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Date:   Thu, 26 Oct 2023 10:24:54 +0800
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Subject: Re: [PATCH 2/3] cpufreq: CPPC: Keep the target core awake when
 reading its cpufreq rate
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To:     Sudeep Holla <sudeep.holla@arm.com>
CC:     <broonie@kernel.org>, <joey.gouly@arm.com>, <will@kernel.org>,
        <amit.kachhap@arm.com>, <rafael@kernel.org>,
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References: <20231025093847.3740104-1-zengheng4@huawei.com>
 <20231025093847.3740104-3-zengheng4@huawei.com>
 <20231025111301.ng5eaeaixfs3jjpg@bogus>
From:   Zeng Heng <zengheng4@huawei.com>
In-Reply-To: <20231025111301.ng5eaeaixfs3jjpg@bogus>
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在 2023/10/25 19:13, Sudeep Holla 写道:
> On Wed, Oct 25, 2023 at 05:38:46PM +0800, Zeng Heng wrote:
>> As ARM AMU's document says, all counters are subject to any changes
>> in clock frequency, including clock stopping caused by the WFI and WFE
>> instructions.
>>
>> Therefore, using smp_call_on_cpu() to trigger target CPU to
>> read self's AMU counters, which ensures the counters are working
>> properly while cstate feature is enabled.
>>
>> Reported-by: Sumit Gupta <sumitg@nvidia.com>
>> Link: https://lore.kernel.org/all/20230418113459.12860-7-sumitg@nvidia.com/
>> Signed-off-by: Zeng Heng <zengheng4@huawei.com>
>> ---
>>   drivers/cpufreq/cppc_cpufreq.c | 39 ++++++++++++++++++++++++++--------
>>   1 file changed, 30 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/cpufreq/cppc_cpufreq.c b/drivers/cpufreq/cppc_cpufreq.c
>> index fe08ca419b3d..321a9dc9484d 100644
>> --- a/drivers/cpufreq/cppc_cpufreq.c
>> +++ b/drivers/cpufreq/cppc_cpufreq.c
> [...]
>
>> @@ -850,18 +871,18 @@ static unsigned int cppc_cpufreq_get_rate(unsigned int cpu)
>>   
>>   	cpufreq_cpu_put(policy);
>>   
>> -	ret = cppc_get_perf_ctrs(cpu, &fb_ctrs_t0);
>> -	if (ret)
>> -		return 0;
>> -
>> -	udelay(2); /* 2usec delay between sampling */
>> +	if (cpu_has_amu_feat(cpu))
> Have you compiled this on x86 ? Even if you have somehow managed to,
> this is not the right place to check the presence of AMU feature on
> the CPU.
> If AMU registers are used in CPPC, they must be using FFH GAS, in which
> case the interpretation of FFH is architecture dependent code.


According to drivers/cpufreq/Makefile, cppc_cpufreq.c is only compiled with

ARM architecture.

But here, I would change cpu_has_amu_feat() with cpc_ffh_supported(), which

belongs to FFH APIs.

Thanks for the suggestion.


Thanks again,

Zeng Heng


> --
> Regards,
> Sudeep
>