Re: [PATCH v6 1/2] ACPI: CPPC: Add ospm_nominal_perf support

[Sumit Gupta <sumitg@nvidia.com>]

Hi Zhongqiu,


On 28/07/26 18:54, Zhongqiu Han wrote:
> External email: Use caution opening links or attachments
>
>
> Hello Sumit,
>
> On 7/18/2026 5:53 AM, Sumit Gupta wrote:
>> Expose the OSPM Nominal Performance register (ACPI 6.6, Section
>> 8.4.6.1.2.6), which conveys the desired nominal performance level
>> at which the platform may run. Unlike the existing read-only
>> Nominal Performance register, it is writable and lets OSPM
>> request a lower nominal level than the platform-reported nominal.
>> The platform classifies performance above this level as boosted
>> and below as throttled for its power/thermal decisions.
>>
>> It is exposed as a per-policy cpufreq sysfs attribute in kHz, to
>> match the cpufreq sysfs unit convention:
>>
>>    /sys/devices/system/cpu/cpuX/cpufreq/ospm_nominal_freq
>>
>> The attribute is documented in
>> Documentation/ABI/testing/sysfs-devices-system-cpu.
>>
>> Writes are converted to perf via cppc_khz_to_perf(), validated
>> against [Lowest Performance, Nominal Performance], and applied to
>> the policy->cpu. The register is assumed shared across the
>> policy->cpus.
>>
>> On read, the current register value is returned, or
>> "<unsupported>" if the platform does not implement the register.
>>
>> Also add the register to the OSPM-set register save/restore
>> table, so its value survives CPU hotplug and reverts to the
>> firmware value on driver unload, like the other registers in
>> the table.
>>
>> Signed-off-by: Sumit Gupta <sumitg@nvidia.com>
>> ---
>>   .../ABI/testing/sysfs-devices-system-cpu      | 26 ++++++++++
>>   drivers/acpi/cppc_acpi.c                      | 32 +++++++++++++
>>   drivers/cpufreq/cppc_cpufreq.c                | 47 +++++++++++++++++++
>>   include/acpi/cppc_acpi.h                      | 10 ++++
>>   4 files changed, 115 insertions(+)
>>
>> diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu 
>> b/Documentation/ABI/testing/sysfs-devices-system-cpu
>> index 82d10d556cc8..a8d592c08823 100644
>> --- a/Documentation/ABI/testing/sysfs-devices-system-cpu
>> +++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
>> @@ -346,6 +346,32 @@ Description:     Performance Limited
>>
>>               This file is only present if the cppc-cpufreq driver is 
>> in use.
>>
>> +What: /sys/devices/system/cpu/cpuX/cpufreq/ospm_nominal_freq
>> +Date:                May 2026
>> +Contact:     linux-pm@vger.kernel.org
>> +Description: OSPM Nominal Performance (kHz)
>> +
>> +             OSPM uses this attribute to request a nominal performance
>> +             level lower than the platform-reported nominal. The
>> +             platform treats performance above this level as boost
>> +             and below as throttle for power and thermal decisions.
>> +
>> +             Read returns the current value in kHz, or "<unsupported>"
>> +             if the platform does not implement the register. Write a
>> +             kHz value in the range [lowest_freq, nominal_freq].
>> +
>> +             Note that tasks may be migrated from one CPU to another
>> +             by the scheduler's load-balancing algorithm, and if
>> +             different OSPM Nominal Performance values are set for
>> +             those CPUs (through different cpufreq policies), that may
>> +             lead to undesirable outcomes. To avoid such issues it is
>> +             better to set the same value across all policies, or to
>> +             pin every task potentially sensitive to it to a specific
>> +             CPU.
>> +
>> +             This file is only present if the cppc-cpufreq driver is
>> +             in use.
>> +
>>   What: /sys/devices/system/cpu/cpu*/cache/index3/cache_disable_{0,1}
>>   Date:               August 2008
>>   KernelVersion:      2.6.27
>> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
>> index a7fec6c93178..681d4fd40c11 100644
>> --- a/drivers/acpi/cppc_acpi.c
>> +++ b/drivers/acpi/cppc_acpi.c
>> @@ -1685,6 +1685,38 @@ int cppc_set_epp(int cpu, u64 epp_val)
>>   }
>>   EXPORT_SYMBOL_GPL(cppc_set_epp);
>>
>> +/**
>> + * cppc_set_ospm_nominal_perf() - Write OSPM Nominal Performance 
>> register.
>> + * @cpu: CPU on which to write register.
>> + * @ospm_nominal_perf: Value to write to the OSPM Nominal 
>> Performance register.
>> + *
>> + * OSPM Nominal Performance conveys the desired nominal performance 
>> level
>> + * at which the platform may run. Per ACPI 6.6, s8.4.6.1.2.6, the value
>> + * must lie within [Lowest Performance, Nominal Performance] and may be
>> + * set independently of Minimum, Maximum and Desired performance. The
>> + * caller is responsible for validating the range.
>> + *
>> + * Return: 0 on success or negative error code.
>> + */
>> +int cppc_set_ospm_nominal_perf(int cpu, u64 ospm_nominal_perf)
>> +{
>> +     return cppc_set_reg_val(cpu, OSPM_NOMINAL_PERF, 
>> ospm_nominal_perf);
>> +}
>> +EXPORT_SYMBOL_GPL(cppc_set_ospm_nominal_perf);
>> +
>> +/**
>> + * cppc_get_ospm_nominal_perf() - Read OSPM Nominal Performance 
>> register.
>> + * @cpu: CPU from which to read register.
>> + * @ospm_nominal_perf: Pointer to store the OSPM Nominal Performance 
>> value.
>> + *
>> + * Return: 0 on success or negative error code.
>> + */
>> +int cppc_get_ospm_nominal_perf(int cpu, u64 *ospm_nominal_perf)
>> +{
>> +     return cppc_get_reg_val(cpu, OSPM_NOMINAL_PERF, 
>> ospm_nominal_perf);
>> +}
>> +EXPORT_SYMBOL_GPL(cppc_get_ospm_nominal_perf);
>> +
>>   /**
>>    * cppc_get_auto_act_window() - Read autonomous activity window 
>> register.
>>    * @cpu: CPU from which to read register.
>> diff --git a/drivers/cpufreq/cppc_cpufreq.c 
>> b/drivers/cpufreq/cppc_cpufreq.c
>> index 9c88512d635c..eb6746810fa6 100644
>> --- a/drivers/cpufreq/cppc_cpufreq.c
>> +++ b/drivers/cpufreq/cppc_cpufreq.c
>> @@ -37,6 +37,7 @@ enum cppc_saved_reg_id {
>>       CPPC_SAVED_AUTO_SEL,
>>       CPPC_SAVED_EPP,
>>       CPPC_SAVED_AUTO_ACT_WINDOW,
>> +     CPPC_SAVED_OSPM_NOMINAL_PERF,
>>       CPPC_NR_SAVED_REGS,
>>   };
>>
>> @@ -55,6 +56,9 @@ static const struct cppc_saved_reg 
>> cppc_saved_regs[CPPC_NR_SAVED_REGS] = {
>>       [CPPC_SAVED_AUTO_ACT_WINDOW] = {
>>               cppc_get_auto_act_window, cppc_set_auto_act_window,
>>       },
>> +     [CPPC_SAVED_OSPM_NOMINAL_PERF] = {
>> +             cppc_get_ospm_nominal_perf, cppc_set_ospm_nominal_perf,
>> +     },
>>   };
>>
>>   /*
>> @@ -1166,11 +1170,53 @@ static int cppc_get_perf_limited_filtered(int 
>> cpu, u64 *perf_limited)
>>   CPPC_CPUFREQ_ATTR_RW_U64(perf_limited, cppc_get_perf_limited_filtered,
>>                        cppc_set_perf_limited)
>>
>> +static ssize_t show_ospm_nominal_freq(struct cpufreq_policy *policy, 
>> char *buf)
>> +{
>> +     struct cppc_cpudata *cpu_data = policy->driver_data;
>> +     u64 perf;
>> +     int ret;
>> +
>> +     ret = cppc_get_ospm_nominal_perf(policy->cpu, &perf);
>> +     if (ret == -EOPNOTSUPP)
>> +             return sysfs_emit(buf, "<unsupported>\n");
>> +     if (ret)
>> +             return ret;
>> +
>> +     return sysfs_emit(buf, "%u\n",
>> + cppc_perf_to_khz(&cpu_data->perf_caps, perf));
>
> In patch 2/2, cppc_cpufreq_get_effective_nominal() reads the OSPM
> Nominal into ospm_nominal via the output pointer of
> cppc_get_ospm_nominal_perf(policy->cpu, &ospm_nominal), then checks
> ospm_nominal for zero ("A zero value means OSPM has not selected a
> nominal level") and substitutes the platform Nominal in that case.
>
> However, show_ospm_nominal_freq() passes the value read through the same
> output pointer straight into cppc_perf_to_khz() without the equivalent
> zero check. This looks a bit inconsistent — is it intentional, or
> should show() apply the same "zero means not selected" handling?
>

Thanks for pointing this out.
In v7, I will make ospm_nominal_freq as write-only and remove sysfs
show() and cppc_get_ospm_nominal_perf(), eliminating this inconsistency.
The effective nominal calculation will use the last successfully
written value, falling back to the platform Nominal Performance if none
has been written.

Thanks,
Sumit


>
>> +}
>> +
>> +static ssize_t store_ospm_nominal_freq(struct cpufreq_policy *policy,
>> +                                    const char *buf, size_t count)
>> +{
>> +     struct cppc_cpudata *cpu_data = policy->driver_data;
>> +     unsigned int freq_khz;
>> +     u32 perf;
>> +     int ret;
>> +
>> +     ret = kstrtouint(buf, 0, &freq_khz);
>> +     if (ret)
>> +             return ret;
>> +
>> +     perf = cppc_khz_to_perf(&cpu_data->perf_caps, freq_khz);
>> +     if (perf < cpu_data->perf_caps.lowest_perf ||
>> +         perf > cpu_data->perf_caps.nominal_perf)
>> +             return -EINVAL;
>> +
>> +     /* The register is assumed shared across the policy's CPUs. */
>> +     ret = cppc_set_ospm_nominal_perf(policy->cpu, perf);
>> +     if (ret)
>> +             return ret;
>> +
>> +     return 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(perf_limited);
>> +cpufreq_freq_attr_rw(ospm_nominal_freq);
>>
>>   static struct freq_attr *cppc_cpufreq_attr[] = {
>>       &freqdomain_cpus,
>> @@ -1178,6 +1224,7 @@ static struct freq_attr *cppc_cpufreq_attr[] = {
>>       &auto_act_window,
>>       &energy_performance_preference_val,
>>       &perf_limited,
>> +     &ospm_nominal_freq,
>>       NULL,
>>   };
>>
>> diff --git a/include/acpi/cppc_acpi.h b/include/acpi/cppc_acpi.h
>> index cd07e1e92bf4..4c9e59643cc0 100644
>> --- a/include/acpi/cppc_acpi.h
>> +++ b/include/acpi/cppc_acpi.h
>> @@ -180,6 +180,8 @@ extern int cpc_write_ffh(int cpunum, struct 
>> cpc_reg *reg, u64 val);
>>   extern int cppc_get_epp_perf(int cpunum, u64 *epp_perf);
>>   extern int cppc_set_epp_perf(int cpu, struct cppc_perf_ctrls 
>> *perf_ctrls, bool enable);
>>   extern int cppc_set_epp(int cpu, u64 epp_val);
>> +extern int cppc_set_ospm_nominal_perf(int cpu, u64 ospm_nominal_perf);
>> +extern int cppc_get_ospm_nominal_perf(int cpu, u64 *ospm_nominal_perf);
>>   extern int cppc_get_auto_act_window(int cpu, u64 *auto_act_window);
>>   extern int cppc_set_auto_act_window(int cpu, u64 auto_act_window);
>>   extern int cppc_get_auto_sel(int cpu, bool *enable);
>> @@ -268,6 +270,14 @@ static inline int cppc_set_epp(int cpu, u64 
>> epp_val)
>>   {
>>       return -EOPNOTSUPP;
>>   }
>> +static inline int cppc_set_ospm_nominal_perf(int cpu, u64 
>> ospm_nominal_perf)
>> +{
>> +     return -EOPNOTSUPP;
>> +}
>> +static inline int cppc_get_ospm_nominal_perf(int cpu, u64 
>> *ospm_nominal_perf)
>> +{
>> +     return -EOPNOTSUPP;
>> +}
>>   static inline int cppc_get_auto_act_window(int cpu, u64 
>> *auto_act_window)
>>   {
>>       return -EOPNOTSUPP;
>
>
> -- 
> Thx and BRs,
> Zhongqiu Han