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

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Date:	Thu, 4 Feb 2016 16:44:28 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Juri Lelli <juri.lelli@....com>
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 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

>
> Thanks,
>
> - Juri