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

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Date:   Thu, 9 Jul 2020 14:39:12 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Ionela Voinescu <ionela.voinescu@....com>
Cc:     Dietmar Eggemann <dietmar.eggemann@....com>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        "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>,
        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 09-07-20, 09:53, Ionela Voinescu wrote:
> On Monday 06 Jul 2020 at 14:14:47 (+0200), Dietmar Eggemann wrote:
> > Why can't we just move the arch_set_freq_scale() call from cpufreq
> > driver to cpufreq core w/o introducing a FIE related driver flag?
> > 
> > Current scenario for Frequency Invariance Engine (FIE) on arm/arm64.
> > 
> > +------------------------------+       +------------------------------+
> > |                              |       |                              |
> > | cpufreq core:                |       | arch: (arm, arm64)           |
> > 
> > |                              |       |                              |
> > | weak arch_set_freq_scale() {}|       |                              |
> > |                              |       |                              |
> > +------------------------------+       |                              |
> >                                        |                              |
> > +------------------------------+       |                              |
> > |                              |       |                              |
> > | cpufreq driver:              |       |                              |
> > |                            +-----------> arch_set_freq_scale()      |
> > |                              |       |   {                          |
> > +------------------------------+       |      if (use counters)       |
> >                                        |        return;               |
> > +------------------------------+       |      ...                     |
> > |                              |       |   }                          |
> > | task scheduler:              |       |                              |
> > |                            +-----------> arch_scale_freq_tick()*    |
> > |                              |       |   {                          |
> > 
> > |                              |       |      if (!use counters)      |
> > |                              |       |        return;               |
> > |                              |       |      ...                     |
> > |                              |       |   }                          |
> > +------------------------------+       +------------------------------+
> > 
> > * defined as topology_scale_freq_tick() in arm64
> > 
> > Only Arm/Arm64 defines arch_set_freq_scale() to get the 'legacy' CPUfreq
> > based FIE. This would still be the case when we move
> > arch_set_freq_scale() from individual cpufreq drivers to cpufreq core.
> > 
> > Arm64 is the only arch which has to runtime-choose between two different
> > FIEs. This is currently done by bailing out early in one of the FIE
> > functions based on 'use counters'.
> > 
> > X86 (and others) will continue to not define arch_set_freq_scale().
> > 
> > The issue with CONFIG_BL_SWITCHER (vexpress-spc-cpufreq.c) could be
> > solved arm/arm64 internally (arch_topology.c) by putting
> > arch_set_freq_scale() under a !CONFIG_BL_SWITCHER guard.
> > I doubt that there are any arm bL systems out there running it. At least
> > I'm not aware of any complaints due to missing FIE support in bl
> > switcher setups so far.

I agree to that.

> Thank you Dietmar, for your review.
> 
> I was trying to suggest the same in my other replies.

I am sorry, I must have overlooked that part in your replies,
otherwise I may agreed to it :)

> Rafael, Viresh, would you mind confirming whether you still consider
> having an 'opt in' flag is preferable here?

Well, we wanted an opt-in flag instead of an opt-out one. And no flag
is certainly better.

-- 
viresh