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

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Date:	Fri, 15 Aug 2014 16:07:41 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ashwin Chaugule <ashwin.chaugule@...aro.org>
Cc:	lkml <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>, mingo@...nel.org,
	len.brown@...el.com, rjw@...ysocki.net,
	"linaro-acpi@...ts.linaro.org" <linaro-acpi@...ts.linaro.org>,
	Arnd Bergmann <arnd@...db.de>, linux-acpi@...r.kernel.org,
	cpufreq@...r.kernel.org, Patch Tracking <patches@...aro.org>,
	Dirk Brandewie <dirk.brandewie@...il.com>
Subject: Re: [RFC 0/3] Experimental patchset for CPPC

On Fri, Aug 15, 2014 at 09:08:50AM -0400, Ashwin Chaugule wrote:
> If the OS only looks at Highest, Lowest, Delivered registers and only
> writes to Desired, then we're not really any different than how we do
> things today in the CPUFreq layer.

The thing is; we're already struggling to make 'sense' of x86 as it
stands today. And it looks like this CPPC stuff makes the behaviour even
less certain.

> Or even in the case of
> intel_pstate, if you map Desired to PERF_CTL and get value of
> Delivered by using aperf/mperf ratios (as my experimental driver
> does), then we can still maintain the existing system performance. It
> seems like if an OS can make use of the additional information then it
> should be net win for overall power savings and performance
> enhancement. Also, using the CPPC descriptors, we should be able to
> have one driver across X86 and ARM64. (possibly others too.)

Yikes, so aaargh64 will go do creative power management too?

And worse; it will go do ACPI? Welcome to the world of guaranteed BIOS
fail :-(

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