linux-kernel - Re: [PATCH v2 02/10] cpufreq: provide data for frequency-invariant load-tracking support

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Date:   Tue, 11 Jul 2017 16:12:57 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:     Viresh Kumar <viresh.kumar@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux PM <linux-pm@...r.kernel.org>,
        Russell King - ARM Linux <linux@....linux.org.uk>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Russell King <rmk+kernel@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Juri Lelli <juri.lelli@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH v2 02/10] cpufreq: provide data for frequency-invariant
 load-tracking support

On 11/07/17 15:59, Rafael J. Wysocki wrote:
> On Tuesday, July 11, 2017 04:06:01 PM Dietmar Eggemann wrote:
>> On 11/07/17 07:01, Viresh Kumar wrote:
>>> On 10-07-17, 13:02, Dietmar Eggemann wrote:
>>>> Yes, I will change this. The #define approach is not really necessary
>>>> here since we're not in the scheduler hot-path and inlining is not
>>>> really required here.
>>>
>>> It would be part of scheduler hot-path for the fast-switching case, isn't it ?
>>> (I am not arguing against using weak functions, just wanted to correct above
>>> statement).
>>
>> Yes you're right here.
>>
>> But in the meantime we're convinced that cpufreq_driver_fast_switch() is
>> not the right place to call arch_set_freq_scale() since for (future)
>> arm/arm64 fast-switch driver, the return value of
>> cpufreq_driver->fast_switch() does not give us the information that the
>> frequency value did actually change.
>>
>> So we probably have to do this soemwhere in the cpufreq driver(s) to
>> support fast-switching until we have aperf/mperf like counters.
> 
> If that's the case, I'd say call arch_set_freq_scale() from drivers in all
> cases or it will get *really* confusing.

Agreed, we should do it for slow-switching drivers from within the
driver as well in this case.