Re: [PATCH 2/2] arm64: topology: read CPPC FFH feedback counters in one operation

On 6/29/2026 11:27 PM, Sumit Gupta wrote:
>
> On 10/04/26 15:11, Pengjie Zhang wrote:
>> External email: Use caution opening links or attachments
>>
>>
>> arm64 implements CPPC FFH feedback-counter reads using AMU counters.
>> Because those counters must be sampled on the target CPU, reading the
>> delivered and reference counters separately widens the observation 
>> window
>> between them.
>>
>> Implement the paired FFH feedback-counter read hook on arm64 and sample
>> both AMU counters together before decoding the requested CPC register
>> values.
>>
>> Also factor the FFH bitfield extraction logic into a helper and reuse
>> it from the existing single-counter FFH read path.
>>
>> Signed-off-by: Pengjie Zhang <[email protected]>
>> ---
>>   arch/arm64/kernel/topology.c | 75 ++++++++++++++++++++++++++++++++----
>>   1 file changed, 67 insertions(+), 8 deletions(-)
>>
>> diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
>> index b32f13358fbb..b90a767b2a1f 100644
>> --- a/arch/arm64/kernel/topology.c
>> +++ b/arch/arm64/kernel/topology.c
>> @@ -50,6 +50,16 @@ struct amu_cntr_sample {
>>          unsigned long   last_scale_update;
>>   };
>>
>> +struct amu_ffh_ctrs {
>> +       u64 corecnt;
>> +       u64 constcnt;
>> +};
>> +
>> +enum cpc_ffh_ctr_id {
>> +       CPC_FFH_CTR_CORE  = 0x0,
>> +       CPC_FFH_CTR_CONST = 0x1,
>> +};
>> +
>>   static DEFINE_PER_CPU_SHARED_ALIGNED(struct amu_cntr_sample, 
>> cpu_amu_samples);
>>
>>   void update_freq_counters_refs(void)
>> @@ -397,7 +407,7 @@ static void cpu_read_constcnt(void *val)
>>   }
>>
>>   static inline
>> -int counters_read_on_cpu(int cpu, smp_call_func_t func, u64 *val)
>> +int counters_read_on_cpu(int cpu, smp_call_func_t func, void *val)
>>   {
>>          /*
>>           * Abort call on counterless CPU.
>> @@ -447,24 +457,73 @@ bool cpc_ffh_supported(void)
>>          return true;
>>   }
>>
>> +static void amu_read_core_const_ctrs(void *val)
>> +{
>> +       struct amu_ffh_ctrs *ctrs = val;
>> +
>> +       cpu_read_constcnt(&ctrs->constcnt);
>> +       cpu_read_corecnt(&ctrs->corecnt);
>> +}
>
> Any reason to flip the order?
> Harmless as they are read back to back, but better to add a comment
> if it's intentional.
>
> Thanks,
> Sumit
> ....
>
Hi Sumit，

Thanks for taking the time to review and test.
The cpu_read_constcnt() function includes the conditional check
this_cpu_has_cap(ARM64_WORKAROUND_2457168), which incurs a latency of
6–12 nanoseconds on the our platform.
If cpu_read_corecnt() is called prior to cpu_read_constcnt(), it will
widen the sampling interval between the corecnt and constcnt counter
readings. To address this, we have adjusted the call order:
cpu_read_constcnt() is executed first, followed by cpu_read_corecnt().
I will add comments later.

Thanks,
   Pengjie


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