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

[luoxueqin]

在 2026/5/29 23:33, srinivas pandruvada 写道:
> 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
Thanks Srinivas.

The intent is not to fix any issue in intel_pstate. It already
implements the fallback correctly today.

My goal was to provide a common helper encapsulating the ACPI
CPPC semantics, with intel_pstate serving as the first user.
That said, I agree the benefit may not justify a new exported
helper at this point.
> > Signed-off-by: Xueqin Luo <luoxueqin@xxxxxxxxxx>
> > ---
> >   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;