linux-kernel - Re: cpuidle and cpufreq coupling?

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Message-ID: <49e8479c-bc47-16fe-0bf9-8a4aba333909@arm.com>
Date:   Thu, 20 Jul 2017 10:23:27 +0100
From:   Sudeep Holla <sudeep.holla@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        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 20/07/17 08:18, Viresh Kumar 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 :)
> 

It doesn't need to be in Linux. E.g. PSCI or any low lever driver can do
that transparently.

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

That's seems hacky and NAK if it's PSCI platform. It's cleaner do such
hacks/workarounds in platform specific PSCI firmware.

> @Florian: It would be more tricky then we anticipate. We don't always
> want to go to low OPP on idle, as we may get out of it very quickly
> and changing OPP twice (before and after idle) in that scenario would
> be a complete waste of time.

Exactly.

-- 
Regards,
Sudeep