linux-kernel - Re: [PATCH V3 1/2] topology: Allow multiple entities to provide sched_freq

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Date:   Fri, 19 Feb 2021 10:28:23 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Ionela Voinescu <ionela.voinescu@....com>
Cc:     Rafael Wysocki <rjw@...ysocki.net>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-pm@...r.kernel.org, Sudeep Holla <sudeep.holla@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH V3 1/2] topology: Allow multiple entities to provide
 sched_freq_tick() callback

On 18-02-21, 16:36, Ionela Voinescu wrote:
> Yes, we don't care if there is no cpufreq driver, as the use of AMUs won't
> get initialised either. But we do care if there is a cpufreq driver that
> does not support frequency invariance, which is the example above.
> 
> The intention with the patches that made cpufreq based invariance generic
> a while back was for it to be present, seamlessly, for as many drivers as
> possible, as a less than accurate invariance default method is still
> better than nothing.

Right.

> So only a few drivers today don't support cpufreq based FI

Only two AFAICT, both x86, and the AMU stuff doesn't conflict with
them.

drivers/cpufreq/intel_pstate.c
drivers/cpufreq/longrun.c

> but it's not a guarantee that it will stay this way.

What do you mean by "no guarantee" here ?

The very core routines (cpufreq_freq_transition_end() and
cpufreq_driver_fast_switch()) of the cpufreq core call
arch_set_freq_scale() today and this isn't going to change anytime
soon. If something gets changed there someone will need to see other
parts of the kernel which may get broken with that.

I don't see any need of complicating other parts of the kernel like,
amu or cppc code for that. They should be kept simple and they should
assume cpufreq invariance will be supported as it is today.

-- 
viresh