linux-kernel - Re: [PATCH] dt-bindings: timer: Add QEMU compatible strings

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Message-ID: <e89478b5-8495-a846-61ff-917bc4e08490@linaro.org>
Date:   Sun, 18 Sep 2022 09:50:58 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Jean-Philippe Brucker <jean-philippe@...aro.org>,
        mark.rutland@....com, robh+dt@...nel.org,
        krzysztof.kozlowski+dt@...aro.org, linux-kernel@...r.kernel.org,
        devicetree@...r.kernel.org
Subject: Re: [PATCH] dt-bindings: timer: Add QEMU compatible strings

On 17/09/2022 19:47, Marc Zyngier wrote:
> On Sat, 17 Sep 2022 17:51:20 +0100,
> Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org> wrote:
>>
>> On 16/09/2022 14:30, Jean-Philippe Brucker wrote:
>>> QEMU uses both "arm,armv8-timer" and "arm,armv7-timer" as compatible
>>> string. Although it is unlikely that any guest relies on this, we can't
>>> be certain of that. Therefore, add these to the schema. Clean up the
>>> compatible list a little while at it.
>>>
>>> Signed-off-by: Jean-Philippe Brucker <jean-philippe@...aro.org>
>>
>> I guess you wanted to say QEMU uses "arm,armv8-timer" followed by
>> "arm,armv7-timer", because otherwise I would understand it that either
>> that or that. Anyway, is it a valid (virtualized) hardware? Is ARMv8
>> timer really, really compatible with ARMv7 one?
> 
> Yes. There isn't a shred of difference between the two in the earlier
> revisions of the ARMv8 architecture, and none of the differences
> introduced in later revisions are exposed to DT anyway.
> 
>> I don't think we should document invalid setups out-of-tree, just
>> because they are there, and something like this was also expressed by Rob:
>> https://lore.kernel.org/all/20220518163255.GE3302100-robh@kernel.org/
> 
> This is, on the contrary, something that is perfectly valid. For
> example, a system running a 32bit OS on a 64bit system is perfectly
> entitled to expose both (v8 because that's what the HW is, v7 because
> that's what the OS is the most likely to understand).
> 
> You may find it odd, but that:
> 
> - expresses something that is actually required
> 
> - is what I, as the original author of this binding, have always
>   considered valid
> 
> - has been valid for a long time (10+ years) before you decided it
>   suddenly wasn't
> 
> I understand that the "DT police" has high standards, but this has
> been around for much longer, and it isn't because the conversion to
> schema is imperfect that you can rewrite history.
> 
> As for the patch, I'd remove the QEMU reference and the deprecation.
> This format is perfectly allowed, and is in use in most VMMs out
> there. Yes, DT is an ABI.

Thanks for the explanation, actually enough was to say that it is
perfectly valid combination describing hardware. :)


Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>


Best regards,
Krzysztof