[PATCH] doc: Add documentation about devicetree usage

[Simon Glass]

Hi Bin,

On Sat, 28 Aug 2021 at 07:15, Bin Meng <bmeng.cn at gmail.com> wrote:
>
> Hi Simon,
>
> On Sat, Aug 28, 2021 at 11:23 AM Simon Glass <sjg at 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 at 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.
>
> Do you have some idea of how to support the QEMU case, if not using
> CONFIG_OF_PRIOR_STAGE?

To be clear, I am not planning to remove this option. It will still work.

But I do want to have an in-tree devicetree for all boards, and
someone will need to update qemu to support adding U-Boot
nodes/properties. It always has an array of options controlling what
it presents to BIOS/UEFI, for example, so this should not be too
controversial.

Regards,
Simon