Re: [PATCH] arm64: dts: qcom: sc7180: Add 'sustainable_power' for CPU thermal zones

[Doug Anderson]

Hi,

On Tue, Sep 1, 2020 at 10:36 PM Rajendra Nayak <rnayak@xxxxxxxxxxxxxx> wrote:
>
>
> > * In terms of the numbers here, I believe that you're claiming that we
> > can dissipate 768 mW * 6 + 1202 mW * 2 = ~7 Watts of power.  My memory
> > of how much power we could dissipate in previous laptops I worked on
> > is a little fuzzy, but that doesn't seem insane for a passively-cooled
> > laptop.  However, I think someone could conceivably put this chip in a
> > smaller form factor.  In such a case, it seems like we'd want these
> > things to sum up to ~2000 (if it would ever make sense for someone to
> > put this chip in a phone) or ~4000 (if it would ever make sense for
> > someone to put this chip in a small tablet).  It seems possible that,
> > to achieve this, we might have to tweak the
> > "dynamic-power-coefficient".
>
> DPC values are calculated (at a SoC) by actually measuring max power at various
> frequency/voltage combinations by running things like dhrystone.
> How would the max power a SoC can generate depend on form factors?
> How much it can dissipate sure is, but then I am not super familiar how
> thermal frameworks end up using DPC for calculating power dissipated,
> I am guessing they don't.
>
> > I don't know how much thought was put
> > into those numbers, but the fact that the little cores have a super
> > round 100 for their dynamic-power-coefficient makes me feel like they
> > might have been more schwags than anything.  Rajendra maybe knows?
>
> FWIK, the values are always scaled and normalized to 100 for silver and
> then used to derive the relative DPC number for gold. If you see the DPC
> for silver cores even on sdm845 is a 100.
> Again these are not estimations but based on actual power measurements.

The scaling to 100 doesn't seem to match how the thermal framework is
using them.  Take a look at of_cpufreq_cooling_register().  It takes
the "dynamic-power-coefficient" and passes it as "capacitance" into
__cpufreq_cooling_register().  That's eventually used to compute
power, which is documented in the code to be in mW.

power = (u64)capacitance * freq_mhz * voltage_mv * voltage_mv;
do_div(power, 1000000000);

/* power is stored in mW */
freq_table[i].power = power;

That's used together with "sustainable-power", which is the attribute
that Matthias is trying to set.  That value is documented to be in mW
as well.

...so if the silver cores are always scaled to 100 regardless of how
much power they actually draw then it'll be impossible to actually
think about "sustainable-power" as a mW value.  Presumably we either
need to accept that fact (and ideally document it) or we need to
change the values for silver / gold cores (we could still keep the
relative values the same and just scale them).

-Doug