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

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Message-ID: <20190118123900.GJ14385@localhost.localdomain>
Date:   Fri, 18 Jan 2019 13:39:00 +0100
From:   Juri Lelli <juri.lelli@...il.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.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 17/01/19 15:55, Rafael J. Wysocki wrote:
> On Thu, Jan 17, 2019 at 2:16 PM Juri Lelli <juri.lelli@...il.com> 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.
> > >
> > > One immediate observation is that it seems to do quite a bit of what
> > > is done in the PM QoS framework, so maybe there is an opportunity for
> > > some consolidation in there.
> >
> > Right, had the same impression. :-)
> >
> > I was also wondering how this new framework is dealing with
> > constraints/request imposed/generated by the scheduler and related
> > interfaces (thinking about schedutil and Patrick's util_clamp).
> 
> My understanding is that it is orthogonal to them, like adding extra
> constraints on top of them etc.

Mmm, ok. But, if that is indeed the case, I now wonder why and how
existing (or hopefully to be added soon) interfaces are not sufficient.
I'm not against this proposal, just trying to understand if this might
create unwanted, hard to manage, overlap.