dt-binding: arm/cpus.txt: fix dynamic-power-coefficient unit

Message ID 1534869685-22221-1-git-send-email-vincent.guittot@linaro.org
State Superseded
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  • dt-binding: arm/cpus.txt: fix dynamic-power-coefficient unit
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Commit Message

Vincent Guittot Aug. 21, 2018, 4:41 p.m.
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(-)

-- 
2.7.4

Comments

Vincent Guittot Aug. 22, 2018, 2:32 p.m. | #1
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):

Patch

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):