Vincent Guittot [off-list ref] writes:

Hi Amit,

On Wed, 22 Aug 2018 at 12:11, Punit Agrawal [off-list ref] wrote:

quoted

Hi Vincent,

Thanks for the patch. One comment about the choice of units below.

Vincent Guittot [off-list ref] writes:

quoted

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.

quoted

quoted

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.


[...]

quoted

quoted

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):