* [PATCH v3] cpufreq: CPPC: Fix performance/frequency conversion
@ 2022-02-08 8:01 Pierre Gondois
2022-02-10 5:27 ` Viresh Kumar
0 siblings, 1 reply; 2+ messages in thread
From: Pierre Gondois @ 2022-02-08 8:01 UTC (permalink / raw)
To: linux-kernel
Cc: Ionela.Voinescu, Lukasz.Luba, Morten.Rasmussen, Pierre Gondois,
Rafael J. Wysocki, Viresh Kumar, linux-pm
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:
1- the functions are not inverse for some values:
(perf_to_khz(khz_to_perf(x)) != x)
2- cppc_cpufreq_perf_to_khz(LOWEST_PERF) can sometimes give
a different value from LOWEST_FREQ due to integer approximation
3- it is implied that performance and frequency are proportional
(NOMINAL_FREQ / NOMINAL_PERF) == (LOWEST_PERF / LOWEST_FREQ)
This patch changes the conversion functions to an affine function.
This fixes the 3 points above.
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 | 43 +++++++++++++++++-----------------
1 file changed, 21 insertions(+), 22 deletions(-)
diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
index d4c27022b9c9..e0ff09d66c96 100644
--- a/drivers/cpufreq/cppc_cpufreq.c
+++ b/drivers/cpufreq/cppc_cpufreq.c
@@ -303,52 +303,48 @@ 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
+ * use them to convert perf to freq and vice versa. The conversion is
+ * extrapolated as an affine function passing by the 2 points:
+ * - (Low perf, Low freq)
+ * - (Nominal perf, Nominal perf)
*/
static unsigned int cppc_cpufreq_perf_to_khz(struct cppc_cpudata *cpu_data,
unsigned int perf)
{
struct cppc_perf_caps *caps = &cpu_data->perf_caps;
+ s64 retval, offset = 0;
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;
- }
+ mul = caps->nominal_freq - caps->lowest_freq;
+ div = caps->nominal_perf - caps->lowest_perf;
+ offset = caps->nominal_freq - div64_u64(caps->nominal_perf * mul, div);
} else {
if (!max_khz)
max_khz = cppc_get_dmi_max_khz();
mul = max_khz;
div = caps->highest_perf;
}
- return (u64)perf * mul / div;
+
+ retval = offset + div64_u64(perf * mul, div);
+ if (retval >= 0)
+ return retval;
+ return 0;
}
static unsigned int cppc_cpufreq_khz_to_perf(struct cppc_cpudata *cpu_data,
unsigned int freq)
{
struct cppc_perf_caps *caps = &cpu_data->perf_caps;
+ s64 retval, offset = 0;
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;
- }
+ mul = caps->nominal_perf - caps->lowest_perf;
+ div = caps->nominal_freq - caps->lowest_freq;
+ offset = caps->nominal_perf - div64_u64(caps->nominal_freq * mul, div);
} else {
if (!max_khz)
max_khz = cppc_get_dmi_max_khz();
@@ -356,7 +352,10 @@ static unsigned int cppc_cpufreq_khz_to_perf(struct cppc_cpudata *cpu_data,
div = max_khz;
}
- return (u64)freq * mul / div;
+ retval = offset + div64_u64(freq * mul, div);
+ if (retval >= 0)
+ return retval;
+ return 0;
}
static int cppc_cpufreq_set_target(struct cpufreq_policy *policy,
--
2.25.1
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH v3] cpufreq: CPPC: Fix performance/frequency conversion
2022-02-08 8:01 [PATCH v3] cpufreq: CPPC: Fix performance/frequency conversion Pierre Gondois
@ 2022-02-10 5:27 ` Viresh Kumar
0 siblings, 0 replies; 2+ messages in thread
From: Viresh Kumar @ 2022-02-10 5:27 UTC (permalink / raw)
To: Pierre Gondois
Cc: linux-kernel, Ionela.Voinescu, Lukasz.Luba, Morten.Rasmussen,
Rafael J. Wysocki, linux-pm
On 08-02-22, 09:01, Pierre Gondois 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:
> 1- the functions are not inverse for some values:
> (perf_to_khz(khz_to_perf(x)) != x)
> 2- cppc_cpufreq_perf_to_khz(LOWEST_PERF) can sometimes give
> a different value from LOWEST_FREQ due to integer approximation
> 3- it is implied that performance and frequency are proportional
> (NOMINAL_FREQ / NOMINAL_PERF) == (LOWEST_PERF / LOWEST_FREQ)
>
> This patch changes the conversion functions to an affine function.
> This fixes the 3 points above.
>
> 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 | 43 +++++++++++++++++-----------------
> 1 file changed, 21 insertions(+), 22 deletions(-)
Applied. Thanks.
--
viresh
^ permalink raw reply [flat|nested] 2+ messages in thread
end of thread, other threads:[~2022-02-10 5:27 UTC | newest]
Thread overview: 2+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-02-08 8:01 [PATCH v3] cpufreq: CPPC: Fix performance/frequency conversion Pierre Gondois
2022-02-10 5:27 ` Viresh Kumar
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.