`Vincent Guittot <vincent.guittot@linaro.org> writes:> Hi Amit,>> On Wed, 22 Aug 2018 at 12:11, Punit Agrawal <punit.agrawal@arm.com> wrote:>>>> Hi Vincent,>>>> Thanks for the patch. One comment about the choice of units below.>>>> Vincent Guittot <vincent.guittot@linaro.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 / 1000Yes, 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 reasoningThe problem I was thinking of doesn't hold as it's the same issue withvoltage.One benefit to go with uW/MHz/V^2 might be the extra resolution that itprovides. I'd prefer to go with uW/MHz/V^2 if there's no compellingreason to go with anything else.[...]>> >>> > 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@linaro.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):`