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

Heinrich Schuchardt xypron.glpk at gmx.de
Fri Sep 10 19:09:30 CEST 2021 


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 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 ?
>>>
>>> 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

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