linux-kernel - Re: [RFC 0/3] Experimental patchset for CPPC

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Message-ID: <CAJvTdK=hYS87BKZ0NcX7U3RJRmUHWGhC6krSTTmN8O2MO9BK=A@mail.gmail.com>
Date:	Fri, 15 Aug 2014 18:22:10 -0400
From:	Len Brown <lenb@...nel.org>
To:	Ashwin Chaugule <ashwin.chaugule@...aro.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Mike Turquette <mike.turquette@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Brown, Len" <len.brown@...el.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.ne>,
	"linaro-acpi@...ts.linaro.org" <linaro-acpi@...ts.linaro.org>,
	Arnd Bergmann <arnd@...db.de>,
	linux acpi <linux-acpi@...r.kernel.org>,
	cpufreq@...r.kernel.org, Patch Tracking <patches@...aro.org>
Subject: Re: [RFC 0/3] Experimental patchset for CPPC

> I verified that CPU freq requests were taken by reading out the PERF_STATUS register.

Don't use the x86 PERF_STATUS register -- it will not tell you what
you want to know.

If you want to see the actual frequency, you need to watch how many
cycles elapse per a known time interval,
which is how intel_pstate and the turbostat utility do it.

cheers,
Len Brown
Intel Open Source Technology Center
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