linux-kernel - Re: [PATCH 0/3] drivers: Frequency constraint infrastructure

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Date:   Fri, 8 Feb 2019 10:53:55 +0100
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Viresh Kumar <vireshk@...nel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Matthias Kaehlcke <mka@...omium.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/3] drivers: Frequency constraint infrastructure

On Fri, Feb 8, 2019 at 10:09 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 11-01-19, 10:47, Rafael J. Wysocki wrote:
> > On Fri, Jan 11, 2019 at 10:18 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > >
> > > Hi,
> > >
> > > This commit introduces the frequency constraint infrastructure, which
> > > provides a generic interface for parts of the kernel to constraint the
> > > working frequency range of a device.
> > >
> > > The primary users of this are the cpufreq and devfreq frameworks. The
> > > cpufreq framework already implements such constraints with help of
> > > notifier chains (for thermal and other constraints) and some local code
> > > (for user-space constraints). The devfreq framework developers have also
> > > shown interest [1] in such a framework, which may use it at a later
> > > point of time.
> > >
> > > The idea here is to provide a generic interface and get rid of the
> > > notifier based mechanism.
> > >
> > > Only one constraint is added for now for the cpufreq framework and the
> > > rest will follow after this stuff is merged.
> > >
> > > Matthias Kaehlcke was involved in the preparation of the first draft of
> > > this work and so I have added him as Co-author to the first patch.
> > > Thanks Matthias.
> > >
> > > FWIW, This doesn't have anything to do with the boot-constraints
> > > framework [2] I was trying to upstream earlier :)
> >
> > This is quite a bit of code to review, so it will take some time.
>
> @Rafael: You are going to provide some more feedback here, right ?

I think I've said something already.

TLDR: I'm not convinced.

PM QoS does similar things in a similar way.  Why does it have to be a
different framework?

Using freq constraints for thermal reasons without coordinating it
with scheduling is questionable in general, because thermal control
may work against scheduling then.

AFAICS, the original reason for notifiers in cpufreq was to avoid
drawing too much power (and frequency was a proxy of power then) and
they allowed the firmware to set the additional limit when going from
AC to battery, for example.  That appears to be a different goal,
though.