linux-kernel - Re: [PATCH v3 7/7] clocksource/drivers/arm_arch_timer: validate arch_timer

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Message-ID: <05e257b6-0a39-135d-8117-7883739538c3@arm.com>
Date:   Wed, 12 Feb 2020 09:30:34 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Ionela Voinescu <ionela.voinescu@....com>, catalin.marinas@....com,
        will@...nel.org, mark.rutland@....com, maz@...nel.org,
        suzuki.poulose@....com, sudeep.holla@....com, lukasz.luba@....com,
        rjw@...ysocki.net
Cc:     peterz@...radead.org, mingo@...hat.com, vincent.guittot@...aro.org,
        viresh.kumar@...aro.org, linux-arm-kernel@...ts.infradead.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Subject: Re: [PATCH v3 7/7] clocksource/drivers/arm_arch_timer: validate
 arch_timer_rate

On 11/02/2020 18:45, Ionela Voinescu wrote:
> From: Valentin Schneider <valentin.schneider@....com>
> 
> Using an arch timer with a frequency of less than 1MHz can result in an
> incorrect functionality of the system which assumes a reasonable rate.
> 
> One example is the use of activity monitors for frequency invariance
> which uses the rate of the arch timer as the known rate of the constant
> cycle counter in computing its ratio compared to the maximum frequency
> of a CPU. For arch timer frequencies less than 1MHz this ratio could
> end up being 0 which is an invalid value for its use.
> 

I'm being pedantic here (as usual), but I'd contrast this a bit more. The
activity monitor code checks by itself that the ratio doesn't end up being
0, which is why we don't slam the brakes if the arch timer freq is < 1MHz.

It's just a CNTFRQ sanity check that goes a bit beyond the 0 value check,
IMO.

> Therefore, warn if the arch timer rate is below 1MHz which contravenes
> the recommended architecture interval of 1 to 50MHz.
> 
> Signed-off-by: Ionela Voinescu <ionela.voinescu@....com>
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Marc Zyngier <maz@...nel.org>

ISTR something somewhere that says the first signoff should be that of the
author of the patch, and seeing as I just provided an untested diff that
ought to be you :)