linux-kernel - Re: [PATCH RFC 2/2] cpufreq: arm_big

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Date:	Thu, 9 Oct 2014 19:38:23 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Turquette <mturquette@...aro.org>
Cc:	Morten Rasmussen <morten.rasmussen@....com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	"pjt@...gle.com" <pjt@...gle.com>,
	"bsegall@...gle.com" <bsegall@...gle.com>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"nicolas.pitre@...aro.org" <nicolas.pitre@...aro.org>,
	"rjw@...ysocki.net" <rjw@...ysocki.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"tuukka.tikkanen@...aro.org" <tuukka.tikkanen@...aro.org>
Subject: Re: [PATCH RFC 2/2] cpufreq: arm_big_little: provide cpu capacity

On Thu, Oct 09, 2014 at 10:25:13AM -0700, Mike Turquette wrote:
> Quoting Peter Zijlstra (2014-10-09 02:02:52)
> > On Wed, Oct 08, 2014 at 03:37:32PM -0700, Mike Turquette wrote:
> > > It creates a dependency such that any ARM platform that wants to have
> > > frequency-invariant load must use CPUfreq. I don't think we want that
> > > dependency. CPUfreq is a likely back-end for many devices, but not all.
> > > 
> > > Consider near-future ARM devices that support ACPI for power management
> > > with no corresponding CPUfreq driver. For example if the CPPC driver is
> > > not hooked up to CPUfreq, platforms using CPPC will not jive with the
> > > ARM arch hook that depends on CPUfreq.
> > 
> > Oh crap no, CPPC will not add yet another interface to cpu frequency
> > stuff.
> 
> Right.
> 
> So let's say the ARM arch hook creates a dependency on CPUfreq to scale
> capacity as a function of cpu frequency (as it does in the ARM scale
> invariance series).
> 
> Then let's say that a hypothetical ARM platform named "foo" uses CPPC
> and not CPUfreq to scale frequency. Foo's implementation of CPPC does
> not use any of the full-auto or hw-auto stuff. It allows the OS to
> request minimum performance levels and the like.
> 
> In this case, how can foo take advantage of the scale invariant stuff?
> 
> Also, feel free to replace "CPPC" with "anything other than CPUfreq".
> The problem is a general one and not specific to CPPC or ACPI.

Well answer #1 is that you simply should not ever bypass cpufreq for
setting cpu frequencies (be this the existing cpufreq or the future
integrated cpufreq).

Answer #2 is that if you were allowed to create a second infrastructure
and you're not calling the right scheduler hooks right along with it,
you're buggy.

In short, your problem, not mine.
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