From a314d524ac6513312a06b56f09fd703ebed15e63 Mon Sep 17 00:00:00 2001 From: Vincent Guittot <vincent.guittot@linaro.org> Date: Wed, 29 Aug 2018 09:08:38 +0200 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 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> Acked-by: Punit Agrawal <punit.agrawal@arm.com> Acked-by: Viresh Kumar <viresh.kumar@linaro.org> Signed-off-by: Rob Herring <robh@kernel.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 96dfccc0faa8d..b0198a1cf4032 100644 --- a/Documentation/devicetree/bindings/arm/cpus.txt +++ b/Documentation/devicetree/bindings/arm/cpus.txt @@ -276,7 +276,7 @@ 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. @@ -287,7 +287,7 @@ 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): -- 2.39.5