Re: [PATCH 0/4] soc: renesas: Identify SoC and register with the SoC bus

[Geert Uytterhoeven]

Hi Arnd,

On Mon, Oct 10, 2016 at 4:28 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
> On Tuesday, October 4, 2016 11:09:23 AM CEST Geert Uytterhoeven wrote:
>> Some Renesas SoCs may exist in different revisions, providing slightly
>> different functionalities (e.g. R-Car H3 ES1.x and ES2.0). This needs to
>> be catered for by drivers and/or platform code.  The recently proposed
>> soc_device_match() API seems like a good fit to handle this.
>>
>> This patch series implements the core infrastructure to provide SoC and
>> revision information through the SoC bus for Renesas ARM SoCs. It
>> consists of 4 patches:
>>   - Patch 1 avoids a crash when SoC revision information is needed and
>>     provided early,
>>   - Patch 2 (from Arnd) introduces the soc_device_match() API.
>>     I don't know if, when, and through which channel this patch is
>>     planned to go upstream,
>>   - Patch 3 fixes a bug in soc_device_match(), causing a crash when
>>     trying to match on an SoC attribute that is not provided (seen on
>>     EMEV2, RZ/A, and R-Car M1A, which lack revision information),
>>   - Patch 4 identifies Renesas SoCs and registers them with the SoC bus.
>>
>> Tested on (family, machine, soc_id, optional revision):
>>
>>     Emma Mobile EV2, EMEV2 KZM9D Board, emev2
>>     RZ/A, Genmai, r7s72100
>>     R-Mobile, APE6EVM, r8a73a4, ES1.0
>>     R-Mobile, armadillo 800 eva, r8a7740, ES2.0
>>     R-Car Gen1, bockw, r8a7778
>>     R-Car Gen1, marzen, r8a7779, ES1.0
>>     R-Car Gen2, Lager, r8a7790, ES1.0
>>     R-Car Gen2, Koelsch, r8a7791, ES1.0
>>     R-Car Gen2, Gose, r8a7793, ES1.0
>>     R-Car Gen2, Alt, r8a7794, ES1.0
>>     R-Car Gen3, Renesas Salvator-X board based on r8a7795, r8a7795, ES1.0
>>     R-Car Gen3, Renesas Salvator-X board based on r8a7796, r8a7796, ES1.0
>>     SH-Mobile, KZM-A9-GT, sh73a0, ES2.0
>
> As mentioned in the comment for the driver patch, I think this makes
> a lot of sense for the machines that have a revision register, in
> particular when the interpretation of that register is always done
> the same way, but I'm a bit skeptical about doing it in the same driver
> for machines that don't have the register.
>
> Matching by a device rather than the SoC platform also has the advantage
> that there is no need to maintain a list of compatible numbers in the
> driver.

Currently we (usually) use:
  - SoC-specific compatible values, to handle known differences within the
    same family now, and handle future unknown differences,
  - Family-specific compatible values, which we define ourselves.

Usually drivers match on the latter.

Every time a new SoC is introduced, we have to update lots of DT binding
docs, to add the new SoC-specific compatible values.

Two-phase matching (driver code matches against "renesas,<foo>",
driver matches against SoC using soc_device_match()) would allow to
remove the burden of updating DT documentation all the time.
The drivers would need updates, though.
Another advantage would be that we can reuse .dtsi snippets for SoCs in
the same family, which we currently can't easily do due to the SoC-specific
compatible values.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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