linux-kernel - Re: [PATCH 0/4] CPUFreq statistics retrieved by drivers

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Date:   Tue, 4 Aug 2020 16:08:57 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Lukasz Luba <lukasz.luba@....com>
Cc:     Sudeep Holla <sudeep.holla@....com>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-pm@...r.kernel.org,
        cristian.marussi@....com, rjw@...ysocki.net
Subject: Re: [PATCH 0/4] CPUFreq statistics retrieved by drivers

On 04-08-20, 11:29, Lukasz Luba wrote:
> On 8/4/20 6:35 AM, Viresh Kumar wrote:
> > IIUC, the only concern right now is to capture stats with fast switch ? Maybe we
> > can do something else in that case and brainstorm a bit..
> 
> Correct, the fast switch is the only concern right now and not tracked. We
> could fill in that information with statistics data from firmware
> with a cpufreq driver help.
> 
> I could make the if from patch 1/4 covering narrowed case, when
> fast switch is present, check for drivers stats.
> Something like:
> -----------8<------------------------------------------------------------
> if (policy->fast_switch_enabled)
> 	if (policy->has_driver_stats)
> 		return cpufreq_stats_present_driver_data(policy, buf);
> 	else
> 		return 0;
> -------------->8----------------------------------------------------------

I don't think doing it with help of firmware is the right thing to do
here then. For another platform we may not have a firmware which can
help us, we need something in the opp core itself for that. Lemme see
if I can do something about it.

> > Why is firmware the governor here ? Aren't you talking about the simple fast
> > switch case only ?
> 
> I used a term 'governor' for the firmware because it makes the final
> set for the frequency. It (FW) should respect the frequency value
> set using the fast switch. I don't know how other firmware (e.g. Intel)
> treats this fast switch value or if they even expose FW stats, though.

For Intel I think, Linux is one of the entities that vote for deciding
the frequency of the CPUs and the firmware (after taking all such
factors into account) chooses a frequency by its own, which must be >=
the frequency requested by Linux.

> You can read about this statistics region in [1] at:
> 4.5.5 Performance domain statistics shared memory region
> 
> > 
> > Over that, I think this cpufreq stats information isn't parsed by any tool right
> > now and tweaking it a bit won't hurt anyone (like if we start capturing things a
> > bit differently). So we may not want to worry about breaking userspace ABI here,
> > if what we are looking to do is the right thing to do.
> 
> So, there is some hope... IMHO it would be better to have this cpufreq
> stats in normal location, rather then in scmi debugfs.

I agree.

> > I am not sure what notifications are we talking about here.
> 
> There is a notification mechanism described in the SCMI spec [1] at
> 4.5.4 Notifications.
> We were referring to that mechanism.

Ahh, I see. All I was thinking was about the cpufreq specific
notifiers :)

-- 
viresh