linux-kernel - Re: cpuidle and cpufreq coupling?

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Date:   Thu, 20 Jul 2017 11:52:41 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     "Rafael J. Wysocki" <rafael@...nel.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>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: cpuidle and cpufreq coupling?

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
>>
>> Generally, the idle driver is expected to manipulate OPPs as suitable
>> for it at the low level.
>
> Does any idle driver do it today ?
>
> I am not sure, but I haven't heard anyone from ARM doing it. Though I
> may have completely missed it :)

You may not, but that's what is recommended.

Had you attended PM sessions at the LPC and similar, you might have
heard about it ...

> So, that must call into cpufreq (somehow) and look for a low power
> OPP?

It should know what OPP to use and then coordinate with cpufreq so
they don't go against each other (on shared policies).