linux-kernel - Re: [PATCH] dt-binding: arm/cpus.txt: fix dynamic-power-coefficient unit

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Date:   Wed, 29 Aug 2018 08:48:29 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Punit Agrawal <punit.agrawal@....com>
Cc:     Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org, Eduardo Valentin <edubezval@...il.com>,
        viresh kumar <viresh.kumar@...aro.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "open list:THERMAL" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH] dt-binding: arm/cpus.txt: fix dynamic-power-coefficient unit

On Tue, 28 Aug 2018 at 16:42, Punit Agrawal <punit.agrawal@....com> wrote:
>
> Vincent Guittot <vincent.guittot@...aro.org> writes:
>
> > Hi Amit,
> >
> > On Wed, 22 Aug 2018 at 12:11, Punit Agrawal <punit.agrawal@....com> wrote:
> >>
> >> Hi Vincent,
> >>
> >> Thanks for the patch. One comment about the choice of units below.
> >>
> >> Vincent Guittot <vincent.guittot@...aro.org> writes:
> >>
> >> > The unit of dynamic-power-coefficient is described as mW/MHz/uV^2 whereas
> >> > its usage in the code assumes that unit is mW/GHz/V^2
> >>
> >> Instead of choosing GHz as the base, I'd prefer to use uW/MHz/V^2. It'll
> >> avoid introducing fractional GHz value for frequency calculations.
> >
> > I don't understand your concern about fractional Ghz value for
> > frequency calculation ?
> > I mean, why it's a problem for frequency with Ghz vs Mhz but not a
> > problem for voltage with V vs mV ?
> > Don't we have the same "problem" in both case ?
>
> You're right. It's the same problem in both cases.
>
> >>
> >> > In drivers/thermal/cpu_cooling.c, the code is :
> >> >
> >> > power = (u64)capacitance * freq_mhz * voltage_mv * voltage_mv;
> >> > do_div(power, 1000000000);
> >> >
> >> > which can be summarized as :
> >> > power (mW) = capacitance * freq_mhz/1000 * (voltage_mv/1000)^2
> >>
> >> Which would then translate to -
> >>
> >> power (mW) = power (uW) / 1000 = capacitance * freq_mhz * (voltage_mv/1000)^2
> >
> > Not sure that the equation above is correct. If we consider uW/MHz/V^2
> > for the unit, the equation becomes :
> > power (mW) = power (uW) / 1000 = capacitance * freq_mhz *
> > (voltage_mv/1000)^2 / 1000
>
> Yes, I missed the "/ 1000" at the end.
>
> > which can be rearranged as
> > power (mW) = power (uW) / 1000 = capacitance * freq_mhz/ 1000 *
> > (voltage_mv/1000)^2
> >
> > TBH, I don't really mind between  mW/GHz/V^2 or uW/MHz/V^2 as they are
> > the same at the end
> > but I don't catch your reasoning
>
> The problem I was thinking of doesn't hold as it's the same issue with
> voltage.
>
> One benefit to go with uW/MHz/V^2 might be the extra resolution that it
> provides. I'd prefer to go with uW/MHz/V^2 if there's no compelling
> reason to go with anything else.

Ok, I'm going to update the patch with uW/MHz/V^2

Thanks
>
>
> [...]
>
> >> >
> >> > Furthermore, if we test basic values like :
> >> > voltage_mv = 1000mV = 1V
> >> > freq_mhz = 1000Mhz = 1Ghz
> >> >
> >> > The minimum possible power, when dynamic-power-coefficient equals 1, will
> >> > be :
> >> > min power = 1 * 1000  * (1000000)^2 = 10^15 mW
> >> > which is not realistic
> >> >
> >> > With the unit used by the code, the min power is
> >> > min power =  1 * 1 * 1^2 = 1mW which is far more realistic
> >> >
> >> > Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> >> > ---
> >> >  Documentation/devicetree/bindings/arm/cpus.txt | 4 ++--
> >> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >> >
> >> > diff --git a/Documentation/devicetree/bindings/arm/cpus.txt b/Documentation/devicetree/bindings/arm/cpus.txt
> >> > index 29e1dc5..0148d7d 100644
> >> > --- a/Documentation/devicetree/bindings/arm/cpus.txt
> >> > +++ b/Documentation/devicetree/bindings/arm/cpus.txt
> >> > @@ -274,7 +274,7 @@ described below.
> >> >               Usage: optional
> >> >               Value type: <prop-encoded-array>
> >> >               Definition: A u32 value that represents the running time dynamic
> >> > -                         power coefficient in units of mW/MHz/uV^2. The
> >> > +                         power coefficient in units of mW/GHz/V^2. The
> >> >                           coefficient can either be calculated from power
> >> >                           measurements or derived by analysis.
> >> >
> >> > @@ -285,7 +285,7 @@ described below.
> >> >
> >> >                           Pdyn = dynamic-power-coefficient * V^2 * f
> >> >
> >> > -                         where voltage is in uV, frequency is in MHz.
> >> > +                         where voltage is in V, frequency is in GHz.
> >> >
> >> >  Example 1 (dual-cluster big.LITTLE system 32-bit):