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

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Date:   Tue, 4 Aug 2020 11:44:10 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
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 8/4/20 11:38 AM, Viresh Kumar wrote:
> 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.

OK, great, I will wait then with this patch series v2 which would change
into debugfs scmi only. Could you please add me on CC, I am very
interested in.

> 
>>> 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 :)
>