# [v2] dt-binding: arm/cpus.txt: fix dynamic-power-coefficient unit

Message ID 1535526518-29022-1-git-send-email-vincent.guittot@linaro.org Accepted 29e5f9441ba730d71cc0f0eda551940af53d2b77 show [v2] dt-binding: arm/cpus.txt: fix dynamic-power-coefficient unit | expand

## Commit Message

Vincent Guittot Aug. 29, 2018, 7:08 a.m. UTC
```The unit of dynamic-power-coefficient is described as mW/MHz/uV^2 whereas
its usage in the code assumes that unit is uW/MHz/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
or
power (mW) = (capacitance * freq_mhz * (voltage_mv/1000)^2) / 1000
then
power (mW) = power (uW) / 1000
so
power (uW) = capacitance * freq_mhz * (voltage_mv/1000)^2

Furthermore, if we test basic values like :
voltage_mv = 1000mV = 1V
freq_mhz = 1000Mhz

The minimum possible power, when dynamic-power-coefficient equals 1, will
be with current unit:
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 * 1000 * 1^2 = 1000uW = 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
```

## Patch

```diff --git a/Documentation/devicetree/bindings/arm/cpus.txt b/Documentation/devicetree/bindings/arm/cpus.txt
index 29e1dc5..71d8cd0 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 uW/MHz/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 MHz.

Example 1 (dual-cluster big.LITTLE system 32-bit):

```