Re: [PATCH] dt-binding: arm/cpus.txt: fix dynamic-power-coefficient unit
From: Punit Agrawal <hidden>
Date: 2018-08-22 10:11:27
Also in:
linux-devicetree, lkml
Hi Vincent, Thanks for the patch. One comment about the choice of units below. Vincent Guittot [off-list ref] writes:
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.
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 Thanks, Punit
quoted hunk
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):