[PATCH v3 3/3] RFC: doc: Add documentation about devicetree usage

[Heinrich Schuchardt]

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 at chromium.org>
>>> Reviewed-by: Marcel Ziswiler <marcel.ziswiler at 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