| Message ID | 1534869685-22221-1-git-send-email-vincent.guittot@linaro.org |
|---|---|
| State | Superseded, archived |
| Headers | show |
| Series | dt-binding: arm/cpus.txt: fix dynamic-power-coefficient unit | expand |
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. > 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 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):
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):
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 / 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@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):
On Tue, 28 Aug 2018 at 16:42, Punit Agrawal <punit.agrawal@arm.com> wrote: > > 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 / 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@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):
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):
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 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 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(-)