Re: [PATCH v4 1/5] dt-bindings: arm: renesas: Document Renesas RZ/G2UL SMARC EVK

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Krzysztof,

On Sat, Apr 2, 2022 at 10:03 PM Krzysztof Kozlowski
<krzysztof.kozlowski@linaro.org> wrote:
> On 02/04/2022 21:54, Biju Das wrote:
> >> I understand that carrier board is the same, so the SoM differs.
> >
> > For R9A07G043 case, even SoM is same, only SOC differs.
>
> I assumed that you cannot have same SoMs with different SoCs...
>
> >
> >> In your
> >> model to figure out what type of hardware is it, your choice is to compare
> >> two compatibles:
> >> renesas,smarc-evk + renesas,r9a07g043u11
> >>
> >> If user-space compares only last compatible, it get's only SMARC, so it
> >> does not know on what hardware it runs.
> >
> > But Here user-space can easily identify the H/W with existing scheme. See the logs from user-space.
> >
> > / # for i in machine family soc_id revision; do echo -n "$i: "; cat /sys/devices/soc0/$i;done
> > machine: Renesas SMARC EVK based on r9a07g043u11
> > family: RZ/G2UL
> > soc_id: r9a07g043
> > revision: 0
>
> User-space is one example. We don't limit to this. Anyway, the
> compatible is the main way to check it. Machine is just test, not
> compatible, not part of ABI. soc_id and revision could help, but these
> are separate ABIs. They can be not compiled-in and then you have only
> compatible.
>
> Regardless whether there is another way for user-space to figure out
> hardware, it does not change the fact that such usage of compatibles
> does not look correct with Devicetree spec.
> "...They
>  allow a device to express its compatibility with a family of similar
> devices, potentially allowing a single
>  device driver to match against several devices."
>
> The "renesas,smarc-evk" compatible is not the most specific one, because
> different configurations have it.

From the letter of the spec, this is indeed not valid.
However, we started doing this several years ago, as the various
variants of the Salvator-X(S) and ULCB boards are identical, and just
differ in SoC (actually SiP) mounted.

E.g. arch/arm64/boot/dts/renesas/r8a77951-salvator-xs.dts has
compatible = "renesas,salvator-xs", "renesas,r8a7795".

While we could add e.g. "renesas,salvator-xs-r8a7791", doing so
would inflate the bindings a lot.

> Again - you intend to use a pair or even a triple of compatibles to
> uniquely identify type of hardware. I don't think it is correct - the
> final, most specific compatible, uniquely identifies the hardware. All
> other compatibles are just for fallback.

And what to do when adding more DT overlays for expansion boards?
This would become unmanageable soon.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds