linux-kernel - Re: [PATCH v3 3/3] [RFC] CPUFreq: Add support for cpu-perf-dependencies

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Date:   Tue, 17 Nov 2020 14:06:44 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Nicola Mazzucato <nicola.mazzucato@....com>,
        Lukasz Luba <lukasz.luba@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <vireshk@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Stephen Boyd <sboyd@...nel.org>, Nishanth Menon <nm@...com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Chris Redpath <chris.redpath@....com>
Subject: Re: [PATCH v3 3/3] [RFC] CPUFreq: Add support for cpu-perf-dependencies

On Tue, Nov 17, 2020 at 11:53 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 17-11-20, 10:47, Nicola Mazzucato wrote:
> > Freq-invariance has been mentioned. I suppose the fix will depend on which
> > strategy we prefer to solve this.
>
> I am not sure how FIE will use this information, as I thought the
> problem is about knowing the current frequency on a CPU instead of
> which CPUs are related. Anyway, EM is good enough to get this stuff
> going.
>
> > As a reminder, two solutions:
> > 1) dependent_cpus cpumask in cpufreq and involved entities pick this info
>
> Yeah, this one. And it will be called freqdomain_cpus. Add support for
> freqdomain_cpus in core, update acpi-cpufreq to reuse it instead of
> adding its own and you can have it in other drivers then.

Is this really a cpufreq thing, though, or is it arch stuff?  I think
the latter, because it is not necessary for anything in cpufreq.

Yes, acpi-cpufreq happens to know this information, because it uses
processor_perflib, but the latter may as well be used by the arch
enumeration of CPUs and the freqdomain_cpus mask may be populated from
there.

As far as cpufreq is concerned, if the interface to the hardware is
per-CPU, there is one CPU per policy and cpufreq has no business
knowing anything about the underlying hardware coordination.