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

[Rob Herring]

On Sat, Jun 11, 2022 at 7:07 AM Krzysztof Kozlowski
<krzysztof.kozlowski@xxxxxxxxxx> wrote:
>
> On 10/06/2022 18:33, Rob Herring wrote:
> > 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.
>
> Bah, I wrote something else than I had in mind. So one more try:
>
> Considering 1+2 above, maybe let's document these properties as
> *deprecated*? Would that solve your point of "endorsement for other
> vendors"?

Yes.

> However the idea to restrict them per-compatible, is also nice. Although
> I cannot guarantee the list will not grow for older SoCs.

No issue with that.

Rob