Re: [PATCH v4] cpufreq: cppc: mask Desired_Excursion when autonomous selection is enabled

[Sumit Gupta <sumitg@nvidia.com>]

On 13/05/26 10:12, Xueqin Luo wrote:
> External email: Use caution opening links or attachments
>
>
> According to the ACPI 6.6 specification, the Desired_Excursion field is not
> utilized when autonomous selection is enabled. In this mode, the bit is
> architecturally ignored and does not carry meaningful information.
>
> Currently, the kernel exposes the raw Performance Limited register
> value to userspace through the cpufreq sysfs interface. This may lead to
> misinterpretation, as userspace may assume Desired_Excursion is valid
> even when autonomous selection is active.
>
> To provide a stable and semantically correct ABI, mask out the
> Desired_Excursion bit when autonomous selection is enabled, so that
> userspace does not observe undefined or misleading values.
>
> Writes are left unchanged, as the field is architecturally ignored in
> this mode and write attempts are harmless.
>
> Signed-off-by: Xueqin Luo <luoxueqin@kylinos.cn>
> Reviewed-by: Pierre Gondois <pierre.gondois@arm.com>

Reviewed-by: Sumit Gupta <sumitg@nvidia.com>>, with a nit


> ---
> v3 -> v4:
>   - Address Sumit Gupta's review comments by reordering the
>     error check and simplifying auto_sel handling in the
>     perf_limited getter
> ---
>   drivers/cpufreq/cppc_cpufreq.c | 29 ++++++++++++++++++++++++++++-
>   1 file changed, 28 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
> index 7e7f9dfb7a24..d2f86df06129 100644
> --- a/drivers/cpufreq/cppc_cpufreq.c
> +++ b/drivers/cpufreq/cppc_cpufreq.c
> @@ -982,7 +982,34 @@ store_energy_performance_preference_val(struct cpufreq_policy *policy,
>          return count;
>   }
>
> -CPPC_CPUFREQ_ATTR_RW_U64(perf_limited, cppc_get_perf_limited,
> +static int cppc_get_perf_limited_filtered(int cpu, u64 *perf_limited)
> +{
> +       struct cpufreq_policy *policy;
> +       struct cppc_cpudata *cpu_data;
> +       int ret;
> +
> +       ret = cppc_get_perf_limited(cpu, perf_limited);
> +       if (ret)
> +               return ret;
> +
> +       policy = cpufreq_cpu_get_raw(cpu);
> +       if (!policy)
> +               return 0;

return -EINVAL ?

Thank you,
Sumit Gupta


> +
> +       cpu_data = policy->driver_data;
> +
> +       /*
> +        * Desired Excursion is ignored when autonomous selection is
> +        * enabled. Clear the bit to avoid exposing meaningless state
> +        * to userspace.
> +        */
> +       if (cpu_data && cpu_data->perf_ctrls.auto_sel)
> +               *perf_limited &= ~CPPC_PERF_LIMITED_DESIRED_EXCURSION;
> +
> +       return 0;
> +}
> +
> +CPPC_CPUFREQ_ATTR_RW_U64(perf_limited, cppc_get_perf_limited_filtered,
>                           cppc_set_perf_limited)
>
>   cpufreq_freq_attr_ro(freqdomain_cpus);
> --
> 2.43.0
>