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| author | Vincent Guittot <vincent.guittot@linaro.org> | 2018-08-29 09:08:38 +0200 |
|---|---|---|
| committer | Rob Herring <robh@kernel.org> | 2018-09-26 18:02:41 -0500 |
| commit | 29e5f9441ba730d71cc0f0eda551940af53d2b77 (patch) | |
| tree | ebacd9c16708691b686f33347c1eeaaf13690e27 /Documentation/devicetree/bindings/arm/cpus.txt | |
| parent | f858927fd6ce394a7f431153d44ad0a09e8f49a1 (diff) | |
| download | linux-29e5f9441ba730d71cc0f0eda551940af53d2b77.tar.bz2 | |
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>
Diffstat (limited to 'Documentation/devicetree/bindings/arm/cpus.txt')
| -rw-r--r-- | Documentation/devicetree/bindings/arm/cpus.txt | 4 |
1 files changed, 2 insertions, 2 deletions
diff --git a/Documentation/devicetree/bindings/arm/cpus.txt b/Documentation/devicetree/bindings/arm/cpus.txt index 96dfccc0faa8..b0198a1cf403 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): |