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

[Beata Michalska <beata.michalska@arm.com>]

Gonna be a bit picky with wording so do bear with me ...

On Fri, Apr 10, 2026 at 05:41:44PM +0800, Pengjie Zhang 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.
> 
> Signed-off-by: Pengjie Zhang <zhangpengjie2@huawei.com>
> ---
>  drivers/acpi/cppc_acpi.c | 58 ++++++++++++++++++++++++++++++++++++----
>  include/acpi/cppc_acpi.h |  7 +++++
>  2 files changed, 60 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/acpi/cppc_acpi.c b/drivers/acpi/cppc_acpi.c
> index 2e91c5a97761..7b3e8b0597dc 100644
> --- a/drivers/acpi/cppc_acpi.c
> +++ b/drivers/acpi/cppc_acpi.c
> @@ -988,6 +988,23 @@ 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:	CPU number to read
This reads bit awkward. Target CPU maybe ?
> + * @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 for success and error code
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
> @@ -1504,6 +1521,40 @@ bool cppc_perf_ctrs_in_pcc(void)
>  }
>  EXPORT_SYMBOL_GPL(cppc_perf_ctrs_in_pcc);
>  
> +static int cppc_read_perf_fb_ctrs(int cpunum,
> +				  struct cpc_register_resource *delivered_reg,
> +				  struct cpc_register_resource *reference_reg,
> +				  u64 *delivered, u64 *reference)
The signature here differs from cpc_read_ffh_fb_ctrs.
It's not an issue but it might be good idea to stay consistent maybe ?
Also ... was about to suggest to stick to either perf_ctrs or feedback_ctrs
in naming but it seems the is no clear pattern  withint the file
...
> +{
> +	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)
> +			return 0;
> +
> +		if (ret != -EOPNOTSUPP)
> +			return ret;
Shouldn't this one be enough ? Don't think you need the first condition.
> +	}
> +
> +	ret = cpc_read(cpunum, delivered_reg, delivered);
> +	if (ret)
> +		return ret;
> +
> +	ret = cpc_read(cpunum, reference_reg, reference);
> +	if (ret)
> +		return ret;
As you are not doing anything with 'ret' this could just be:
	ret = cpc_read(cpunum, delivered_reg, delivered);
	if (ret) return ret;
	return cpc_read(cpunum, reference_reg, reference);

Though that's minor.

---
BR
Beata
> +
> +	return 0;
> +}
> +
>  /**
>   * cppc_get_perf_ctrs - Read a CPU's performance feedback counters.
>   * @cpunum: CPU from which to read counters.
> @@ -1547,11 +1598,8 @@ 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_perf_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 d1f02ceec4f9..006b42dbbd4b 100644
> --- a/include/acpi/cppc_acpi.h
> +++ b/include/acpi/cppc_acpi.h
> @@ -172,6 +172,8 @@ 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);
> @@ -246,6 +248,11 @@ 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
>