Re: [PATCH v2 1/2] ACPI: CPPC: add paired FFH feedback-counter read hook

[Rafael J. Wysocki (Intel)]

On Wed, Jul 8, 2026 at 10:28 AM Pengjie Zhang <zhangpengjie2@xxxxxxxxxx> wrote:
>
> cppc_get_perf_ctrs() reads the delivered and reference performance
> counters one at a time.
>
> Allow architectures to provide both FFH feedback counters in one
> operation when that either narrows the sampling window or avoids extra
> cross-CPU reads. Add a small FFH-specific hook for that case and fall
> back to the existing per-register reads when unsupported.
>
> Tested-by: Sumit Gupta <sumitg@xxxxxxxxxx>
> Reviewed-by: Sumit Gupta <sumitg@xxxxxxxxxx>
> Tested-by: Vanshidhar Konda <vanshikonda@xxxxxxxxxxxxxxxxxxxxxx>
> Reviewed-by: Vanshidhar Konda <vanshikonda@xxxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Pengjie Zhang <zhangpengjie2@xxxxxxxxxx>

This is fine with me, so

Acked-by: Rafael J. Wysocki (Intel) <rafael@xxxxxxxxxx>

and please route it along with the second patch in the series.

Thanks!

> ---
>  drivers/acpi/cppc_acpi.c | 50 ++++++++++++++++++++++++++++++++++++----
>  include/acpi/cppc_acpi.h |  7 ++++++
>  2 files changed, 52 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index 9f572f481241..b6356476a46e 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -1004,6 +1004,22 @@ int __weak cpc_read_ffh(int cpunum, struct cpc_reg *reg, u64 *val)
>         return -ENOTSUPP;
>  }
>
> +/**
> + * cpc_read_ffh_fb_ctrs() - Read FFH feedback counters together
> + * @cpunum:    Target CPU
> + * @reg1:      first CPPC register information
> + * @val1:      place holder for first return value
> + * @reg2:      second CPPC register information
> + * @val2:      place holder for second return value
> + *
> + * Return: 0 on success, error code otherwise
> + */
> +int __weak cpc_read_ffh_fb_ctrs(int cpunum, struct cpc_reg *reg1,
> +                               u64 *val1, struct cpc_reg *reg2, u64 *val2)
> +{
> +       return -EOPNOTSUPP;
> +}
> +
>  /**
>   * cpc_write_ffh() - Write FFH register
>   * @cpunum:    CPU number to write
> @@ -1496,6 +1512,33 @@ bool cppc_perf_ctrs_in_pcc_cpu(unsigned int cpu)
>  }
>  EXPORT_SYMBOL_GPL(cppc_perf_ctrs_in_pcc_cpu);
>
> +static int cppc_read_fb_ctrs(int cpunum,
> +                            struct cpc_register_resource *delivered_reg,
> +                            struct cpc_register_resource *reference_reg,
> +                            u64 *delivered, u64 *reference)
> +{
> +       int ret;
> +
> +       /*
> +        * For FFH feedback counters, try a paired read first to reduce
> +        * sampling skew between delivered and reference counters. Fall
> +        * back to the existing per-register reads if unsupported.
> +        */
> +       if (CPC_IN_FFH(delivered_reg) && CPC_IN_FFH(reference_reg)) {
> +               ret = cpc_read_ffh_fb_ctrs(cpunum,
> +                                       &delivered_reg->cpc_entry.reg, delivered,
> +                                       &reference_reg->cpc_entry.reg, reference);
> +               if (ret != -EOPNOTSUPP)
> +                       return ret;
> +       }
> +
> +       ret = cpc_read(cpunum, delivered_reg, delivered);
> +       if (ret)
> +               return ret;
> +
> +       return cpc_read(cpunum, reference_reg, reference);
> +}
> +
>  /**
>   * cppc_perf_ctrs_in_pcc - Check if any perf counters are in a PCC region.
>   *
> @@ -1561,11 +1604,8 @@ int cppc_get_perf_ctrs(int cpunum, struct cppc_perf_fb_ctrs *perf_fb_ctrs)
>                 }
>         }
>
> -       ret = cpc_read(cpunum, delivered_reg, &delivered);
> -       if (ret)
> -               goto out_err;
> -
> -       ret = cpc_read(cpunum, reference_reg, &reference);
> +       ret = cppc_read_fb_ctrs(cpunum, delivered_reg, reference_reg,
> +                               &delivered, &reference);
>         if (ret)
>                 goto out_err;
>
> diff --git a/include/acpi/cppc_acpi.h b/include/acpi/cppc_acpi.h
> index 8693890a7275..5acebe62feac 100644
> --- a/include/acpi/cppc_acpi.h
> +++ b/include/acpi/cppc_acpi.h
> @@ -176,6 +176,8 @@ extern int cppc_get_transition_latency(int cpu);
>  extern bool cpc_ffh_supported(void);
>  extern bool cpc_supported_by_cpu(void);
>  extern int cpc_read_ffh(int cpunum, struct cpc_reg *reg, u64 *val);
> +extern int cpc_read_ffh_fb_ctrs(int cpu, struct cpc_reg *reg1, u64 *val1,
> +                               struct cpc_reg *reg2, u64 *val2);
>  extern int cpc_write_ffh(int cpunum, struct cpc_reg *reg, u64 val);
>  extern int cppc_get_epp_perf(int cpunum, u64 *epp_perf);
>  extern int cppc_set_epp_perf(int cpu, struct cppc_perf_ctrls *perf_ctrls, bool enable);
> @@ -250,6 +252,11 @@ static inline int cpc_read_ffh(int cpunum, struct cpc_reg *reg, u64 *val)
>  {
>         return -EOPNOTSUPP;
>  }
> +static inline int cpc_read_ffh_fb_ctrs(int cpu, struct cpc_reg *reg1, u64 *val1,
> +                                      struct cpc_reg *reg2, u64 *val2)
> +{
> +       return -EOPNOTSUPP;
> +}
>  static inline int cpc_write_ffh(int cpunum, struct cpc_reg *reg, u64 val)
>  {
>         return -EOPNOTSUPP;
> --
> 2.33.0
>