On Tue, 28 Aug 2018 at 16:42, Punit Agrawal <punit.agrawal@xxxxxxx> wrote: > > 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. 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@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):
