Re: [PATCH v3] cpufreq: CPPC: mask Desired_Excursion when autonomous selection is enabled

[Pierre Gondois]

Hello Xueqin,

Thanks for the update,

Reviewed-by: Pierre Gondois <pierre.gondois@xxxxxxx>


On 5/6/26 12:18, luoxueqin wrote:
> From: Xueqin Luo <luoxueqin@xxxxxxxxxx>
>
> According to the ACPI 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@xxxxxxxxxx>
> ---
> v2->v3:
> - Use cached auto_sel from cpufreq driver data
> - Fall back to raw value when policy is unavailable
> - Keep sysfs ABI unchanged by using a wrapper around cppc_get_perf_limited()
> ---
>   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..a439d9621eed 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;
> +	bool auto_sel_enabled = false;
> +	int ret;
> +
> +	policy = cpufreq_cpu_get_raw(cpu);
> +	if (policy && policy->driver_data) {
> +		cpu_data = policy->driver_data;
> +		auto_sel_enabled = cpu_data->perf_ctrls.auto_sel;
> +	}
> +
> +	ret = cppc_get_perf_limited(cpu, perf_limited);
> +	if (ret)
> +		return ret;
> +
> +	/*
> +	 * Desired_Excursion is ignored when Autonomous Selection is enabled.
> +	 * Mask it to avoid exposing misleading values to userspace.
> +	 */
> +	if (auto_sel_enabled)
> +		*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);