[PATCH RFC v3 0/9] dt-bindings: hwinfo: Introduce board-id

[Elliot Berman]

Device manufacturers frequently ship multiple boards or SKUs under a
single software package. These software packages will ship multiple
devicetree blobs and require some mechanism to pick the correct DTB for
the board the software package was deployed. Introduce a common
definition for adding board identifiers to device trees. board-id
provides a mechanism for bootloaders to select the appropriate DTB which
is vendor/OEM-agnostic.

This series is based off a talk I gave at EOSS NA 2024 [1]. There is
some further discussion about how to do devicetree selection in the
boot-architecture mailing list [2].

[1]: https://sched.co/1aBFy
[2]: https://lists.linaro.org/archives/list/boot-architecture@xxxxxxxxxxxxxxxx/thread/DZCZSOCRH5BN7YOXEL2OQKSDIY7DCW2M/

Quick summary
-------------
This series introduces a new subnode in the root:
/ {
	board-id {
		some-hw-id = <value>;
		other-hw-id = <val1>, <val2>;
	};
};

Firmware provides a mechanism to fetch the values of "some-hw-id" and
"other-hw-id" based on the property name. I'd like to leave exact
mechanism data out of the scope of this proposal to keep this proposal 
flexible because it seems architecture specific, although I think we we
should discuss possible approaches. A DTB matches if firmware can
provide a matching value for every one of the properties under
/board-id. In the above example, val1 and val2 are both valid values and
firmware only provides the one that actually describes the board. 

It's expected that devicetree's board-id don't describe all the
properties firmware could provide. For instance, a devicetree overlay
may only care about "other-hw-id" and not "some-hw-id". Thus, it need 
only mention "other-hw-id" in its board-id node.

Isn't that what the compatible property is for?
-----------------------------------------------
The compatible property can be used for board matching, but requires
bootloaders and/or firmware to maintain a database of possible strings
to match against or implement complex compatible string matching.
Compatible string matching becomes complicated when there are multiple
versions of board: the device tree selector should recognize a DTB that
cares to distinguish between v1/v2 and a DTB that doesn't make the
distinction.  An eeprom either needs to store the compatible strings
that could match against the board or the bootloader needs to have
vendor-specific decoding logic for the compatible string. Neither
increasing eeprom storage nor adding vendor-specific decoding logic is
desirable.

How is this better than Qualcomm's qcom,msm-id/qcom,board-id?
-------------------------------------------------------------
The selection process for devicetrees was Qualcomm-specific and not
useful for other devices and bootloaders that were not developed by
Qualcomm because a complex algorithm was used to implement. Board-ids
provide a matching solution that can be implemented by bootloaders
without introducing vendor-specific code. Qualcomm uses three
devicetree properties: msm-id (interchangeably: soc-id), board-id, and
pmic-id.  This does not scale well for use casese which use identifiers,
for example, to distinguish between a display panel. For a display
panel, an approach could be to add a new property: display-id, but now
bootloaders need to be updated to also read this property. We want to
avoid requiring to update bootloaders with new hardware identifiers: a
bootloader need only recognize the identifiers it can handle.

Notes about the patches
-----------------------
In my opinion, most of the patches in this series should be submitted to
libfdt and/or dtschema project. I've made them apply on the kernel tree
to be easier for other folks to pick them up and play with them. As the
patches evolve, I can send them to the appropriate projects.

Signed-off-by: Elliot Berman <quic_eberman@xxxxxxxxxxx>
---
Changes in v3:
 - Follow new "/board-id {}" approach, which uses key-value pairs
 - Add match algorithm in libfdt and some examples to demo how the
   selection could work in tools/board-id

Changes in V2:
 - Addressed few comments related to board-id, and DDR type.
 - Link to V2:  https://lore.kernel.org/all/a930a3d6-0846-a709-8fe9-44335fec92ca@xxxxxxxxxxx/#r

---
Amrit Anand (1):
      dt-bindings: arm: qcom: Update Devicetree identifiers

Elliot Berman (8):
      libfdt: board-id: Implement board-id scoring
      dt-bindings: board: Introduce board-id
      fdt-select-board: Add test tool for selecting dtbs based on board-id
      dt-bindings: board: Document board-ids for Qualcomm devices
      arm64: boot: dts: sm8650: Add board-id
      arm64: boot: dts: qcom: Use phandles for thermal_zones
      arm64: boot: dts: qcom: sm8550: Split into overlays
      tools: board-id: Add test suite

 .../devicetree/bindings/board/board-id.yaml        |  24 ++++
 .../devicetree/bindings/board/qcom,board-id.yaml   | 144 ++++++++++++++++++++
 arch/arm64/boot/dts/qcom/Makefile                  |   4 +
 arch/arm64/boot/dts/qcom/pm8010.dtsi               |  62 ++++-----
 arch/arm64/boot/dts/qcom/pm8550.dtsi               |  32 ++---
 arch/arm64/boot/dts/qcom/pm8550b.dtsi              |  36 +++--
 arch/arm64/boot/dts/qcom/pm8550ve.dtsi             |  38 +++---
 arch/arm64/boot/dts/qcom/pm8550vs.dtsi             | 128 +++++++++--------
 arch/arm64/boot/dts/qcom/pmr735d_a.dtsi            |  38 +++---
 arch/arm64/boot/dts/qcom/pmr735d_b.dtsi            |  38 +++---
 .../dts/qcom/{sm8550-mtp.dts => sm8550-mtp.dtso}   |  24 +++-
 .../dts/qcom/{sm8550-qrd.dts => sm8550-qrd.dtso}   |  22 ++-
 .../boot/dts/qcom/{sm8550.dtsi => sm8550.dts}      |  10 +-
 arch/arm64/boot/dts/qcom/sm8650-mtp.dts            |   6 +
 arch/arm64/boot/dts/qcom/sm8650-qrd.dts            |   6 +
 arch/arm64/boot/dts/qcom/sm8650.dtsi               |   2 +-
 include/dt-bindings/arm/qcom,ids.h                 |  86 ++++++++++--
 scripts/dtc/.gitignore                             |   1 +
 scripts/dtc/Makefile                               |   3 +-
 scripts/dtc/fdt-select-board.c                     | 126 +++++++++++++++++
 scripts/dtc/libfdt/fdt_ro.c                        |  76 +++++++++++
 scripts/dtc/libfdt/libfdt.h                        |  54 ++++++++
 tools/board-id/test.py                             | 151 +++++++++++++++++++++
 23 files changed, 901 insertions(+), 210 deletions(-)
---
base-commit: e8f897f4afef0031fe618a8e94127a0934896aba
change-id: 20240112-board-ids-809ff0281ee5

Best regards,
-- 
Elliot Berman <quic_eberman@xxxxxxxxxxx>