linux-kernel - Re: [PATCH 1/8] cpufreq: allow drivers to flag custom support for freq invariance

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Date:   Thu, 2 Jul 2020 08:28:18 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Ionela Voinescu <ionela.voinescu@....com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Catalin Marinas <catalin.marinas@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Will Deacon <will@...nel.org>,
        Russell King - ARM Linux <linux@...linux.org.uk>,
        Valentin Schneider <valentin.schneider@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/8] cpufreq: allow drivers to flag custom support for
 freq invariance

On 01-07-20, 18:05, Rafael J. Wysocki wrote:
> On Wed, Jul 1, 2020 at 3:33 PM Ionela Voinescu <ionela.voinescu@....com> wrote:
> > On Wednesday 01 Jul 2020 at 16:16:17 (+0530), Viresh Kumar wrote:
> > > I will rather suggest CPUFREQ_SKIP_SET_FREQ_SCALE as the name and
> > > functionality. We need to give drivers a choice if they do not want
> > > the core to do it on their behalf, because they are doing it on their
> > > own or they don't want to do it.
> 
> Well, this would go backwards to me, as we seem to be designing an
> opt-out flag for something that's not even implemented already.
> 
> I would go for an opt-in instead.  That would be much cleaner and less
> prone to regressions IMO.

That's fine, I just wanted an option for drivers to opt-out of this
thing. I felt okay with the opt-out flag as this should be enabled for
most of the drivers and so enabling by default looked okay as well.

> > In this case we would not be able to tell if cpufreq (driver or core)
> > can provide the frequency scale factor, so we would not be able to tell
> > if the system is really frequency invariant; CPUFREQ_SKIP_SET_FREQ_SCALE

That is easy to fix. Let the drivers call
enable_cpufreq_freq_invariance() and set the flag.

> > would be set if either:
> >  - the driver calls arch_set_freq_scale() on its own
> >  - the driver does not want arch_set_freq_scale() to be called.
> >
> > So at the core level we would not be able to distinguish between the
> > two, and return whether cpufreq-based invariance is supported.
> >
> > I don't really see a reason why a driver would not want to set the
> > frequency scale factor

A simple case where the driver doesn't have any idea what the real
freq of the CPU is and it doesn't have counters to guess it as well.

There can be other reasons which we aren't able to imagine at this
point of time.

-- 
viresh