[PATCH v2 3/3] RFC: doc: Add documentation about devicetree usage

[Tom Rini]

On Mon, Aug 30, 2021 at 04:30:55PM +0200, Heinrich Schuchardt wrote:
> 
> 
> On 8/28/21 6:46 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.

I feel like I should emphasize that this is "document what we have
today" at least as much, if not more-so, than "document what we want to
move to tomorrow".

In that this highlights some design disagreements that need to be
settled, good.  But lets perhaps start separate threads on those areas?

> > Signed-off-by: Simon Glass <sjg at chromium.org>
> > ---
> > 
> > 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 | 563 +++++++++++++++++++++++++++++
> >   doc/develop/package/index.rst      |   1 +
> >   3 files changed, 565 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..d922d3f87ae
> > --- /dev/null
> > +++ b/doc/develop/package/devicetree.rst
> > @@ -0,0 +1,563 @@
> > +.. 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.
> > +
> > +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 linst, 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, but it is not clear what controls affect this generation.
> > +
> > +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.
> > +
> > +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`.
> > +
> > +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.
> > +
> > +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.
> 
> For many boards we lag far behind Linux' device-tree.

Which is a huge problem that needs to be fixed.  The intention has never
been to "commit and forget".

[snip]
> > +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:
> 
> Currently it is Linux that sets the standards not U-Boot.

Well no, Linux isn't supposed to set "the standard" here either.  It's
OS-agnostic.

> U-Boot can apply an overlay to the devicetree provided by the prior boot
> stage. We should not try to force any U-Boot specific stuff onto other
> projects.

We should, when applicable, submit our bindings upstream just like any
other project.  We also want to make sure that when we do so, we hold
ourselves to a high standard.

> > +- 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.
> 
> This paragraph is incomprehensible for me.
> 
> If both the prior boot stage and U-Boot comply to the standards set by
> Linux we are fine. Any U-Boot quirks should be kept out of other projects.

It's not the "Linux" device tree.  It's the device tree for the system.

-- 
Tom
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