linux-kernel - Re: [PATCH V2] cpufreq: Disallow ->resolve_freq() for drivers providing ->target

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Date:	Thu, 21 Jul 2016 17:34:05 -0700
From:	Steve Muckle <steve.muckle@...aro.org>
To:	"Rafael J. Wysocki" <rafael@...nel.org>
Cc:	Steve Muckle <steve.muckle@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	Linux PM <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH V2] cpufreq: Disallow ->resolve_freq() for drivers
 providing ->target_index()

On Fri, Jul 22, 2016 at 02:18:54AM +0200, Rafael J. Wysocki wrote:
> > My thinking was that one of these two would be preferable:
> >
> > - Forcing ->target() drivers to install a ->resolve_freq callback,
> >   enforcing this at cpufreq driver init time.
> 
> That would have been possible, but your series didn't do that.
> 
> >   My understanding is
> >   ->target() drivers are deprecated anyway
> 
> No, they aren't.

Ok. I didn't follow Documentation/cpu-freq/cpu-drivers.txt section 1.5
then - it suggests something about target() is deprecated, perhaps it's
out of date.

> There simply are cases in which frequency tables are not workable
> (like the ACPI CPPC one).

Sure that makes sense.

> >  and theren't aren't many of
> >   them, though I don't know offhand exactly how many or how hard it
> >   would be to do for each one.
> >
> > - Forcing callers (schedutil in this case) to check that either
> >   ->target() or ->resolve_freq() is implemented. It means
> >   catching and scrutinizing future callers of resolve_freq.
> 
> But that doesn't reduce the number of checks in cpufreq_driver_resolve_freq().
> 
> There still are three choices in there: return a frequency from the
> table (if present), or call ->resolve_freq (if implemented), or return
> target_freq (as the last resort).

Sorry, that should've been "check that either ->target_index() or
->resolve_freq() is implemented." 

Implementing resolve_freq for the target() drivers and requiring it at
driver init time is probably the better way to go though. Perhaps I can
work on this at some point.