, Tom Rini trini@konsulko.com wrote:

...

On Sat, Sep 11, 2021 at 12:09:40AM +0200, Mark Kettenis wrote:

...

...

Date: Fri, 10 Sep 2021 17:17:37 -0400 From: Tom Rini trini@konsulko.com

On Fri, Sep 10, 2021 at 11:12:20PM +0200, Mark Kettenis wrote:

...

> Date: Fri, 10 Sep 2021 08:34:20 -0400 > From: Tom Rini trini@konsulko.com > > On Fri, Sep 10, 2021 at 10:38:17AM +0200, Heinrich Schuchardt

wrote:

...

...

...

...

...

> > > > > > On 9/9/21 10:10 PM, Simon Glass wrote: > > > At present some of the ideas and techniques behind

devicetree in U-Boot

...

...

...

...

...

> > > are assumed, implied or unsaid. Add some documentation to

cover how

...

...

...

...

...

> > > devicetree is build, how it can be modified and the rules

about using

...

...

...

...

...

> > > the various CONFIG_OF_... options. > > > > > > Signed-off-by: Simon Glass sjg@chromium.org > > > Reviewed-by: Marcel Ziswiler marcel.ziswiler@toradex.com > > > --- > > > > > > Changes in v3: > > > - Fix typos linst suppled receive EFL > > > - Drop 'and' before 'self-defeating' > > > - Reword mention of control of QEMU's devicetree generation > > > - Add mention of dropping CONFIG_OF_BOARD > > > - Clarify the 'Once this bug is fixed' paragraph a bit > > > - Expand ways that CONFIG_OF_PRIOR_STAGE can support the

U-Boot devicetree

...

...

...

...

...

> > > - Add a note at the top explaining that his patch covers

'now', not 'future'

...

...

...

...

...

> > > - Add note 'Note: Some boards use a devicetree in U-Boot

which does not match'

...

...

...

...

...

> > > > > > Changes in v2: > > > - Fix typos per Sean (thank you!) and a few others > > > - Add a 'Use of U-Boot /config node' section > > > - Drop mention of dm-verity since that actually uses the

kernel cmdline

...

...

...

...

...

> > > - Explain that OF_BOARD will still work after these

changes (in

...

...

...

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...

> > > 'Once this bug is fixed...' paragraph) > > > - Expand a bit on the reason why the 'Current situation'

is bad

...

...

...

...

...

> > > - Clarify in a second place that Linux and U-Boot use the

same devicetree

...

...

...

...

...

> > > in 'To be clear, while U-Boot...' > > > - Expand on why we should have rules for other projects in > > > 'Devicetree in another project' > > > - Add a comment as to why devicetree in U-Boot is not 'bad

design'

...

...

...

...

...

> > > - Reword 'in-tree U-Boot devicetree' to 'devicetree source

in U-Boot'

...

...

...

...

...

> > > - Rewrite 'Devicetree generated on-the-fly in another

project' to cover

...

...

...

...

...

> > > points raised on v1 > > > - Add 'Why does U-Boot have its nodes and properties?' > > > - Add 'Why not have two devicetrees?' > > > > > > doc/develop/index.rst | 1 + > > > doc/develop/package/devicetree.rst | 583

+++++++++++++++++++++++++++++

...

...

...

...

...

> > > doc/develop/package/index.rst | 1 + > > > 3 files changed, 585 insertions(+) > > > create mode 100644 doc/develop/package/devicetree.rst > > > > > > diff --git a/doc/develop/index.rst b/doc/develop/index.rst > > > index 83c929babda..d5ad8f9fe53 100644 > > > --- a/doc/develop/index.rst > > > +++ b/doc/develop/index.rst > > > @@ -36,6 +36,7 @@ Packaging > > > :maxdepth: 1 > > > > > > package/index > > > + package/devicetree > > > > > > Testing > > > ------- > > > diff --git a/doc/develop/package/devicetree.rst

b/doc/develop/package/devicetree.rst

...

...

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...

...

> > > new file mode 100644 > > > index 00000000000..b1bd310d906 > > > --- /dev/null > > > +++ b/doc/develop/package/devicetree.rst > > > @@ -0,0 +1,583 @@ > > > +.. SPDX-License-Identifier: GPL-2.0+ > > > + > > > +Updating the devicetree > > > +======================= > > > + > > > +Note: This documentation describes how things are today,

mostly, with some

...

...

...

...

...

> > > +mention of things that need to be fixed. It is not

intended to point the way to

...

...

...

...

...

> > > +what might be done in the future. That should be the

subject of discussions on

...

...

...

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> > > +the mailing list. > > > + > > > +U-Boot uses devicetree for runtime configuration and

storing required blobs or

...

...

...

...

...

> > > +any other information it needs to operate. It is possible

to update the

...

...

...

...

...

> > > +devicetree separately from actually building U-Boot. This

provides a good degree

...

...

...

...

...

> > > +of control and flexibility for firmware that uses U-Boot

in conjunction with

...

...

...

...

...

> > > +other project. > > > + > > > +There are many reasons why it is useful to modify the

devicetree after building

...

...

...

...

...

> > > +it: > > > + > > > +- Configuration can be changed, e.g. which UART to use > > > +- A serial number can be added > > > +- Public keys can be added to allow image verification > > > +- Console output can be changed (e.g. to select serial or

vidconsole)

...

...

...

...

...

> > > + > > > +This section describes how to work with devicetree to

accomplish your goals.

...

...

...

...

...

> > > + > > > +See also :doc:`../devicetree/control` for a basic summary

of the available

...

...

...

...

...

> > > +features. > > > + > > > + > > > +Devicetree source > > > +----------------- > > > + > > > +Every board in U-Boot must include a devicetree

sufficient to build and boot

...

...

...

...

...

> > > +that board on suitable hardware (or emulation). This is

specified using the

...

...

...

...

...

> > > +`CONFIG DEFAULT_DEVICE_TREE` option. > > > + > > > + > > > +Current situation (August 2021) > > > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > > + > > > +As an aside, at present U-Boot allows

`CONFIG_DEFAULT_DEVICE_TREE` to be empty,

...

...

...

...

...

> > > +e.g. if `CONFIG_OF_BOARD` or `CONFIG_OF_PRIOR_STAGE` are

used. This has

...

...

...

...

...

> > > +unfortunately created an enormous amount of confusion and

some wasted effort.

...

...

...

...

...

> > > +This was not intended and this bug will be fixed soon. > > > + > > > +Some of the problems created are: > > > + > > > +- It is not obvious that the devicetree is coming from

another project

...

...

...

...

...

> > > + > > > +- There is no way to see even a sample devicetree for

these platform in U-Boot,

...

...

...

...

...

> > > + so it is hard to know what is going on, e.g. which

devices are typically

...

...

...

...

...

> > > + present > > > + > > > +- The other project may not provide a way to support

U-Boot's requirements for

...

...

...

...

...

> > > + devicetree, such as the /config node. Note: On the

U-Boot mailing list, this

...

...

...

...

...

> > > + was only discovered after weeks of discussion and

confusion

...

...

...

...

...

> > > + > > > +- For QEMU specifically, consulting two QEMU source files

is required, for which

...

...

...

...

...

> > > + there are no references in U-Boot documentation. The

code is generating a

...

...

...

...

...

> > > + devicetree, with some control from command-line args,

but it is not clear

...

...

...

...

...

> > > + how to add properties required by U-Boot. > > > + > > > +Specifically on the changes in U-Boot: > > > + > > > +- `CONFIG_OF_BOARD` was added in rpi_patch_ for Raspberry

Pi, which does have

...

...

...

...

...

> > > + an in-tree devicetree, but this feature has since been

used for boards that

...

...

...

...

...

> > > + don't > > > +- `CONFIG_OF_PRIOR_STAGE` was added in bcm_patch_ as part

of a larger Broadcom

...

...

...

...

...

> > > + change with a tag indicating it only affected one

board, so the change in

...

...

...

...

...

> > > + behaviour was not noticed at the time. It has since

been used by RISC-V qemu

...

...

...

...

...

> > > + boards. > > > + > > > +Note: It is not clear that we actually need both of

these. Possibly

...

...

...

...

...

> > > +`CONFIG_OF_BOARD` can be dropped. > > > + > > > +Once this bug is fixed, CONFIG_OF_BOARD and

CONFIG_OF_PRIOR_STAGE will override

...

...

...

...

...

> > > > What does "bug" refer to? Above you describe the current

design not a bug.

...

...

...

...

...

> > The bug is that we have two options to provide seemingly the

same

...

...

...

...

...

> functionality. Is there a functional difference between

CONFIG_OF_BOARD

...

...

...

...

...

> and CONFIG_OF_PRIOR_STAGE ?

With CONFIG_OF_BOARD there is a function that returns the

pointer to

...

...

...

...

...

the DTB, so you can do all sort of things with it.

With CONFIG_OF_PRIOR_STAGE there is a variable that you need to

set in

...

...

...

...

...

low-level code to point at the DTB and there is a pre-defined

function

...

...

...

...

...

that returns that pointer.

CONFIG_OF_BOARD is more flexible than CONFIG_OF_PRIOR_STAGE, but

if

...

...

...

...

...

the only thing you want to do is to pass on a DTB that is passed

in a

...

...

...

...

...

CPU register to U-Boot then CONFIG_OF_PRIOR_STAGE is probably

easier

...

...

...

...

...

to use.

I'm not convinced there is a bug here.

Thanks for explaining. Couldn't CONFIG_OF_PRIOR_STAGE be

rewritten as

...

...

...

...

an implementation of CONFIG_OF_BOARD, possibly at the same or less overall code size? That I think is the potential bug.

Probably a little bit more code:

void * board_fdt_blob_setup(void) { return (void *)(uintptr_t)prior_stage_fdt_address; }

Tiny bit more. Probably worth doing to make the choices clearer on which to select when? Bin, Rick, thoughts on this since riscv is the main user of CONFIG_OF_PRIOR_STAGE at this point?

Bin, Rick?

What is the prior stage in the RISC-V stage? Could we get it to set up a bloblist? Then we can add a devicetree in there, with the option to add more things in future.

I'm suggesting we don't need to do anything upstream of us, just rework things to use the other hook for "provided a DTB by caller, use it", so that we have a single hook for that.

-- Tom

What was the rationale in posting in kernel.org ( https://lore.kernel.org/all/20211003125134.2.I7733f5a849476e908cc51f0c71b8a5... ) and not in U-Boot knowing there is still no consensus on the big picture ? We agreed you would defer the device tree documentation patch you proposed because we did not agree on the painted overall picture. So I was surprised by your post. I agree standardization of U-Boot bindings is a good thing. Trustedfirnware.org does it internally and U-Boot can get inspiration from this. https://trustedfirmware-a.readthedocs.io/en/latest/components/cot-binding.ht...

--

François-Frédéric Ozog | *Director Business Development* T: +33.67221.6485 francois.ozog@linaro.org | Skype: ffozog

Le mer. 13 oct. 2021 à 14:55, Tom Rini trini@konsulko.com a écrit :

On Wed, Oct 13, 2021 at 10:15:02AM +0200, François Ozog wrote:

...

Le sam. 18 sept. 2021 à 15:18, Tom Rini trini@konsulko.com a écrit :

...

On Sat, Sep 18, 2021 at 03:38:45AM -0600, Simon Glass wrote:

...

Hi,

On Fri, 10 Sept 2021 at 16:44, Tom Rini trini@konsulko.com wrote:

...

On Sat, Sep 11, 2021 at 12:09:40AM +0200, Mark Kettenis wrote:

...

> Date: Fri, 10 Sep 2021 17:17:37 -0400 > From: Tom Rini trini@konsulko.com > > On Fri, Sep 10, 2021 at 11:12:20PM +0200, Mark Kettenis wrote: > > > Date: Fri, 10 Sep 2021 08:34:20 -0400 > > > From: Tom Rini trini@konsulko.com > > > > > > On Fri, Sep 10, 2021 at 10:38:17AM +0200, Heinrich

Schuchardt

...

...

wrote:

...

...

...

> > > > > > > > > > > > On 9/9/21 10:10 PM, Simon Glass wrote: > > > > > At present some of the ideas and techniques behind

devicetree in U-Boot

...

...

...

> > > > > are assumed, implied or unsaid. Add some documentation

to

...

...

cover how

...

...

...

> > > > > devicetree is build, how it can be modified and the

rules

...

...

about using

...

...

...

> > > > > the various CONFIG_OF_... options. > > > > > > > > > > Signed-off-by: Simon Glass sjg@chromium.org > > > > > Reviewed-by: Marcel Ziswiler <

marcel.ziswiler@toradex.com>

...

...

...

...

...

> > > > > --- > > > > > > > > > > Changes in v3: > > > > > - Fix typos linst suppled receive EFL > > > > > - Drop 'and' before 'self-defeating' > > > > > - Reword mention of control of QEMU's devicetree

generation

...

...

...

...

...

> > > > > - Add mention of dropping CONFIG_OF_BOARD > > > > > - Clarify the 'Once this bug is fixed' paragraph a bit > > > > > - Expand ways that CONFIG_OF_PRIOR_STAGE can support

the

...

...

U-Boot devicetree

...

...

...

> > > > > - Add a note at the top explaining that his patch

covers

...

...

'now', not 'future'

...

...

...

> > > > > - Add note 'Note: Some boards use a devicetree in

U-Boot

...

...

which does not match'

...

...

...

> > > > > > > > > > Changes in v2: > > > > > - Fix typos per Sean (thank you!) and a few others > > > > > - Add a 'Use of U-Boot /config node' section > > > > > - Drop mention of dm-verity since that actually uses

the

...

...

kernel cmdline

...

...

...

> > > > > - Explain that OF_BOARD will still work after these

changes (in

...

...

...

> > > > > 'Once this bug is fixed...' paragraph) > > > > > - Expand a bit on the reason why the 'Current

situation'

...

...

is bad

...

...

...

> > > > > - Clarify in a second place that Linux and U-Boot use

the

...

...

same devicetree

...

...

...

> > > > > in 'To be clear, while U-Boot...' > > > > > - Expand on why we should have rules for other

projects in

...

...

...

...

...

> > > > > 'Devicetree in another project' > > > > > - Add a comment as to why devicetree in U-Boot is not

'bad

...

...

design'

...

...

...

> > > > > - Reword 'in-tree U-Boot devicetree' to 'devicetree

source

...

...

in U-Boot'

...

...

...

> > > > > - Rewrite 'Devicetree generated on-the-fly in another

project' to cover

...

...

...

> > > > > points raised on v1 > > > > > - Add 'Why does U-Boot have its nodes and properties?' > > > > > - Add 'Why not have two devicetrees?' > > > > > > > > > > doc/develop/index.rst | 1 + > > > > > doc/develop/package/devicetree.rst | 583

+++++++++++++++++++++++++++++

...

...

...

> > > > > doc/develop/package/index.rst | 1 + > > > > > 3 files changed, 585 insertions(+) > > > > > create mode 100644 doc/develop/package/devicetree.rst > > > > > > > > > > diff --git a/doc/develop/index.rst

b/doc/develop/index.rst

...

...

...

...

...

> > > > > index 83c929babda..d5ad8f9fe53 100644 > > > > > --- a/doc/develop/index.rst > > > > > +++ b/doc/develop/index.rst > > > > > @@ -36,6 +36,7 @@ Packaging > > > > > :maxdepth: 1 > > > > > > > > > > package/index > > > > > + package/devicetree > > > > > > > > > > Testing > > > > > ------- > > > > > diff --git a/doc/develop/package/devicetree.rst

b/doc/develop/package/devicetree.rst

...

...

...

> > > > > new file mode 100644 > > > > > index 00000000000..b1bd310d906 > > > > > --- /dev/null > > > > > +++ b/doc/develop/package/devicetree.rst > > > > > @@ -0,0 +1,583 @@ > > > > > +.. SPDX-License-Identifier: GPL-2.0+ > > > > > + > > > > > +Updating the devicetree > > > > > +======================= > > > > > + > > > > > +Note: This documentation describes how things are

today,

...

...

mostly, with some

...

...

...

> > > > > +mention of things that need to be fixed. It is not

intended to point the way to

...

...

...

> > > > > +what might be done in the future. That should be the

subject of discussions on

...

...

...

> > > > > +the mailing list. > > > > > + > > > > > +U-Boot uses devicetree for runtime configuration and

storing required blobs or

...

...

...

> > > > > +any other information it needs to operate. It is

possible

...

...

to update the

...

...

...

> > > > > +devicetree separately from actually building U-Boot.

This

...

...

provides a good degree

...

...

...

> > > > > +of control and flexibility for firmware that uses

U-Boot

...

...

in conjunction with

...

...

...

> > > > > +other project. > > > > > + > > > > > +There are many reasons why it is useful to modify the

devicetree after building

...

...

...

> > > > > +it: > > > > > + > > > > > +- Configuration can be changed, e.g. which UART to use > > > > > +- A serial number can be added > > > > > +- Public keys can be added to allow image verification > > > > > +- Console output can be changed (e.g. to select

serial or

...

...

vidconsole)

...

...

...

> > > > > + > > > > > +This section describes how to work with devicetree to

accomplish your goals.

...

...

...

> > > > > + > > > > > +See also :doc:`../devicetree/control` for a basic

summary

...

...

of the available

...

...

...

> > > > > +features. > > > > > + > > > > > + > > > > > +Devicetree source > > > > > +----------------- > > > > > + > > > > > +Every board in U-Boot must include a devicetree

sufficient to build and boot

...

...

...

> > > > > +that board on suitable hardware (or emulation). This

is

...

...

specified using the

...

...

...

> > > > > +`CONFIG DEFAULT_DEVICE_TREE` option. > > > > > + > > > > > + > > > > > +Current situation (August 2021) > > > > > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > > > > + > > > > > +As an aside, at present U-Boot allows

`CONFIG_DEFAULT_DEVICE_TREE` to be empty,

...

...

...

> > > > > +e.g. if `CONFIG_OF_BOARD` or `CONFIG_OF_PRIOR_STAGE`

are

...

...

used. This has

...

...

...

> > > > > +unfortunately created an enormous amount of confusion

and

...

...

some wasted effort.

...

...

...

> > > > > +This was not intended and this bug will be fixed soon. > > > > > + > > > > > +Some of the problems created are: > > > > > + > > > > > +- It is not obvious that the devicetree is coming from

another project

...

...

...

> > > > > + > > > > > +- There is no way to see even a sample devicetree for

these platform in U-Boot,

...

...

...

> > > > > + so it is hard to know what is going on, e.g. which

devices are typically

...

...

...

> > > > > + present > > > > > + > > > > > +- The other project may not provide a way to support

U-Boot's requirements for

...

...

...

> > > > > + devicetree, such as the /config node. Note: On the

U-Boot mailing list, this

...

...

...

> > > > > + was only discovered after weeks of discussion and

confusion

...

...

...

> > > > > + > > > > > +- For QEMU specifically, consulting two QEMU source

files

...

...

is required, for which

...

...

...

> > > > > + there are no references in U-Boot documentation. The

code is generating a

...

...

...

> > > > > + devicetree, with some control from command-line

args,

...

...

but it is not clear

...

...

...

> > > > > + how to add properties required by U-Boot. > > > > > + > > > > > +Specifically on the changes in U-Boot: > > > > > + > > > > > +- `CONFIG_OF_BOARD` was added in rpi_patch_ for

Raspberry

...

...

Pi, which does have

...

...

...

> > > > > + an in-tree devicetree, but this feature has since

been

...

...

used for boards that

...

...

...

> > > > > + don't > > > > > +- `CONFIG_OF_PRIOR_STAGE` was added in bcm_patch_ as

part

...

...

of a larger Broadcom

...

...

...

> > > > > + change with a tag indicating it only affected one

board, so the change in

...

...

...

> > > > > + behaviour was not noticed at the time. It has since

been used by RISC-V qemu

...

...

...

> > > > > + boards. > > > > > + > > > > > +Note: It is not clear that we actually need both of

these. Possibly

...

...

...

> > > > > +`CONFIG_OF_BOARD` can be dropped. > > > > > + > > > > > +Once this bug is fixed, CONFIG_OF_BOARD and

CONFIG_OF_PRIOR_STAGE will override

...

...

...

> > > > > > > > What does "bug" refer to? Above you describe the current

design not a bug.

...

...

...

> > > > > > The bug is that we have two options to provide seemingly

the

...

...

same

...

...

...

> > > functionality. Is there a functional difference between

CONFIG_OF_BOARD

...

...

...

> > > and CONFIG_OF_PRIOR_STAGE ? > > > > With CONFIG_OF_BOARD there is a function that returns the

pointer to

...

...

...

> > the DTB, so you can do all sort of things with it. > > > > With CONFIG_OF_PRIOR_STAGE there is a variable that you need

to

...

...

set in

...

...

...

> > low-level code to point at the DTB and there is a pre-defined

function

...

...

...

> > that returns that pointer. > > > > CONFIG_OF_BOARD is more flexible than CONFIG_OF_PRIOR_STAGE,

but

...

...

if

...

...

...

> > the only thing you want to do is to pass on a DTB that is

passed

...

...

in a

...

...

...

> > CPU register to U-Boot then CONFIG_OF_PRIOR_STAGE is probably

easier

...

...

...

> > to use. > > > > I'm not convinced there is a bug here. > > Thanks for explaining. Couldn't CONFIG_OF_PRIOR_STAGE be

rewritten as

...

...

...

> an implementation of CONFIG_OF_BOARD, possibly at the same or

less

...

...

...

...

...

> overall code size? That I think is the potential bug.

Probably a little bit more code:

void * board_fdt_blob_setup(void) { return (void *)(uintptr_t)prior_stage_fdt_address; }

Tiny bit more. Probably worth doing to make the choices clearer on which to select when? Bin, Rick, thoughts on this since riscv is

the

...

...

...

...

main user of CONFIG_OF_PRIOR_STAGE at this point?

Bin, Rick?

What is the prior stage in the RISC-V stage? Could we get it to set

up

...

...

...

a bloblist? Then we can add a devicetree in there, with the option to add more things in future.

I'm suggesting we don't need to do anything upstream of us, just rework things to use the other hook for "provided a DTB by caller, use it", so that we have a single hook for that.

-- Tom

What was the rationale in posting in kernel.org (

https://lore.kernel.org/all/20211003125134.2.I7733f5a849476e908cc51f0c71b8a5...

...

) and not in U-Boot knowing there is still no consensus on the big

picture ?

Well, because we need to get our bindings reviewed and made official, and that looked like a reasonable place and choice to start with? v2 cc'd a different set of lists, at Rob's suggestion.

making it “ Official” does not necessarily mean in the Linux DT. What would you think with a hardware description file that comes with U-Boot, LinuxBoot and EDK2 config options ? No need for Linux to know about those U-Boot internal config? Why would those U-Boot elements be passed down to any OS ? Do you expect to have bootloader specific behavior in distros ? TF-A makes the binding official and standardized in its own documentation. U-Boot can and should do the same.

...

We agreed you would defer the device tree documentation patch you

proposed

...

because we did not agree on the painted overall picture. So I was

surprised

...

by your post. I agree standardization of U-Boot bindings is a good thing. Trustedfirnware.org does it internally and U-Boot can get inspiration

from

...

this.

https://trustedfirmware-a.readthedocs.io/en/latest/components/cot-binding.ht...

Note that your example there should also be reviewed and sent upstream as the problem is less "U-Boot's config binding isn't documented" but more "U-Boot's config binding isn't official".

-- Tom

Le mer. 13 oct. 2021 à 15:30, Tom Rini trini@konsulko.com a écrit :

On Wed, Oct 13, 2021 at 03:12:02PM +0200, François Ozog wrote:

...

Le mer. 13 oct. 2021 à 14:55, Tom Rini trini@konsulko.com a écrit :

...

On Wed, Oct 13, 2021 at 10:15:02AM +0200, François Ozog wrote:

...

Le sam. 18 sept. 2021 à 15:18, Tom Rini trini@konsulko.com a

écrit :

...

...

...

...

On Sat, Sep 18, 2021 at 03:38:45AM -0600, Simon Glass wrote:

...

Hi,

On Fri, 10 Sept 2021 at 16:44, Tom Rini trini@konsulko.com

wrote:

...

...

...

...

...

> > On Sat, Sep 11, 2021 at 12:09:40AM +0200, Mark Kettenis wrote: > > > Date: Fri, 10 Sep 2021 17:17:37 -0400 > > > From: Tom Rini trini@konsulko.com > > > > > > On Fri, Sep 10, 2021 at 11:12:20PM +0200, Mark Kettenis

wrote:

...

...

...

...

...

> > > > > Date: Fri, 10 Sep 2021 08:34:20 -0400 > > > > > From: Tom Rini trini@konsulko.com > > > > > > > > > > On Fri, Sep 10, 2021 at 10:38:17AM +0200, Heinrich

Schuchardt

...

...

wrote:

...

> > > > > > > > > > > > > > > > > > On 9/9/21 10:10 PM, Simon Glass wrote: > > > > > > > At present some of the ideas and techniques behind

devicetree in U-Boot

...

> > > > > > > are assumed, implied or unsaid. Add some

documentation

...

...

to

...

...

cover how

...

> > > > > > > devicetree is build, how it can be modified and the

rules

...

...

about using

...

> > > > > > > the various CONFIG_OF_... options. > > > > > > > > > > > > > > Signed-off-by: Simon Glass sjg@chromium.org > > > > > > > Reviewed-by: Marcel Ziswiler <

marcel.ziswiler@toradex.com>

...

...

...

> > > > > > > --- > > > > > > > > > > > > > > Changes in v3: > > > > > > > - Fix typos linst suppled receive EFL > > > > > > > - Drop 'and' before 'self-defeating' > > > > > > > - Reword mention of control of QEMU's devicetree

generation

...

...

...

> > > > > > > - Add mention of dropping CONFIG_OF_BOARD > > > > > > > - Clarify the 'Once this bug is fixed' paragraph a

bit

...

...

...

...

...

> > > > > > > - Expand ways that CONFIG_OF_PRIOR_STAGE can

support

...

...

the

...

...

U-Boot devicetree

...

> > > > > > > - Add a note at the top explaining that his patch

covers

...

...

'now', not 'future'

...

> > > > > > > - Add note 'Note: Some boards use a devicetree in

U-Boot

...

...

which does not match'

...

> > > > > > > > > > > > > > Changes in v2: > > > > > > > - Fix typos per Sean (thank you!) and a few others > > > > > > > - Add a 'Use of U-Boot /config node' section > > > > > > > - Drop mention of dm-verity since that actually

uses

...

...

the

...

...

kernel cmdline

...

> > > > > > > - Explain that OF_BOARD will still work after these

changes (in

...

> > > > > > > 'Once this bug is fixed...' paragraph) > > > > > > > - Expand a bit on the reason why the 'Current

situation'

...

...

is bad

...

> > > > > > > - Clarify in a second place that Linux and U-Boot

use

...

...

the

...

...

same devicetree

...

> > > > > > > in 'To be clear, while U-Boot...' > > > > > > > - Expand on why we should have rules for other

projects in

...

...

...

> > > > > > > 'Devicetree in another project' > > > > > > > - Add a comment as to why devicetree in U-Boot is

not

...

...

'bad

...

...

design'

...

> > > > > > > - Reword 'in-tree U-Boot devicetree' to 'devicetree

source

...

...

in U-Boot'

...

> > > > > > > - Rewrite 'Devicetree generated on-the-fly in

another

...

...

...

...

project' to cover

...

> > > > > > > points raised on v1 > > > > > > > - Add 'Why does U-Boot have its nodes and

properties?'

...

...

...

...

...

> > > > > > > - Add 'Why not have two devicetrees?' > > > > > > > > > > > > > > doc/develop/index.rst | 1 + > > > > > > > doc/develop/package/devicetree.rst | 583

+++++++++++++++++++++++++++++

...

> > > > > > > doc/develop/package/index.rst | 1 + > > > > > > > 3 files changed, 585 insertions(+) > > > > > > > create mode 100644

doc/develop/package/devicetree.rst

...

...

...

...

...

> > > > > > > > > > > > > > diff --git a/doc/develop/index.rst

b/doc/develop/index.rst

...

...

...

> > > > > > > index 83c929babda..d5ad8f9fe53 100644 > > > > > > > --- a/doc/develop/index.rst > > > > > > > +++ b/doc/develop/index.rst > > > > > > > @@ -36,6 +36,7 @@ Packaging > > > > > > > :maxdepth: 1 > > > > > > > > > > > > > > package/index > > > > > > > + package/devicetree > > > > > > > > > > > > > > Testing > > > > > > > ------- > > > > > > > diff --git a/doc/develop/package/devicetree.rst

b/doc/develop/package/devicetree.rst

...

> > > > > > > new file mode 100644 > > > > > > > index 00000000000..b1bd310d906 > > > > > > > --- /dev/null > > > > > > > +++ b/doc/develop/package/devicetree.rst > > > > > > > @@ -0,0 +1,583 @@ > > > > > > > +.. SPDX-License-Identifier: GPL-2.0+ > > > > > > > + > > > > > > > +Updating the devicetree > > > > > > > +======================= > > > > > > > + > > > > > > > +Note: This documentation describes how things are

today,

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mostly, with some

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> > > > > > > +mention of things that need to be fixed. It is not

intended to point the way to

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> > > > > > > +what might be done in the future. That should be

the

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subject of discussions on

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> > > > > > > +the mailing list. > > > > > > > + > > > > > > > +U-Boot uses devicetree for runtime configuration

and

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storing required blobs or

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> > > > > > > +any other information it needs to operate. It is

possible

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to update the

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> > > > > > > +devicetree separately from actually building

U-Boot.

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This

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provides a good degree

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> > > > > > > +of control and flexibility for firmware that uses

U-Boot

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in conjunction with

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> > > > > > > +other project. > > > > > > > + > > > > > > > +There are many reasons why it is useful to modify

the

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devicetree after building

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> > > > > > > +it: > > > > > > > + > > > > > > > +- Configuration can be changed, e.g. which UART

to use

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> > > > > > > +- A serial number can be added > > > > > > > +- Public keys can be added to allow image

verification

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> > > > > > > +- Console output can be changed (e.g. to select

serial or

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vidconsole)

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> > > > > > > + > > > > > > > +This section describes how to work with

devicetree to

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accomplish your goals.

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> > > > > > > + > > > > > > > +See also :doc:`../devicetree/control` for a basic

summary

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of the available

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> > > > > > > +features. > > > > > > > + > > > > > > > + > > > > > > > +Devicetree source > > > > > > > +----------------- > > > > > > > + > > > > > > > +Every board in U-Boot must include a devicetree

sufficient to build and boot

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> > > > > > > +that board on suitable hardware (or emulation).

This

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is

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specified using the

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> > > > > > > +`CONFIG DEFAULT_DEVICE_TREE` option. > > > > > > > + > > > > > > > + > > > > > > > +Current situation (August 2021) > > > > > > > +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ > > > > > > > + > > > > > > > +As an aside, at present U-Boot allows

`CONFIG_DEFAULT_DEVICE_TREE` to be empty,

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> > > > > > > +e.g. if `CONFIG_OF_BOARD` or

`CONFIG_OF_PRIOR_STAGE`

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are

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used. This has

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> > > > > > > +unfortunately created an enormous amount of

confusion

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and

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some wasted effort.

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> > > > > > > +This was not intended and this bug will be fixed

soon.

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> > > > > > > + > > > > > > > +Some of the problems created are: > > > > > > > + > > > > > > > +- It is not obvious that the devicetree is coming

from

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another project

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> > > > > > > + > > > > > > > +- There is no way to see even a sample devicetree

for

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these platform in U-Boot,

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> > > > > > > + so it is hard to know what is going on, e.g.

which

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devices are typically

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> > > > > > > + present > > > > > > > + > > > > > > > +- The other project may not provide a way to

support

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U-Boot's requirements for

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> > > > > > > + devicetree, such as the /config node. Note: On

the

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U-Boot mailing list, this

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> > > > > > > + was only discovered after weeks of discussion

and

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confusion

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> > > > > > > + > > > > > > > +- For QEMU specifically, consulting two QEMU

source

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files

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is required, for which

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> > > > > > > + there are no references in U-Boot

documentation. The

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code is generating a

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> > > > > > > + devicetree, with some control from command-line

args,

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but it is not clear

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> > > > > > > + how to add properties required by U-Boot. > > > > > > > + > > > > > > > +Specifically on the changes in U-Boot: > > > > > > > + > > > > > > > +- `CONFIG_OF_BOARD` was added in rpi_patch_ for

Raspberry

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Pi, which does have

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> > > > > > > + an in-tree devicetree, but this feature has

since

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been

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used for boards that

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> > > > > > > + don't > > > > > > > +- `CONFIG_OF_PRIOR_STAGE` was added in bcm_patch_

as

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part

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of a larger Broadcom

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> > > > > > > + change with a tag indicating it only affected

one

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board, so the change in

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> > > > > > > + behaviour was not noticed at the time. It has

since

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been used by RISC-V qemu

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> > > > > > > + boards. > > > > > > > + > > > > > > > +Note: It is not clear that we actually need both

of

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these. Possibly

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> > > > > > > +`CONFIG_OF_BOARD` can be dropped. > > > > > > > + > > > > > > > +Once this bug is fixed, CONFIG_OF_BOARD and

CONFIG_OF_PRIOR_STAGE will override

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> > > > > > > > > > > > What does "bug" refer to? Above you describe the

current

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design not a bug.

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> > > > > > > > > > The bug is that we have two options to provide

seemingly

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the

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same

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> > > > > functionality. Is there a functional difference

between

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CONFIG_OF_BOARD

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> > > > > and CONFIG_OF_PRIOR_STAGE ? > > > > > > > > With CONFIG_OF_BOARD there is a function that returns the

pointer to

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> > > > the DTB, so you can do all sort of things with it. > > > > > > > > With CONFIG_OF_PRIOR_STAGE there is a variable that you

need

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to

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set in

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> > > > low-level code to point at the DTB and there is a

pre-defined

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function

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> > > > that returns that pointer. > > > > > > > > CONFIG_OF_BOARD is more flexible than

CONFIG_OF_PRIOR_STAGE,

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but

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if

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> > > > the only thing you want to do is to pass on a DTB that is

passed

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in a

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> > > > CPU register to U-Boot then CONFIG_OF_PRIOR_STAGE is

probably

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easier

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> > > > to use. > > > > > > > > I'm not convinced there is a bug here. > > > > > > Thanks for explaining. Couldn't CONFIG_OF_PRIOR_STAGE be

rewritten as

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> > > an implementation of CONFIG_OF_BOARD, possibly at the same

or

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less

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> > > overall code size? That I think is the potential bug. > > > > Probably a little bit more code: > > > > void * > > board_fdt_blob_setup(void) > > { > > return (void *)(uintptr_t)prior_stage_fdt_address; > > } > > Tiny bit more. Probably worth doing to make the choices

clearer on

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> which to select when? Bin, Rick, thoughts on this since riscv

is

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the

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> main user of CONFIG_OF_PRIOR_STAGE at this point?

Bin, Rick?

What is the prior stage in the RISC-V stage? Could we get it to

set

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up

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a bloblist? Then we can add a devicetree in there, with the

option to

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add more things in future.

I'm suggesting we don't need to do anything upstream of us, just

rework

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things to use the other hook for "provided a DTB by caller, use

it", so

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that we have a single hook for that.

What was the rationale in posting in kernel.org (

https://lore.kernel.org/all/20211003125134.2.I7733f5a849476e908cc51f0c71b8a5...

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) and not in U-Boot knowing there is still no consensus on the big

picture ?

Well, because we need to get our bindings reviewed and made official, and that looked like a reasonable place and choice to start with? v2 cc'd a different set of lists, at Rob's suggestion.

making it “ Official” does not necessarily mean in the Linux DT. What

would

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you think with a hardware description file that comes with U-Boot,

No? A project-specific binding is only valid within the project. So unless it's a linux,foo binding it's not "Linux DT", it's generic. Part of the point in pushing this up for real review is that IF we want to be passed this information then it needs to be a real official and reviewed binding. And that process in turn meanings convincing other (generally skeptical) people it's a good idea. And maybe that it should take a more generic approach.

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LinuxBoot and EDK2 config options ? No need for Linux to know about those U-Boot internal config? Why would those U-Boot elements be passed down to any OS ?

I don't see the harm?