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>
---
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;
+
+	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

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
>