Re: [PATCH v4 5/8] ACPI: CPPC: add APIs and sysfs interface for perf_limited register

[Pierre Gondois <pierre.gondois@arm.com>]

On 11/5/25 12:38, Sumit Gupta wrote:
> Add sysfs interface to read/write the Performance Limited register.
>
> The Performance Limited register indicates to the OS that an
> unpredictable event (like thermal throttling) has limited processor
> performance. This register is sticky and remains set until reset or
> OS clears it by writing 0.
>
> The interface is exposed as:
>   /sys/devices/system/cpu/cpuX/cpufreq/perf_limited
>
> Signed-off-by: Sumit Gupta<sumitg@nvidia.com>
> ---
>   drivers/acpi/cppc_acpi.c       | 26 ++++++++++++++++++++++++++
>   drivers/cpufreq/cppc_cpufreq.c | 12 ++++++++++++
>   include/acpi/cppc_acpi.h       | 10 ++++++++++
>   3 files changed, 48 insertions(+)
>
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index ef53eb8a1feb..9b8da3ef06db 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -1810,6 +1810,32 @@ int cppc_set_max_perf(int cpu, u64 max_perf)
>   }
>   EXPORT_SYMBOL_GPL(cppc_set_max_perf);
>   
> +/**
> + * cppc_get_perf_limited - Get the Performance Limited register value.
> + * @cpu: CPU from which to get Performance Limited register.
> + * @perf_limited: Pointer to store the Performance Limited value.
> + *
> + * Return: 0 for success, -EIO on register access failure, -EOPNOTSUPP if not supported.
> + */
> +int cppc_get_perf_limited(int cpu, u64 *perf_limited)
> +{
> +	return cppc_get_reg_val(cpu, PERF_LIMITED, perf_limited);
> +}
> +EXPORT_SYMBOL_GPL(cppc_get_perf_limited);
> +
> +/**
> + * cppc_set_perf_limited() - Write the Performance Limited register.
> + * @cpu: CPU on which to write register.
> + * @perf_limited: Value to write to the perf_limited register.
> + *
> + * Return: 0 for success, -EIO on register access failure, -EOPNOTSUPP if not supported.
> + */
> +int cppc_set_perf_limited(int cpu, u64 perf_limited)
> +{
> +	return cppc_set_reg_val(cpu, PERF_LIMITED, perf_limited);
> +}

There are currently only 2 bits used:
- 0 Desired_Excursion
- 1 Minimum_Excursion
It might be worth defining these bits and mask the values when trying to 
set the register.

------

Also NIT:

The spec. says:
" All accesses to the Performance Limited Register must be made using 
interlocked operations, by both accessing entities."

I am not sure I understand which synchronization issues are faced.
It's just to report the comment from the spec.


> +EXPORT_SYMBOL_GPL(cppc_set_perf_limited);
> +
>   /**
>    * cppc_get_perf - Get a CPU's performance controls.
>    * @cpu: CPU for which to get performance controls.
> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
> index cde6202e9c51..a425ad575aa6 100644
> --- a/drivers/cpufreq/cppc_cpufreq.c
> +++ b/drivers/cpufreq/cppc_cpufreq.c
> @@ -1043,12 +1043,23 @@ static ssize_t store_max_perf(struct cpufreq_policy *policy, const char *buf, si
>   	return count;
>   }
>   
> +static ssize_t show_perf_limited(struct cpufreq_policy *policy, char *buf)
> +{
> +	return cppc_cpufreq_sysfs_show_u64(policy->cpu, cppc_get_perf_limited, buf);
> +}
> +
> +static ssize_t store_perf_limited(struct cpufreq_policy *policy, const char *buf, size_t count)
> +{
> +	return cppc_cpufreq_sysfs_store_u64(policy->cpu, cppc_set_perf_limited, buf, count);
> +}
> +
>   cpufreq_freq_attr_ro(freqdomain_cpus);
>   cpufreq_freq_attr_rw(auto_select);
>   cpufreq_freq_attr_rw(auto_act_window);
>   cpufreq_freq_attr_rw(energy_performance_preference_val);
>   cpufreq_freq_attr_rw(min_perf);
>   cpufreq_freq_attr_rw(max_perf);
> +cpufreq_freq_attr_rw(perf_limited);
>   
>   static struct freq_attr *cppc_cpufreq_attr[] = {
>   	&freqdomain_cpus,
> @@ -1057,6 +1068,7 @@ static struct freq_attr *cppc_cpufreq_attr[] = {
>   	&energy_performance_preference_val,
>   	&min_perf,
>   	&max_perf,
> +	&perf_limited,
>   	NULL,
>   };
>   
> diff --git a/include/acpi/cppc_acpi.h b/include/acpi/cppc_acpi.h
> index be7de1222eee..8baff46f2ac7 100644
> --- a/include/acpi/cppc_acpi.h
> +++ b/include/acpi/cppc_acpi.h
> @@ -177,6 +177,8 @@ extern int cppc_get_min_perf(int cpu, u64 *min_perf);
>   extern int cppc_set_min_perf(int cpu, u64 min_perf);
>   extern int cppc_get_max_perf(int cpu, u64 *max_perf);
>   extern int cppc_set_max_perf(int cpu, u64 max_perf);
> +extern int cppc_get_perf_limited(int cpu, u64 *perf_limited);
> +extern int cppc_set_perf_limited(int cpu, u64 perf_limited);
>   extern int amd_get_highest_perf(unsigned int cpu, u32 *highest_perf);
>   extern int amd_get_boost_ratio_numerator(unsigned int cpu, u64 *numerator);
>   extern int amd_detect_prefcore(bool *detected);
> @@ -285,6 +287,14 @@ static inline int cppc_set_max_perf(int cpu, u64 max_perf)
>   {
>   	return -EOPNOTSUPP;
>   }
> +static inline int cppc_get_perf_limited(int cpu, u64 *perf_limited)
> +{
> +	return -EOPNOTSUPP;
> +}
> +static inline int cppc_set_perf_limited(int cpu, u64 perf_limited)
> +{
> +	return -EOPNOTSUPP;
> +}
>   static inline int amd_get_highest_perf(unsigned int cpu, u32 *highest_perf)
>   {
>   	return -ENODEV;