Re: [PATCH] ACPI: RISC-V: CPPC: Implement cpc_read_ffh_fb_ctrs()

On 9/7/2026 9:17 PM, Yicong Yang wrote:
> On 9/7/26 7:29 PM, Jie Zhan wrote:
>>
>>
>> On 9/2/2026 4:34 PM, Yufan Dou wrote:
>>> On RISC-V, each cpc_read_ffh() for a CSR-type FFH register sends a
>>> separate IPI to the target hart via smp_call_function_single().
>>> cppc_get_perf_ctrs() therefore samples the delivered and reference
>>> counters in two separate IPIs, at two different instants. The skew
>>> between the two sampling points distorts the delivered/reference
>>> ratio and thus the frequency reported by cpufreq.
>>>
>>> Implement the cpc_read_ffh_fb_ctrs() hook to read both CSR counters
>>> back-to-back in a single IPI, which narrows the sampling window to a
>>> few instructions and halves the number of cross-CPU calls. This
>>> improves the accuracy of the reported frequency in the same way as
>>> the arm64 implementation of the hook, which reads both AMU counters
>>> in a single counters_read_on_cpu() call.
>>>
>>> SBI-type FFH registers still fall back to individual reads, since
>>> pairing them would not reduce the number of SBI calls on the target
>>> hart.
>>>
>>> Testing was performed while CPU1 was kept busy with:
>>>
>>>   # stress-ng --cpu 1 --taskset 1
>>>
>>> On a CPU with cpuinfo_max_freq of 3000000 kHz:
>>>
>>> Before:
>>>
>>>   Maximum observed cpuinfo_cur_freq: 3201369 kHz
>>>   Maximum observed error:            +201369 kHz (+6.71%)
>>>
>>> After:
>>>
>>>   Maximum observed cpuinfo_cur_freq: 3009646 kHz
>>>   Maximum observed error:              +9646 kHz (+0.32%)
>>>
>>> The maximum observed error is reduced by 95.2%.
>> Glad to see that works for RISC-V as well.
>>>
>>> Signed-off-by: Yufan Dou <[email protected]>
>>> ---
>>>  drivers/acpi/riscv/cppc.c | 47 +++++++++++++++++++++++++++++++++++++++
>>>  1 file changed, 47 insertions(+)
>>>
>>> diff --git a/drivers/acpi/riscv/cppc.c b/drivers/acpi/riscv/cppc.c
>>> index 42c1a9052470..2ca98fa69e9a 100644
>>> --- a/drivers/acpi/riscv/cppc.c
>>> +++ b/drivers/acpi/riscv/cppc.c
>>> @@ -85,6 +85,20 @@ static void cppc_ffh_csr_write(void *write_data)
>>>  	data->ret.error = -EINVAL;
>>>  }
>>>  
>>> +struct sbi_cppc_fb_ctrs_data {
>>> +	struct sbi_cppc_data first;
>>> +	struct sbi_cppc_data second;
>>> +};
>>> +
>>> +static void cppc_ffh_csr_read_fb_ctrs(void *read_data)
>>> +{
>>> +	struct sbi_cppc_fb_ctrs_data *data = read_data;
>>> +
>>> +	cppc_ffh_csr_read(&data->first);
>>> +	if (!data->first.ret.error)
>>> +		cppc_ffh_csr_read(&data->second);
>>> +}
>>> +
>>>  /*
>>>   * Refer to drivers/acpi/cppc_acpi.c for the description of the functions
>>>   * below.
>>> @@ -125,6 +139,39 @@ int cpc_read_ffh(int cpu, struct cpc_reg *reg, u64 *val)
>>>  	return -EINVAL;
>>>  }
>>>  
>>> +int cpc_read_ffh_fb_ctrs(int cpu, struct cpc_reg *reg1, u64 *val1,
>>> +			 struct cpc_reg *reg2, u64 *val2)
>>> +{
>>> +	struct sbi_cppc_fb_ctrs_data data;
>>> +	int ret;
>>> +
>>> +	if (WARN_ON_ONCE(irqs_disabled()))
>>> +		return -EPERM;
>> AFAICS, this may cause CPPC FIE to constantly fail to read counters because
>> FIE runs in hardirq context.
> 
> I suppose it's mainly due to the smp_call_function_single() cannot work
> with irqs_disabled().
> 
> seems we need to do the similar handling like arm64's
> counters_read_on_cpu() - read the counters locally when irqs_disabled()
> if local cpu is the target cpu.
Exactly.
> 
> so is cpc_read_ffh() of riscv. but I suppose this should be a fix of below
> commit, as it were in the kthread for FIE...
> 997c021abc6e ("cpufreq: CPPC: Update FIE arch_freq_scale in ticks for non-PCC regs")
> 
The fix on riscv has been posted.
https://lore.kernel.org/all/[email protected]/

cpc_read_ffh_fb_ctrs() and cpc_read_ffh() both call counters_read_on_cpu()
on arm64, so fixing counters_read_on_cpu() is fine.
But the two ABIs are implemented separately here on riscv, so copy the same
fix to cpc_read_ffh_fb_ctrs() or factor out a common process.
>> Any test on CPPC FIE?
>>> +
>>> +	/* Only CSR counters can be paired within a single IPI. */
>>> +	if (FFH_CPPC_TYPE(reg1->address) != FFH_CPPC_CSR ||
>>> +	    FFH_CPPC_TYPE(reg2->address) != FFH_CPPC_CSR)
>>> +		return -EOPNOTSUPP;
>>> +
>>> +	data.first.reg = FFH_CPPC_CSR_NUM(reg1->address);
>>> +	data.second.reg = FFH_CPPC_CSR_NUM(reg2->address);
>>> +
>>> +	ret = smp_call_function_single(cpu, cppc_ffh_csr_read_fb_ctrs,
>>> +				       &data, 1);
>>> +	if (ret)
>>> +		return ret;
>>> +
>>> +	if (data.first.ret.error)
>>> +		return data.first.ret.error;
>>> +	if (data.second.ret.error)
>>> +		return data.second.ret.error;
>>> +
>>> +	*val1 = data.first.ret.value;
>>> +	*val2 = data.second.ret.value;
>>> +
>>> +	return 0;
>>> +}
>>> +
>>>  int cpc_write_ffh(int cpu, struct cpc_reg *reg, u64 val)
>>>  {
>>>  	struct sbi_cppc_data data;
>>>
>>> base-commit: 89a312991dc6e638a36adc43ccb91dbc25504c04