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Date: Wed, 10 Apr 2013 11:14:01 +0100
From: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
To: Lukasz Majewski <l.majewski@samsung.com>
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Subject: Re: [RFC PATCH 0/2] cpufreq: Introduce LAB cpufreq governor.
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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