Re: [PATCH 1/4] dt-bindings: arm: qcom: document qcom,msm-id and qcom,board-id

[Rob Herring <robh@kernel.org>]

On Tue, Jun 07, 2022 at 01:15:51PM +0200, Krzysztof Kozlowski wrote:
> On 05/06/2022 17:07, Rob Herring wrote:
> > On Sun, May 29, 2022 at 10:26:26PM +0200, Krzysztof Kozlowski wrote:
> >> The top level qcom,msm-id and qcom,board-id properties are utilized by
> >> bootloaders on Qualcomm MSM platforms to determine which device tree
> >> should be used and passed to the kernel.
> >>
> >> The commit b32e592d3c28 ("devicetree: bindings: Document qcom board
> >> compatible format") from 2015 was a consensus during discussion about
> >> upstreaming qcom,msm-id and qcom,board-id fields.  There are however still
> >> problems with that consensus:
> >> 1. It was reached 7 years ago but it turned out its implementation did
> >>    not reach all possible products.
> >>
> >> 2. Initially additional tool (dtbTool) was needed for parsing these
> >>    fields to create a QCDT image consisting of multiple DTBs, later the
> >>    bootloaders were improved and they use these qcom,msm-id and
> >>    qcom,board-id properties directly.
> >>
> >> 3. Extracting relevant information from the board compatible requires
> >>    this additional tool (dtbTool), which makes the build process more
> >>    complicated and not easily reproducible (DTBs are modified after the
> >>    kernel build).
> >>
> >> 4. Some versions of Qualcomm bootloaders expect these properties even
> >>    when booting with a single DTB.  The community is stuck with these
> >>    bootloaders thus they require properties in the DTBs.
> >>
> >> Since several upstreamed Qualcomm SoC-based boards require these
> >> properties to properly boot and the properties are reportedly used by
> >> bootloaders, document them.
> > 
> > My primary issue here is accepting this will be an endorsement for 
> > other vendors doing something similar. I'm not against an ID 
> > property(ies) in the root node, but would rather see something common 
> > if we do anything.
> 
> Hi Rob,
> 
> A more common approach was merged back in 2015 - encoding this ID
> information in the board compatibles. If I understood previous
> discussion correctly, this common method was later used by Qualcomm DTB
> post-processing tool. At least for some of the cases.
> 
> Other cases (several Qualcomm boards from different vendors) still use
> these ID properties. It even turns out they use it differently between
> vendors (e.g. Xiaomi vs OnePlus).
> 
> Important arguments for documenting these properties:
> 1. These ID properties are already on released boards where changing
> bootloader is non-trivial or even not possible. It will not be possible
> to remove these properties, without seriously affecting the community
> working with them.

Accepting things because they are already in use is also not a path we 
want to go down. If it's the color of the bike shed, then fine.

> 2. According to Konrad [1] (second paragraph), newer chipsets (starting
> with sm8350 released in 2021) do not use these properties. These newer
> DTS do not have them.
> 
> Considering 1+2 above, maybe let's document these properties as
> compatible? Would that solve your point of "endorsement for other vendors"?

What do you mean? Only allow them for certain root compatible strings? I 
suppose that would be okay by me. It would also be useful documentation 
of where they are needed.

Rob