linux-kernel - Re: [PATCH 4/4] cpu_cooling: Drop static-power related stuff

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Date:   Tue, 21 Nov 2017 17:08:34 +0100
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Lukasz Luba <llu.ker.dev@...il.com>
Cc:     Daniel Lezcano <daniel.lezcano@...aro.org>,
        Ionela Voinescu <ionela.voinescu@....com>,
        Punit Agrawal <punit.agrawal@....com>,
        Eduardo Valentin <edubezval@...il.com>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Amit Daniel Kachhap <amit.kachhap@...il.com>,
        Javi Merino <javi.merino@...nel.org>,
        Zhang Rui <rui.zhang@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        Lukasz Luba <lukasz.luba@....com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Javi Merino <javi.merino@....com>
Subject: Re: [PATCH 4/4] cpu_cooling: Drop static-power related stuff

Hi Lukasz

On 21 November 2017 at 16:56, Lukasz Luba <llu.ker.dev@...il.com> wrote:
> On 21/11/17 14:06, Daniel Lezcano wrote:
>>
>> On 21/11/2017 12:30, Ionela Voinescu wrote:
>>
>> [ ... ]
>>
>>> A DT model would be easy to support with the current code but it would
>>> be very inaccurate.
>>
>>
>> Why ?
>>
>> [ ... ]
>>
> Hi all,
>
> The DT solution won't fly, the reason can be found below.
>
> I agree with Ionela and Punit that the Juno board is not
> the best platform to test the static power impact on IPA.
> In some other platforms the static power can be 50% or more
> of the total power, so it cannot be neglected.
>
> These are the issues.
> The static power equation is complicated, here is one known to me.
> The leakage function is exponentially influenced by current circuit
> supply voltage, body-bias and some constants K_{4,5}.
>
> P_{leak} = L_{g}*V_{dd}*K_{3}*e^{K_{4}*V_{dd}}*e^{K_{5}*V_{bs}}+|
> V_{bs}|*I_{Ju}

You forgot one main contributor of static leakage: the temperature

>
> It can also vary depending on technology (CMOS, FinFET, etc).
>
> It would be really hard to approximate by i.e. a polynomial
> function with inputs from DT. One size does not fit all.

But can't we linearized around the target temp ? that were we want to
be accurate

Regards,
>
> The equation can also tell you some interesting things about
> the manufacturing process. Exposing such information might be the last
> thing the vendors want to.
> That's why the vendors might want to implement whole
> thermal management in the firmware or skip static power and
> rely on IPA adaptation.
> They can also use a different api in IPA, when they have some mechanism
> to measure power in firmware, it can be feed into IPA.
>
> Anyway, I would recommend to keep it as is, to have a complete
> power model in the kernel.
> The code without static power routines looks awkward to me.
> From my side - NACK for the patch which removes static power.
>
> Regards,
> Lukasz Luba