linux-kernel - Re: [PATCH 1/2] cpufreq: Introduce target min and max frequency hints

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Message-ID: <20201109043912.7zvfhi42yhr7goy4@vireshk-i7>
Date:   Mon, 9 Nov 2020 10:09:12 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Zhang Rui <rui.zhang@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] cpufreq: Introduce target min and max frequency hints

On 06-11-20, 18:02, Rafael J. Wysocki wrote:
> On Fri, Nov 6, 2020 at 11:07 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
> >
> > On 05-11-20, 19:23, Rafael J. Wysocki wrote:
> > > Index: linux-pm/include/linux/cpufreq.h
> > > ===================================================================
> > > --- linux-pm.orig/include/linux/cpufreq.h
> > > +++ linux-pm/include/linux/cpufreq.h
> > > @@ -63,6 +63,8 @@ struct cpufreq_policy {
> > >
> > >       unsigned int            min;    /* in kHz */
> > >       unsigned int            max;    /* in kHz */
> > > +     unsigned int            target_min; /* in kHz */
> > > +     unsigned int            target_max; /* in kHz */
> > >       unsigned int            cur;    /* in kHz, only needed if cpufreq
> > >                                        * governors are used */
> > >       unsigned int            suspend_freq; /* freq to set during suspend */
> >
> > Rafael, honestly speaking I didn't like this patch very much.
> 
> So what's the concern, specifically?
> 
> > We need to fix a very specific problem with the intel-pstate driver when it is
> > used with powersave/performance governor to make sure the hard limits
> > are enforced. And this is something which no one else may face as
> > well.
> 
> Well, I predict that the CPPC driver will face this problem too at one point.
> 
> As well as any other driver which doesn't select OPPs directly for
> that matter, at least to some extent (note that intel_pstate in the
> "passive" mode without HWP has it too, but since there is no way to
> enforce the target max in that case, it is not relevant).
> 
> > What about doing something like this instead in the intel_pstate
> > driver only to get this fixed ?
> >
> >         if (!strcmp(policy->governor->name, "powersave") ||
> >             !strcmp(policy->governor->name, "performance"))
> >                 hard-limit-to-be-enforced;
> >
> > This would be a much simpler and contained approach IMHO.
> 
> I obviously prefer to do it the way I did in this series, because it
> is more general and it is based on the governor telling the driver
> what is needed instead of the driver trying to figure out what the
> governor is and guessing what may be needed because of that.
> 
> But if you have a very specific technical concern regarding my
> approach, I can do it the other way too.

I was concerned about adding those fields in the policy structure, but
I get that you want to do it in a more generic way.

What about adding a field name "fixed" (or something else) in the
governor's structure which tells us that the frequency is fixed and
must be honored by the driver.

-- 
viresh