linux-kernel - Re: [PATCH] arm64: dts: qcom: sc7180: Add 'sustainable

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Date:   Wed, 2 Sep 2020 11:06:06 +0530
From:   Rajendra Nayak <rnayak@...eaurora.org>
To:     Doug Anderson <dianders@...omium.org>,
        Matthias Kaehlcke <mka@...omium.org>
Cc:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "open list:OPEN FIRMWARE AND FLATTENED DEVICE TREE BINDINGS" 
        <devicetree@...r.kernel.org>,
        linux-arm-msm <linux-arm-msm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Amit Kucheria <amit.kucheria@...aro.org>,
        Sai Prakash Ranjan <saiprakash.ranjan@...eaurora.org>
Subject: Re: [PATCH] arm64: dts: qcom: sc7180: Add 'sustainable_power' for CPU
 thermal zones


> * 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.


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