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.