Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Fri, 5 Nov 2021 16:40:24 +0100 | Subject | Re: [PATCH v1] cpufreq: CPPC: Fix performance/frequency conversion |
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On Fri, Oct 22, 2021 at 9:51 AM Pierre Gondois <Pierre.Gondois@arm.com> wrote: > > CPUfreq governors request CPU frequencies using information > on current CPU usage. The CPPC driver converts them to > performance requests. Frequency targets are computed as: > target_freq = (util / cpu_capacity) * max_freq > target_freq is then clamped between [policy->min, policy->max]. > > The CPPC driver converts performance values to frequencies > (and vice-versa) using cppc_cpufreq_perf_to_khz() and > cppc_cpufreq_khz_to_perf(). These functions both use two different > factors depending on the range of the input value. For > cppc_cpufreq_khz_to_perf(): > - (NOMINAL_PERF / NOMINAL_FREQ) or > - (LOWEST_PERF / LOWEST_FREQ) > and for cppc_cpufreq_perf_to_khz(): > - (NOMINAL_FREQ / NOMINAL_PERF) or > - ((NOMINAL_PERF - LOWEST_FREQ) / (NOMINAL_PERF - LOWEST_PERF)) > > This means the functions are not inverse for some values: > (perf_to_khz(khz_to_perf(x)) != x) > > This patch makes use of one single conversion factor, being > (MAX_PERF / MAX_FREQ). > > As LOWEST_FREQ is not used during conversion, the LOWEST_FREQ > advertised through policy->cpuinfo.min_freq might be different > from the LOWEST_FREQ value available in the CPPC object, > but the conversion will be correct.
Well, this assumes that there is a linear perf <-> freq mapping which is a change in behavior.
While I agree that consistency is a good thing in general, won't this cause the values visible via sysfs to change? And if it does, won't it confuse anyone or break anything in user space?
> Suggested-by: Lukasz Luba <lukasz.luba@arm.com> > Suggested-by: Morten Rasmussen <morten.rasmussen@arm.com> > Signed-off-by: Pierre Gondois <Pierre.Gondois@arm.com> > --- > drivers/cpufreq/cppc_cpufreq.c | 33 ++++++++++----------------------- > 1 file changed, 10 insertions(+), 23 deletions(-) > > diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c > index d4c27022b9c9..d2ac74e7701e 100644 > --- a/drivers/cpufreq/cppc_cpufreq.c > +++ b/drivers/cpufreq/cppc_cpufreq.c > @@ -302,13 +302,10 @@ static u64 cppc_get_dmi_max_khz(void) > } > > /* > - * If CPPC lowest_freq and nominal_freq registers are exposed then we can > - * use them to convert perf to freq and vice versa > - * > - * If the perf/freq point lies between Nominal and Lowest, we can treat > - * (Low perf, Low freq) and (Nom Perf, Nom freq) as 2D co-ordinates of a line > - * and extrapolate the rest > - * For perf/freq > Nominal, we use the ratio perf:freq at Nominal for conversion > + * The CPPC driver receives frequency requests and translates them to performance > + * requests. Thus, frequency values are actually performance values on a frequency > + * scale. The conversion is done as: > + * target_freq = target_perf * (nominal_freq / nominal_perf) > */ > static unsigned int cppc_cpufreq_perf_to_khz(struct cppc_cpudata *cpu_data, > unsigned int perf) > @@ -317,14 +314,9 @@ static unsigned int cppc_cpufreq_perf_to_khz(struct cppc_cpudata *cpu_data, > static u64 max_khz; > u64 mul, div; > > - if (caps->lowest_freq && caps->nominal_freq) { > - if (perf >= caps->nominal_perf) { > - mul = caps->nominal_freq; > - div = caps->nominal_perf; > - } else { > - mul = caps->nominal_freq - caps->lowest_freq; > - div = caps->nominal_perf - caps->lowest_perf; > - } > + if (caps->nominal_freq) { > + mul = caps->nominal_freq; > + div = caps->nominal_perf; > } else { > if (!max_khz) > max_khz = cppc_get_dmi_max_khz(); > @@ -341,14 +333,9 @@ static unsigned int cppc_cpufreq_khz_to_perf(struct cppc_cpudata *cpu_data, > static u64 max_khz; > u64 mul, div; > > - if (caps->lowest_freq && caps->nominal_freq) { > - if (freq >= caps->nominal_freq) { > - mul = caps->nominal_perf; > - div = caps->nominal_freq; > - } else { > - mul = caps->lowest_perf; > - div = caps->lowest_freq; > - } > + if (caps->nominal_freq) { > + mul = caps->nominal_perf; > + div = caps->nominal_freq; > } else { > if (!max_khz) > max_khz = cppc_get_dmi_max_khz(); > -- > 2.17.1 >
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