Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Thu, 20 Jul 2017 11:52:41 +0200 | Subject | Re: cpuidle and cpufreq coupling? |
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On Thu, Jul 20, 2017 at 9:18 AM, Viresh Kumar <viresh.kumar@linaro.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@gmail.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).
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