Re: [PATCH v3 7/7] clocksource/drivers/arm_arch_timer: validate arch_timer_rate

From: Ionela Voinescu <ionela.voinescu@arm.com>
To: Valentin Schneider <valentin.schneider@arm.com>
Cc: catalin.marinas@arm.com, will@kernel.org, mark.rutland@arm.com,
	maz@kernel.org, suzuki.poulose@arm.com, sudeep.holla@arm.com,
	lukasz.luba@arm.com, rjw@rjwysocki.net, peterz@infradead.org,
	mingo@redhat.com, vincent.guittot@linaro.org,
	viresh.kumar@linaro.org, linux-arm-kernel@lists.infradead.org,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-pm@vger.kernel.org
Subject: Re: [PATCH v3 7/7] clocksource/drivers/arm_arch_timer: validate arch_timer_rate
Date: Wed, 12 Feb 2020 10:32:49 +0000	[thread overview]
Message-ID: <20200212103249.GA19041@arm.com> (raw)
In-Reply-To: <05e257b6-0a39-135d-8117-7883739538c3@arm.com>

Hi Valentin,

On Wednesday 12 Feb 2020 at 09:30:34 (+0000), Valentin Schneider wrote:
> On 11/02/2020 18:45, Ionela Voinescu wrote:
> > From: Valentin Schneider <valentin.schneider@arm.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.
> 

I agree, but this part was just given as an example of functionality
that relies on a reasonable arch timer rate. The AMU code checks for the
ratio not to be 0 so it does not end up breaking frequency invariance.
But if the ratio does end up being 0 due to the value of arch_time_rate,
we bypass the use of activity monitors which I'd argue it's incorrect
functionality by disabling a potential better source of information for
frequency invariance.

But I can rewrite this part for more clarity.

> > 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@arm.com>
> > Cc: Mark Rutland <mark.rutland@arm.com>
> > Cc: Marc Zyngier <maz@kernel.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 :)

Will do!

Thanks,
Ionela.