linux-kernel - Re: cpuidle and cpufreq coupling?

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Message-ID: <20170720144530.da6bqpdvt2cvbjjs@hirez.programming.kicks-ass.net>
Date:   Thu, 20 Jul 2017 16:45:30 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Florian Fainelli <f.fainelli@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Markus Mayer <code@...yer.net>,
        Daniel Lezcano <daniel.lezcano@...aro.org>
Subject: Re: cpuidle and cpufreq coupling?

On Thu, Jul 20, 2017 at 11:52:41AM +0200, Rafael J. Wysocki wrote:
> On Thu, Jul 20, 2017 at 9:18 AM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> > On 20-07-17, 01:17, Rafael J. Wysocki wrote:
> >> On Thu, Jul 20, 2017 at 12:54 AM, Florian Fainelli <f.fainelli@...il.com> wrote:
> >> > Hi,
> >> >
> >> > We have a particular ARM CPU design that is drawing quite a lot of
> >> > current upon exit from WFI, and it does so in a way even before the
> >> > first instruction out of WFI is executed. That means we cannot influence
> >> > directly the exit from WFI other than by changing the state in which it
> >> > would be previously entered because of this "dead" time during which the
> >> > internal logic needs to ramp up back where it left.
> >> >
> >> > A naive approach to solving this problem because we have CPU frequency
> >> > scaling available would be to do the following:
> >> >
> >> > - just before entering WFI, switch to a low frequency OPP
> >> > - enter WFI
> >> > - upon exit from WFI, ramp up the frequency back to e.g: highest OPP
> >> >
> >> > Some of the parts that I am not exactly clear on would be:
> >> >
> >> > - would that qualify as a cpuidle governor of some kind that ties in
> >> > which cpufreq?
> >> > - would using cpufreq_driver_fast_switch() be an appropriate API to use
> >> > from outside

Can your ARM part change OPP without scheduling? Because (for obvious
reasons) the idle thread is not supposed to block.