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Subject: Re: [PATCH v3 7/7] clocksource/drivers/arm_arch_timer: validate
 arch_timer_rate
To:     Lukasz Luba <lukasz.luba@arm.com>, Marc Zyngier <maz@kernel.org>
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From:   Valentin Schneider <valentin.schneider@arm.com>
Message-ID: <eeafe486-e408-b229-24ef-329649a13f70@arm.com>
Date:   Wed, 12 Feb 2020 11:12:33 +0000
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On 12/02/2020 10:55, Lukasz Luba wrote:
>> Because, as the commit message outlines it, such a frequency is terribly
>> out of spec?
> 
> I don't see in the RM that < 1MHz is terribly out of spec.
> 'Frequency
> Increments at a fixed frequency, typically in the range 1-50MHz.
> Can support one or more alternative operating modes in which it increments by larger amounts at a
> lower frequency, typically for power-saving.'
> 
> There is even an example how to operate at 20kHz and increment by 500.
> 
> I don't know the code if it's supported, thought.
> 

For that one case the value reported by CNTFRQ shouldn't change - it's still
a timer that looks like is operating at 10MHz, but under the hood is doing
bigger increments at lower freq.

As I was trying to get to, this patch isn't validating the actual frequency
the timer operates on, rather that whatever is reported by CNTFRQ is
somewhat sane (which here means [1, 50]MHz, although we just check the
lower bound).

[...]

>> And? It seems to address a potential issue where the time frequency
>> is out of spec, and makes sure we don't end up with additional problems
>> in the AMU code.
> 
> This patch just prints warning, does not change anything in booting or
> in any code related to AMU.
> 

Right, but it should still be worth having - at least it shows up in
dmesg, and when someone reports something fishy we get a hint that we can
blame the hardware.

>>
>> On its own, it is perfectly sensible and could be merged as part of this
>> series with my
>>
>> Acked-by: Marc Zyngier <maz@kernel.org>
>>
>>          M.
> 
> Regards,
> Lukasz