Date:	Fri, 28 Jun 2013 08:54:57 +0200
From:	Lukasz Majewski <l.majewski@...sung.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	"Rafael J. Wysocky" <rjw@...k.pl>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Jonghwa Lee <jonghwa3.lee@...sung.com>,
	Myungjoo Ham <myungjoo.ham@...sung.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Lukasz Majewski <l.majewski@...ess.pl>,
	Andre Przywara <andre.przywara@...aro.org>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Kukjin Kim <kgene.kim@...sung.com>,
	Zhang Rui <rui.zhang@...el.com>,
	Eduardo Valentin <eduardo.valentin@...com>
Subject: Re: [PATCH v4 5/7] cpufreq: Calculate number of busy CPUs

On Fri, 28 Jun 2013 09:20:40 +0530, Viresh Kumar wrote:
> On 27 June 2013 20:12, Lukasz Majewski <l.majewski@...sung.com> wrote:
> > On Thu, 27 Jun 2013 16:46:44 +0530, Viresh Kumar wrote:
> >> @Rafael: We need you to jump into this discussion now, I don't
> >> have a good idea about what we should do :)
> >>
> >> On 27 June 2013 16:28, Lukasz Majewski <l.majewski@...sung.com>
> >> wrote:
> >> > Do you have any idea of how to precisely set the load threshold?
> >>
> >> I thought we are talking about cpu being in idle state.
> >
> > If we _drop_ the idea with thermal subsystem to disable the boost,
> > the logic as far as I've understood shall here be as follow:
> >
> > Only enable BOOST when one CPU load > THRESHOLD_MAX and other CPUs <
> > THRESHOLD_MIN
> 
> Again, I thought that we are talking about cpus being completely idle.
> i.e. in WFI (wait for interrupt) or deeper states.
> 
> > THRESHOLD_MIN & THRESHOLD_MAX are SoC specific.
> >
> > In my opinion the above constrain imposes policy to the cpufreq
> > driver.
> 
> Hmm.
> 
> > So thermal or "other solution" [*] shall disable boost when
> > overheated and enable it back when things cool down.
> 
> yeah..

For me thermal is a good candidate to enable boost again. I only need
to find a proper place for it.

> 
> > [*] @ Viresh & Rafael do you have any idea about the "other
> > solution" here?
> 
> Not really sure :)

Not any single one? Then I would like to propose thermal.

> 
> >> There might be platforms where overheating isn't a issue with
> >> boost, if it is only enabled while only one cpu is in use.
> >
> > Could you elaborate more on this?
> 
> I meant platforms where chip doesn't heat up much when only one core
> is in use and is using boost frequency. So, they may not require
> support for thermal layer at all.. But I am not aware of what the
> ground reality is. If such systems can be possible or not.

Ok.

> 
> > This would prevent situation when somebody made a mistake and
> > had enabled boost, but for some reason had forgotten to
> > configure/enable thermal subsystem.
> >
> > Moreover Kconfig's CONFIG_CPUFREQ_BOOST flag would indicate that
> > user enabled boost for some reason and he/she (presumably) knows
> > what is doing.
> 
> Yeah.. And drivers like ACPI cpufreq and exynos can simply do a select
> from their Kconfig entries so that user isn't required to select them.

Automatic select is not a good option. My goal would be here to define
BOOST as NO by default (at lease for SW managed ones). And allow user
to enable it explicitly.


-- 
Best regards,

Lukasz Majewski

Samsung R&D Institute Poland (SRPOL) | Linux Platform Group
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