Re: [PATCH] ARM: virt: Relax arch timer version check during early boot

[Marc Zyngier <maz@kernel.org>]

On 2020-01-20 12:34, Vladimir Murzin wrote:
> Hi Marc,
> 
> On 1/20/20 11:14 AM, Marc Zyngier wrote:
>> Hi Vladimir,
>> 
>> On 2020-01-20 11:46, Vladimir Murzin wrote:
>>> + Marc
>>> + kvmarm@lists.cs.columbia.edu
>>> 
>>> On 1/15/20 2:16 PM, Vladimir Murzin wrote:
>>>> Updates to the Generic Timer architecture allow ID_PFR1.GenTimer to
>>>> have values other than 0 or 1. At the moment, Linux is quite strict 
>>>> in
>>>> the way it handles this field at early boot and will not configure
>>>> arch timer if it doesn't find the value 1.
>>>> 
>>>> Since here use ubfx for arch timer version extraction (hyb-stub 
>>>> build
>>>> with -march=armv7-a, so it is safe)
>>>> 
>>>> To help backports (even though the code was correct at the time of 
>>>> writing)
>>>> Fixes: 8ec58be9f3ff ("ARM: virt: arch_timers: enable access to 
>>>> physical timers")
>>>> Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
>> 
>> I'm not opposed to such a change, but it'd be good to document what 
>> other values
>> are expected here, as the current (Rev E_a) ARM ARM only mentions 
>> values 0 and 1.
> 
> That true, ARM ARM doesn't mention it yet. OTOH, should we really care
> about exact values as soon it stays compatible?

That's for you to say, really. But given that you hint at some changes,
it'd be good to have at least a short sentence explaining that, for 
example,
"upcoming revisions of the architecture will allow different 
ID_PFR1.GenTimer
values while preserving backward compatibility".

Other than that, feel free to add my

Acked-by: Marc Zyngier <maz@kernel.org>

Thanks,

         M.
-- 
Jazz is not dead. It just smells funny...
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