Re: is 'dynamic-power-coefficient' expected to be based on 'real' power measurements?

[Matthias Kaehlcke <mka@chromium.org>]

On Tue, Sep 15, 2020 at 02:46:16PM -0700, Doug Anderson wrote:
> Hi,
> 
> On Tue, Sep 15, 2020 at 1:55 PM Daniel Lezcano
> <daniel.lezcano@linaro.org> wrote:
> >
> > On 15/09/2020 19:58, Matthias Kaehlcke wrote:
> > > On Tue, Sep 15, 2020 at 07:50:10PM +0200, Daniel Lezcano wrote:
> > >> On 15/09/2020 19:24, Matthias Kaehlcke wrote:
> > >>> +Thermal folks
> > >>>
> > >>> Hi Rajendra,
> > >>>
> > >>> On Tue, Sep 15, 2020 at 11:14:00AM +0530, Rajendra Nayak wrote:
> > >>>> Hi Rob,
> > >>>>
> > >>>> There has been some discussions on another thread [1] around the DPC (dynamic-power-coefficient) values
> > >>>> for CPU's being relative vs absolute (based on real power) and should they be used to derive 'real' power
> > >>>> at various OPPs in order to calculate things like 'sustainable-power' for thermal zones.
> > >>>> I believe relative values work perfectly fine for scheduling decisions, but with others using this for
> > >>>> calculating power values in mW, is there a need to document the property as something that *has* to be
> > >>>> based on real power measurements?
> > >>>
> > >>> Relative values may work for scheduling decisions, but not for thermal
> > >>> management with the power allocator, at least not when CPU cooling devices
> > >>> are combined with others that specify their power consumption in absolute
> > >>> values. Such a configuration should be supported IMO.
> > >>
> > >> The energy model is used in the cpufreq cooling device and if the
> > >> sustainable power is consistent with the relative values then there is
> > >> no reason it shouldn't work.
> > >
> > > Agreed on thermal zones that exclusively use CPUs as cooling devices, but
> > > what when you have mixed zones, with CPUs with their pseudo-unit and e.g. a
> > > GPU that specifies its power in mW?
> >
> > Well, if a SoC vendor decides to mix the units, then there is nothing we
> > can do.
> 
> I mean, there is something someone could do.  They could buy one of
> these devices, measure the power (which wouldn't actually be that hard
> to do), then submit a patch to adjust all the numbers.  ;-)

In case they look for a recipe:

commit ac60c5e33df4ec2b69c7e3ebbc0ccf1557e7bd5e
Author: Matthias Kaehlcke <mka@chromium.org>
Date:   Thu Apr 11 17:01:58 2019 -0700

    ARM: dts: rockchip: Add dynamic-power-coefficient for rk3288

    The value was determined with the following method:

    - take CPUs 1-3 offline
    - for each OPP
      - set cpufreq min and max freq to OPP freq
      - start dhrystone benchmark
      - measure CPU power consumption during 10s
      - calculate Cx for OPPx
      - Cx = (Px - P1) / (Vx²fx - V1²f1)          [1]
        using the following units: mW / Ghz / V   [2]
    - C = avg(C2, ..., Cn)

   [1] see commit 4daa001a1773 ("arm64: dts: juno: Add cpu
       dynamic-power-coefficient information")
   [2] https://patchwork.kernel.org/patch/10493615/#22158551