Re: [PATCH] ACPI: CPPC: Add helper to get effective guaranteed performance

On Fri, 2026-05-29 at 16:18 +0800, Xueqin Luo wrote:
> According to the ACPI CPPC specification, if the Guaranteed
> Performance register is not implemented, OSPM assumes the
> guaranteed performance is equal to nominal performance.
> 
> Add cppc_get_effective_guaranteed_perf() helper to provide a
> spec-compliant effective guaranteed performance value while
> preserving the raw values returned by cppc_get_perf_caps().
> 
Hi Luo,

Are you trying to fix any issue where you have issue with
"base_frequency" display? intel_pstate will revert to native MSR, which
is always present when we are using CPPC.

I worry about some bad nominal perf in some platforms as we will be
dependent on OEM BIOS.

Thanks,
Srinivas


> Signed-off-by: Xueqin Luo <[email protected]>
> ---
>  drivers/acpi/cppc_acpi.c       | 26 ++++++++++++++++++++++++++
>  drivers/cpufreq/intel_pstate.c | 10 ++++------
>  include/acpi/cppc_acpi.h       |  5 +++++
>  3 files changed, 35 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index f370be8715ae..6dc11030707a 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -1328,6 +1328,32 @@ int cppc_get_highest_perf(int cpunum, u64
> *highest_perf)
>  }
>  EXPORT_SYMBOL_GPL(cppc_get_highest_perf);
>  
> +/**
> + * cppc_get_effective_guaranteed_perf - get effective guaranteed
> performance
> + * @cpunum: CPU number
> + * @guaranteed_perf: Effective guaranteed performance value
> + *
> + * The ACPI CPPC specification states that if the Guaranteed
> + * Performance register is not implemented, OSPM assumes the
> + * guaranteed performance is equal to nominal performance.
> + *
> + * Return: 0 on success, negative errno on failure.
> + */
> +int cppc_get_effective_guaranteed_perf(int cpunum, u64
> *guaranteed_perf)
> +{
> +	struct cppc_perf_caps perf_caps;
> +	int ret;
> +
> +	ret = cppc_get_perf_caps(cpunum, &perf_caps);
> +	if (ret)
> +		return ret;
> +	*guaranteed_perf = perf_caps.guaranteed_perf ?:
> +			   perf_caps.nominal_perf;
> +
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(cppc_get_effective_guaranteed_perf);
> +
>  /**
>   * cppc_get_epp_perf - Get the epp register value.
>   * @cpunum: CPU from which to get epp preference value.
> diff --git a/drivers/cpufreq/intel_pstate.c
> b/drivers/cpufreq/intel_pstate.c
> index 8dc22a65e025..8934e48f71a2 100644
> --- a/drivers/cpufreq/intel_pstate.c
> +++ b/drivers/cpufreq/intel_pstate.c
> @@ -404,17 +404,15 @@ static void intel_pstate_set_itmt_prio(int cpu)
>  
>  static int intel_pstate_get_cppc_guaranteed(int cpu)
>  {
> -	struct cppc_perf_caps cppc_perf;
> +	u64 guaranteed_perf;
>  	int ret;
>  
> -	ret = cppc_get_perf_caps(cpu, &cppc_perf);
> +	ret = cppc_get_effective_guaranteed_perf(cpu,
> &guaranteed_perf);
> +
>  	if (ret)
>  		return ret;
>  
> -	if (cppc_perf.guaranteed_perf)
> -		return cppc_perf.guaranteed_perf;
> -
> -	return cppc_perf.nominal_perf;
> +	return guaranteed_perf;
>  }
>  
>  static int intel_pstate_cppc_get_scaling(int cpu)
> diff --git a/include/acpi/cppc_acpi.h b/include/acpi/cppc_acpi.h
> index d1f02ceec4f9..c02f3bc1fc1a 100644
> --- a/include/acpi/cppc_acpi.h
> +++ b/include/acpi/cppc_acpi.h
> @@ -155,6 +155,7 @@ struct cppc_cpudata {
>  extern int cppc_get_desired_perf(int cpunum, u64 *desired_perf);
>  extern int cppc_get_nominal_perf(int cpunum, u64 *nominal_perf);
>  extern int cppc_get_highest_perf(int cpunum, u64 *highest_perf);
> +extern int cppc_get_effective_guaranteed_perf(int cpunum, u64
> *guaranteed_perf);
>  extern int cppc_get_perf_ctrs(int cpu, struct cppc_perf_fb_ctrs
> *perf_fb_ctrs);
>  extern int cppc_get_perf(int cpu, struct cppc_perf_ctrls
> *perf_ctrls);
>  extern int cppc_set_perf(int cpu, struct cppc_perf_ctrls
> *perf_ctrls);
> @@ -198,6 +199,10 @@ static inline int cppc_get_highest_perf(int
> cpunum, u64 *highest_perf)
>  {
>  	return -EOPNOTSUPP;
>  }
> +static inline int cppc_get_effective_guaranteed_perf(int cpunum, u64
> *guaranteed_perf)
> +{
> +	return -EOPNOTSUPP;
> +}
>  static inline int cppc_get_perf_ctrs(int cpu, struct
> cppc_perf_fb_ctrs *perf_fb_ctrs)
>  {
>  	return -EOPNOTSUPP;