linux-kernel - [PATCH] dt-binding: arm/cpus.txt: fix dynamic-power-coefficient unit

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Date:   Tue, 21 Aug 2018 18:41:25 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     robh+dt@...nel.org, mark.rutland@....com,
        devicetree@...r.kernel.org, punit.agrawal@....com
Cc:     edubezval@...il.com, viresh.kumar@...aro.org,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH] dt-binding: arm/cpus.txt: fix dynamic-power-coefficient unit

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@...aro.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):
 
-- 
2.7.4