linux-kernel - Re: [PATCH 0/4] Add sustainable OPP concept

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Date:   Fri, 30 Oct 2020 09:19:48 +0000
From:   Lukasz Luba <lukasz.luba@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, devicetree@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, vireshk@...nel.org,
        robh+dt@...nel.org, sboyd@...nel.org, nm@...com, rafael@...nel.org,
        sudeep.holla@....com, daniel.lezcano@...aro.org,
        Dietmar.Eggemann@....com
Subject: Re: [PATCH 0/4] Add sustainable OPP concept



On 10/30/20 8:29 AM, Viresh Kumar wrote:
> On 29-10-20, 09:56, Lukasz Luba wrote:
>> There were discussions about Energy Model (EM), scale of values (mW or
>> abstract scale) and relation to EAS and IPA. You can find quite long
>> discussion below v2 [1] (there is also v3 send after agreement [2]).
>> We have in thermal DT binding: 'sustainable-power' expressed in mW,
>> which is used by IPA, but it would not support bogoWatts.
> 
> Why so ? (I am sorry, can't dig into such long threads without knowing
> which message I am looking for :( ). Lets assume if that same property
> can be used for bogoWatts, will that be sufficient for you ? Or you
> will still need this patch set ?

I had a patch for that, but I know Rob's opinion on this one [1] (which
is below in that thread).

> 
>> The sustainable power is used for estimation of internal coefficients
>> (also for power budget), which I am trying to change to work with
>> 'abstract scale' [3][4].
>>
>> This would allow to estimate sustainable power of the system based on
>> CPUs, GPU opp-sustainable points, where we don't have
>> 'sustainable-power' or devices using bogoWatts.
> 
> Then maybe we should ahve sustainable-power in those cases too instead
> of adding a meaningless (IMHO) binding.

How about dropping the DT binding, but just adding this new field into
dev_pm_opp? There will be no DT parsing code, just the get/set
functions, which will be used in SCMI patch 4/4 and in IPA?
That would not require to change any DT bindings.

> 
> Honestly speaking, as Nishanth said, there is nothing like a
> sustainable OPP in reality. Moreover, the DT needs to describe the
> hardware as it is (and in some cases the behavior of the firmware).
> And what you are trying to add here is none of them and so it should
> not go in DT as such. There are too many factors which play a part
> here, ambient temperature is one of the biggest ones, and the software
> needs to find the sustainable OPP by itself based on the current
> situation.
> 
> So I don't really see a good reason why such a property should be
> added here.

I see. Just for your information SCMI supports 'Sustained Performance'
  expressed in kHz.

> 
> Coming to properties like suspend-opp, it made sense for some of the
> platforms as the last configured frequency of the CPU plays a part in
> deciding the power consumed by the SoC even when the system is
> suspended. And finding an optimal OPP (normally the lowest) there
> would make sense and so was that property added.
> 

I also found that suspend-opp (83f8ca45afbf041e312909).
I hope you wouldn't mind if I add this new field into dev_pm_opp (no DT
support, just FW).


[1] https://lore.kernel.org/lkml/20201002114426.31277-4-lukasz.luba@arm.com/