Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 22 Aug 2018 16:32:39 +0200 | Subject | Re: [PATCH] dt-binding: arm/cpus.txt: fix dynamic-power-coefficient unit |
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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 ?
> > > 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 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
Regards, Vincent
> > Thanks, > Punit > > > > > 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):
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