linux-kernel - Re: [PATCH v3 2/6] drivers/cpufreq: implement init_cpu_capacity

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Message-ID: <20160205093040.GG29586@e106622-lin>
Date:	Fri, 5 Feb 2016 09:30:40 +0000
From:	Juri Lelli <juri.lelli@....com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Morten Rasmussen <morten.rasmussen@....com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	LAK <linux-arm-kernel@...ts.infradead.org>,
	"devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Rob Herring <robh+dt@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Russell King - ARM Linux <linux@....linux.org.uk>,
	Sudeep Holla <sudeep.holla@....com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Mark Brown <broonie@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Viresh Kumar <viresh.kumar@...aro.org>
Subject: Re: [PATCH v3 2/6] drivers/cpufreq: implement
 init_cpu_capacity_default()

On 04/02/16 16:46, Vincent Guittot wrote:
> On 4 February 2016 at 16:44, Vincent Guittot <vincent.guittot@...aro.org> wrote:
> > On 4 February 2016 at 15:13, Juri Lelli <juri.lelli@....com> wrote:
> >> On 04/02/16 13:35, Vincent Guittot wrote:
> >>> On 4 February 2016 at 13:16, Juri Lelli <juri.lelli@....com> wrote:
> >>> > Hi Vincent,
> >>> >
> >>> > On 04/02/16 13:03, Vincent Guittot wrote:
> >>> >> On 4 February 2016 at 10:36, Morten Rasmussen <morten.rasmussen@....com> wrote:
> >>> >> > On Wed, Feb 03, 2016 at 10:04:37PM +0100, Vincent Guittot wrote:
> >>> >> >> On 3 February 2016 at 12:59, Juri Lelli <juri.lelli@....com> wrote:
> >>> >>
> >>> >> [snip]
> >>> >>
> >>> >> >> > diff --git a/drivers/cpufreq/Makefile b/drivers/cpufreq/Makefile
> >>> >> >> > index 9e63fb1..c4025fd 100644
> >>> >> >> > --- a/drivers/cpufreq/Makefile
> >>> >> >> > +++ b/drivers/cpufreq/Makefile
> >>> >> >> > @@ -1,5 +1,5 @@
> >>> >> >> >  # CPUfreq core
> >>> >> >> > -obj-$(CONFIG_CPU_FREQ)                 += cpufreq.o freq_table.o
> >>> >> >> > +obj-$(CONFIG_CPU_FREQ)                 += cpufreq.o freq_table.o cpufreq_capacity.o
> >>> >> >>
> >>> >> >> Do you really want to have the calibration of capacity dependent of
> >>> >> >> cpufreq ? It means that we can't use it without a cpufreq driver.
> >>> >> >> IMHO, this creates a unnecessary dependency. I understand that you
> >>> >> >> must ensure that core runs at max fequency if a driver is present but
> >>> >> >> you should be able to calibrate the capacity if cpufreq is not
> >>> >> >> available but you have different capacity because micro architecture
> >>> >> >
> >>> >> > We could remove the dependency on cpufreq, but it would make things more
> >>> >> > complicated for systems which do have frequency scaling as we would have
> >>> >> > to either:
> >>> >> >
> >>> >> > 1) Run the calibration again once cpufreq has been initialized.
> >>> >>
> >>> >> or wait and let time for a driver to initialize and trig the
> >>> >> calibration. If calibration has not been done at the end of the boot,
> >>> >> you can force a calibration. If the cpufeq driver is a module and is
> >>> >> loaded far later for any good or bad reason, we will have to run the
> >>> >> calibration once again but at least the capacity will reflect he
> >>> >> current capacity of the CPUs.
> >>> >> I'm mainly worried that the compilation of the calibration is
> >>> >> dependent of CONFIG_CPU_FREQ not that cpufreq can trig the calibration
> >>> >> sequence
> >>> >>
> >>> >
> >>> > Yes, I guess we can make this work in some way. Out of curiosity,
> >>> > though, are out there heterogenous platforms that don't use cpufreq?
> >>>
> >>> At least, you can find several heterogeneous platforms without OPP
> >>> table for CPUs in the kernel. That's probably a temporary situation
> >>> but which can become a permanent one. It means that we can't calibrate
> >>> the CPUs for these platforms.
> >>>
> >>
> >> Sorry, can you make some examples so that I'm sure I understand what you
> >> are referring to?
> >
> > As an example, the uniphier arm64 Soc doesn't have a cpufreq driver so far
> >
> >>
> >> Anyway, don't these platform still make use of cpufreq (even if without
> >> an OPP table) so that we can still control policy->max and min?
> >
> > AFAICT, They don't have a dedicated cpufreq driver.
> >
> > More generally speaking, it can take time before having
> 
> email sent before the ne d of the sentence ...
> 
> More generally speaking, it can take time before having a cpufreq
> driver whereas we want to run and test scheduler behavior of these
> heterogenous platform
> 

I'm not familiar with this platform, but from what you are saying and
what I could find online, it looks like full Linux support is not
finished yet. Can we consider that as still in development? And if we
can do that, maybe is fair enough that we use the sysfs interface to
play with that platform until support is complete.

Do others have any opinion on this point?

Thanks,

- Juri