Merge patch series "An effort to bring DT bindings compliance within U-Boot"

Sumit Garg <sumit.garg@linaro.org> says:

Prerequisite
------------

This patch series requires devicetree-rebasing git repo to be added as a
subtree to the main U-Boot repo via:

$ git subtree add --prefix dts/upstream \
      https://git.kernel.org/pub/scm/linux/kernel/git/devicetree/devicetree-rebasing.git \
      v6.7-dts --squash

Background
----------

This effort started while I was reviewing patch series corresponding to
Qcom platforms [1] which was about to import modified devicetree source
files from Linux kernel. I suppose keeping devicetree files sync with
Linux kernel without any DT bindings schema validation has been a pain
for U-Boot SoC/platform maintainers. There has been past discussions
about a single DT repo but that hasn't come up and Linux kernel remained
the place where DT source files as well as bindings are placed and
maintained.

However, Linux kernel DT maintainers proposed [2] for U-Boot to rather
use devicetree-rebasing repo [3] which is a forked copy from Linux
kernel for DT source files as well as bindings. It is tagged at every
Linux kernel major release or intermideate release candidates. So here I
have tried to reuse that to bring DT bingings compliance as well as a
standard way to maintain a regular sync of DT source files with Linux
kernel.

In order to maintain devicetree files sync, U-Boot will maintains a Git
subtree for devicetee-rebasing repo as `dts/upstream` sub-directory.
U-Boot will regularly sync `dts/upstream/` subtree whenever the next window
opens with the next available kernel major release.
`dts/update-dts-subtree.sh` script provides a wrapper around git subtree
pull command, usage from the top level U-Boot source tree, run:

$ ./dts/update-dts-subtree.sh pull <devicetree-rebasing-release-tag>

If required it is also possible to cherry-pick fixes from
devicetree-rebasing tree prior to next sync, usage:

$ ./dts/update-dts-subtree.sh pick <devicetree-rebasing-commit-id>

The RFC/prototype for this series has been discussed with Linux DT
maintainers as well as U-Boot maintainers here [4]. Now we would like to
reach out to wider U-Boot community to seek feedback.

[1] https://lore.kernel.org/all/CAFA6WYMLUD9cnkr=R0Uur+1UeTMkKjM2zDdMJtXb3nmrLk+pDg@mail.gmail.com/
[2] https://lore.kernel.org/all/CAL_JsqKEjv2tSGmT+0ZiO7_qbBfhTycbGnhJhYpKDFzfO9jzDg@mail.gmail.com/
[3] https://git.kernel.org/pub/scm/linux/kernel/git/devicetree/devicetree-rebasing.git/
[4] https://github.com/u-boot/u-boot/pull/451

Changes
-------

Traditionally, U-Boot placed copies of devicetree source files from Linux
kernel into `arch/<arch>/dts/<name>.dts` which can be selected via setting
"<name>" when prompted for `DEFAULT_DEVICE_TREE` by Kconfig.

SoC/board maintainers are encouraged to migrate to use synced copies from
`dts/upstream/src/<arch>/<vendor>`. To do that enable `OF_UPSTREAM` for the
SoC being used via Kconfig and set up "<vendor>/<name>" when prompted for
`DEFAULT_DEVICE_TREE` by Kconfig.

An example have been shown for Amlogic meson-gxbb SoC and corresponding
derived boards via patch #10 and #11.

Devicetree bindings schema checks
---------------------------------

With devicetee-rebasing Git subtree, the devicetree bindings are also
regularly synced with Linux kernel as `dts/upstream/Bindings/`
sub-directory. This allows U-Boot to run devicetree bindings schema checks
which will bring compliance to U-Boot core/drivers regarding usage of
devicetree.

Dependencies
------------

The DT schema project must be installed in order to validate the DT schema
binding documents and validate DTS files using the DT schema. The DT schema
project can be installed with pip:

$ pip3 install dtschema

Note that 'dtschema' installation requires 'swig' and Python development
files installed first. On Debian/Ubuntu systems:

$ apt install swig python3-dev

Several executables (dt-doc-validate, dt-mk-schema, dt-validate) will be
installed. Ensure they are in your PATH (~/.local/bin by default).

Recommended is also to install yamllint (used by dtschema when present).

$ apt install yamllint

Running checks
--------------

In order to perform validation of DTB files, use the ``dtbs_check`` target:

$ make dtbs_check

It is also possible to run checks with a subset of matching schema files by
setting the ``DT_SCHEMA_FILES`` variable to 1 or more specific schema files
or patterns (partial match of a fixed string). Each file or pattern should
be separated by ':'.

$ make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml:rtc.yaml
$ make dtbs_check DT_SCHEMA_FILES=/gpio/
$ make dtbs_check DT_SCHEMA_FILES=trivial-devices.yaml