Re: [PATCH v9 1/5] cpufreq: Allow arch_freq_get_on_cpu to return an error

Vanshidhar Konda
2025-01-21T18:41:01Z

On Tue, Jan 21, 2025 at 04:14:32PM +0100, Beata Michalska wrote:
>On Tue, Jan 21, 2025 at 04:17:06PM +0530, Viresh Kumar wrote:
>> On 21-01-25, 08:44, Beata Michalska wrote:
>> > Allow arch_freq_get_on_cpu to return an error for cases when retrieving
>> > current CPU frequency is not possible, whether that being due to lack of
>> > required arch support or due to other circumstances when the current
>> > frequency cannot be determined at given point of time.
>> >
>> > Signed-off-by: Beata Michalska <beata.michalska@arm.com>
>> > ---
>> > arch/x86/kernel/cpu/aperfmperf.c | 2 +-
>> > arch/x86/kernel/cpu/proc.c | 7 +++++--
>> > drivers/cpufreq/cpufreq.c | 8 ++++----
>> > include/linux/cpufreq.h | 2 +-
>> > 4 files changed, 11 insertions(+), 8 deletions(-)
>> >
>> > diff --git a/arch/x86/kernel/cpu/aperfmperf.c b/arch/x86/kernel/cpu/aperfmperf.c
>> > index f642de2ebdac..6cf31a1649c4 100644
>> > --- a/arch/x86/kernel/cpu/aperfmperf.c
>> > +++ b/arch/x86/kernel/cpu/aperfmperf.c
>> > @@ -498,7 +498,7 @@ void arch_scale_freq_tick(void)
>> > */
>> > #define MAX_SAMPLE_AGE ((unsigned long)HZ / 50)
>> >
>> > -unsigned int arch_freq_get_on_cpu(int cpu)
>> > +int arch_freq_get_on_cpu(int cpu)
>> > {
>> > struct aperfmperf *s = per_cpu_ptr(&cpu_samples, cpu);
>> > unsigned int seq, freq;
>> > diff --git a/arch/x86/kernel/cpu/proc.c b/arch/x86/kernel/cpu/proc.c
>> > index 41ed01f46bd9..d79f5845a463 100644
>> > --- a/arch/x86/kernel/cpu/proc.c
>> > +++ b/arch/x86/kernel/cpu/proc.c
>> > @@ -86,9 +86,12 @@ static int show_cpuinfo(struct seq_file *m, void *v)
>> > seq_printf(m, "microcode\t: 0x%x\n", c->microcode);
>> >
>> > if (cpu_has(c, X86_FEATURE_TSC)) {
>> > - unsigned int freq = arch_freq_get_on_cpu(cpu);
>> > + int freq = arch_freq_get_on_cpu(cpu);
>> >
>> > - seq_printf(m, "cpu MHz\t\t: %u.%03u\n", freq / 1000, (freq % 1000));
>> > + if (freq <= 0)
>> > + seq_puts(m, "cpu MHz\t\t: Unknown\n");
>> > + else
>> > + seq_printf(m, "cpu MHz\t\t: %u.%03u\n", freq / 1000, (freq % 1000));
>> > }
>> >
>> > /* Cache size */
>> > diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>> > index 418236fef172..6f45684483c4 100644
>> > --- a/drivers/cpufreq/cpufreq.c
>> > +++ b/drivers/cpufreq/cpufreq.c
>> > @@ -728,18 +728,18 @@ show_one(cpuinfo_transition_latency, cpuinfo.transition_latency);
>> > show_one(scaling_min_freq, min);
>> > show_one(scaling_max_freq, max);
>> >
>> > -__weak unsigned int arch_freq_get_on_cpu(int cpu)
>> > +__weak int arch_freq_get_on_cpu(int cpu)
>> > {
>> > - return 0;
>> > + return -EOPNOTSUPP;
>> > }
>> >
>> > static ssize_t show_scaling_cur_freq(struct cpufreq_policy *policy, char *buf)
>> > {
>> > ssize_t ret;
>> > - unsigned int freq;
>> > + int freq;
>> >
>> > freq = arch_freq_get_on_cpu(policy->cpu);
>> > - if (freq)
>> > + if (freq > 0)
>>
>> >= ?
>>
>> Since we can return error now, 0 should be considered a valid
>> frequency value ?
>Theoretically speaking - it should, though what would 0 actually
>represent then ?
I would think the value of 0 would be valid and should be interpreted in a
product/architecture specific manner? From silicon behavior, to me only negative
frequency values wouldn't make any sense.

In Patch 1 of this series we interpret 0 as "Unknown" on a x86 system though. So
for the sake of consistency should we consider 0 a valid value everywhere?

Regards,
Vanshi

>
>---
>BR
>Beata
>>
>> > ret = sysfs_emit(buf, "%u\n", freq);
>> > else if (cpufreq_driver->setpolicy && cpufreq_driver->get)
>> > ret = sysfs_emit(buf, "%u\n", cpufreq_driver->get(policy->cpu));
>> > diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
>> > index 7fe0981a7e46..02fd4746231d 100644
>> > --- a/include/linux/cpufreq.h
>> > +++ b/include/linux/cpufreq.h
>> > @@ -1184,7 +1184,7 @@ static inline int of_perf_domain_get_sharing_cpumask(int pcpu, const char *list_
>> > }
>> > #endif
>> >
>> > -extern unsigned int arch_freq_get_on_cpu(int cpu);
>> > +extern int arch_freq_get_on_cpu(int cpu);
>> >
>> > #ifndef arch_set_freq_scale
>> > static __always_inline
>> > --
>> > 2.25.1
>>
>> --
>> viresh