Re: [PATCH v2 1/4] PM: EM: convert power field to micro-Watts precision and align drivers

[Lukasz Luba]

On 7/7/22 11:53, Viresh Kumar wrote:
> On 07-07-22, 08:15, Lukasz Luba wrote:
> > The milli-Watts precision causes rounding errors while calculating
> > efficiency cost for each OPP. This is especially visible in the 'simple'
> > Energy Model (EM), where the power for each OPP is provided from OPP
> > framework. This can cause some OPPs to be marked inefficient, while
> > using micro-Watts precision that might not happen.
> >
> > Update all EM users which access 'power' field and assume the value is
> > in milli-Watts.
> >
> > Solve also an issue with potential overflow in calculation of energy
> > estimation on 32bit machine. It's needed now since the power value
> > (thus the 'cost' as well) are higher.
> >
> > Example calculation which shows the rounding error and impact:
> >
> > power = 'dyn-power-coeff' * volt_mV * volt_mV * freq_MHz
> >
> > power_a_uW = (100 * 600mW * 600mW * 500MHz) / 10^6 = 18000
> > power_a_mW = (100 * 600mW * 600mW * 500MHz) / 10^9 = 18
> >
> > power_b_uW = (100 * 605mW * 605mW * 600MHz) / 10^6 = 21961
> > power_b_mW = (100 * 605mW * 605mW * 600MHz) / 10^9 = 21
> >
> > max_freq = 2000MHz
> >
> > cost_a_mW = 18 * 2000MHz/500MHz = 72
> > cost_a_uW = 18000 * 2000MHz/500MHz = 72000
> >
> > cost_b_mW = 21 * 2000MHz/600MHz = 70 // <- artificially better
> > cost_b_uW = 21961 * 2000MHz/600MHz = 73203
> >
> > The 'cost_b_mW' (which is based on old milli-Watts) is misleadingly
> > better that the 'cost_b_uW' (this patch uses micro-Watts) and such
> > would have impact on the 'inefficient OPPs' information in the Cpufreq
> > framework. This patch set removes the rounding issue.
> >
> > Signed-off-by: Lukasz Luba <lukasz.luba@xxxxxxx>
> > ---
> >   drivers/cpufreq/mediatek-cpufreq-hw.c |  7 ++--
> >   drivers/cpufreq/scmi-cpufreq.c        |  6 +++
> >   drivers/opp/of.c                      | 15 ++++----
> >   drivers/powercap/dtpm_cpu.c           |  5 +--
> >   drivers/thermal/cpufreq_cooling.c     | 13 ++++++-
> >   drivers/thermal/devfreq_cooling.c     | 19 ++++++++--
> >   include/linux/energy_model.h          | 54 +++++++++++++++++++--------
> >   kernel/power/energy_model.c           | 24 ++++++++----
> >   8 files changed, 100 insertions(+), 43 deletions(-)
>
> Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>

Thank you Viresh for the ACKs!