linux-kernel - Re: [PATCH 8/8] sched: prefer cpufreq_scale_freq

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Message-ID: <6960637.XgeniQrqfM@vostro.rjw.lan>
Date:	Wed, 16 Mar 2016 22:32:46 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Dietmar Eggemann <dietmar.eggemann@....com>,
	Michael Turquette <mturquette@...libre.com>,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
	Juri.Lelli@....com, steve.muckle@...aro.org,
	morten.rasmussen@....com, vincent.guittot@...aro.org,
	Michael Turquette <mturquette+renesas@...libre.com>
Subject: Re: [PATCH 8/8] sched: prefer cpufreq_scale_freq_capacity

On Wednesday, March 16, 2016 09:07:52 PM Peter Zijlstra wrote:
> On Wed, Mar 16, 2016 at 07:44:33PM +0000, Dietmar Eggemann wrote:
> > intel_pstate (setpolicy()) is an exception but my humble guess is that
> > systems with intel_pstate driver have X86_FEATURE_APERFMPERF support.
> 
> A quick browse of the Intel SDM says you're right. It looks like
> everything after Pentium-M; so Core-Solo/Core-Duo and onwards have
> APERF/MPERF.
> 
> And it looks like P6 class systems didn't have DVFS support at all,
> which basically leaves P4 and Pentium-M as the only chips to have DVFS
> support lacking APERF/MPERF.
> 
> And while I haven't got any P4 based space heaters left, I might still
> have a Pentium-M class laptop somewhere (if it still boots).

intel_pstate depends on APERF/MPERF, so it won't work with anything
without them.