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

[Pengjie Zhang <zhangpengjie2@huawei.com>]

Hi Beata,

Thanks for taking the time to review this patch.
Please see my inline comments below.

On 6/30/2026 3:37 PM, Beata Michalska wrote:
> 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 ?

Will update the wording as suggested.

>> + */
>> +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
> ...
You're right about the mixed naming.
I will use the _fb_ctrs suffix for all new functions in v2.
>> +{
>> +	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.
right，I will drop the redundant if (!ret) check in v2
>> +	}
>> +
>> +	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.
Good catch, this is much cleaner. I will apply this change.
Thanks again for the constructive feedback!

Best regards,
  Pengjie
>
> ---
> 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
>>