linux-kernel - Re: [RFC PATCH 0/2] cpufreq: Introduce LAB cpufreq governor.

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Message-ID: <20130410101400.GA4981@e102568-lin.cambridge.arm.com>
Date:	Wed, 10 Apr 2013 11:14:01 +0100
From:	Lorenzo Pieralisi <lorenzo.pieralisi@....com>
To:	Lukasz Majewski <l.majewski@...sung.com>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Jonghwa Lee <jonghwa3.lee@...sung.com>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	MyungJoo Ham <myungjoo.ham@...sung.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Chanwoo Choi <cw00.choi@...sung.com>,
	"sw0312.kim@...sung.com" <sw0312.kim@...sung.com>,
	Marek Szyprowski <m.szyprowski@...sung.com>
Subject: Re: [RFC PATCH 0/2] cpufreq: Introduce LAB cpufreq governor.

On Wed, Apr 10, 2013 at 09:44:52AM +0100, Lukasz Majewski wrote:

[...]

> > Have you also looked at the power clamp driver that have similar
> > target ?
> 
> I might be wrong here, but in my opinion the power clamp driver is a bit
> different:
> 
> 1. It is dedicated to Intel SoCs, which provide special set of
> registers (i.e. MSR_PKG_Cx_RESIDENCY [*]), which forces a processor to
> enter certain C state for a given duration. Idle duration is calculated
> by per CPU set of high priority kthreads (which also program [*]
> registers). 
> 

Those registers are used for compensation (ie user asked a given idle
ratio but HW stats show a mismatch) and they are not "programmed"
they are just read. That code is Intel specific but it can be easily ported
to ARM, I did that and most of the code is common with zero dependency
on the architecture.

> 2. ARM SoCs don't have such infrastructure, so we depend on SW here.

Well, it is true that most of the SoCs I am working on do not have
a programming interface to monitor C-state residency, granted, this is
a problem. If those stats can be retrieved somehow (I did that on our TC2
platform) then power clamp can be used on ARM with minor modifications.

Lorenzo

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