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

[Sumit Gupta <sumitg@nvidia.com>]

On 28/07/26 02:53, Christian Loehle wrote:
> External email: Use caution opening links or attachments
>
>
> On 7/27/26 18:49, Christian Loehle wrote:
>> On 7/27/26 15:01, Christian Loehle wrote:
>>> On 7/17/26 22:53, 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);
>>> It's a write-only register, we need to track everything in the driver.
>>>
>> So just reread Pierre's comments, TBH I don't see the point of ever
>> reading it, even for sysfs reads, but I don't think
>> reading it for cppc_cpufreq_get_effective_nominal() would be valid
>> in any case?
> FWIW desired_perf was read/write up until ACPI 6.5 and only 6.6 marked
> it as write-only. I don't think we should be reading it either on
> platforms advertising 6.6.
> I will clarify and send a patch.

Good catch that ACPI 6.6 makes Desired Performance write-only,
and thanks for the patches preventing those reads.

The same reasoning applies to OSPM Nominal Performance, so I will treat
it as write-only: make ospm_nominal_freq write-only, remove show(), and
drop cppc_get_ospm_nominal_perf().
Boost and policy limits will use the last value the driver successfully
wrote, falling back to the platform reported Nominal Performance when
nothing has been set.

On unload, I will reset it to the platform Nominal Performance only if
the driver wrote it during its lifetime. Otherwise the register is left
untouched, since the pre-driver value cannot be read back.

Thanks,
Sumit