• Vincent Guittot's avatar
    dt-binding: arm/cpus.txt: fix dynamic-power-coefficient unit · 29e5f944
    Vincent Guittot authored
    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: default avatarVincent Guittot <vincent.guittot@linaro.org>
    Acked-by: default avatarPunit Agrawal <punit.agrawal@arm.com>
    Acked-by: default avatarViresh Kumar <viresh.kumar@linaro.org>
    Signed-off-by: default avatarRob Herring <robh@kernel.org>
    29e5f944
cpus.txt 11.9 KB