+Ilias Apalodimas
On Fri, 27 Aug 2021 at 21:23, Simon Glass sjg@chromium.org 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
doc/develop/index.rst | 1 + doc/develop/package/devicetree.rst | 315 +++++++++++++++++++++++++++++ doc/develop/package/index.rst | 1 + 3 files changed, 317 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..fccbb182f3e --- /dev/null +++ b/doc/develop/package/devicetree.rst @@ -0,0 +1,315 @@ +.. SPDX-License-Identifier: GPL-2.0+
+Updating the devicetree +=======================
+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. Specifically:
+- `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.
+Once this bug is fixed, CONFIG_OF_BOARD and CONFIG_OF_PRIOR_STAGE will override +(at runtime) the devicetree suppled 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`
+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 useul 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_EMBD
- This is tricky, since the devicetree cannot easily be located. If the EFL
- 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
+- 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.
+Devicetree in another project +-----------------------------
+In some cases U-Boot receive 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. It 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 must be sufficient to build and boot, so this is not a +way to bypass that requirement.
+If binman is used, the in-tree U-Boot devicetree must contain the binman +definition so that a valid image can be build.
+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 this case, the devicetree in the other project must track U-Boot's use of +device tree, so that it remains compatible. 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. 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. 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.
+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 root hash for dm-verify 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 in 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.
+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 alowing 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.
+.. _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 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
-- 2.33.0.259.gc128427fd7-goog