Vincent Guittot <vincent.guittot@xxxxxxxxxx> writes: > Hi Amit, > > On Wed, 22 Aug 2018 at 12:11, Punit Agrawal <punit.agrawal@xxxxxxx> wrote: >> >> Hi Vincent, >> >> Thanks for the patch. One comment about the choice of units below. >> >> Vincent Guittot <vincent.guittot@xxxxxxxxxx> 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. [...] >> > >> > 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@xxxxxxxxxx> >> > --- >> > 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):
