linux-kernel - Re: [PATCH v2 1/2] cpufreq: CPPC: keep target core awake when reading its cpufreq rate

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Message-ID: <cde1d8a9-3a21-e82b-7895-40603a14d898@nvidia.com>
Date:   Wed, 17 May 2023 20:33:30 +0530
From:   Sumit Gupta <sumitg@...dia.com>
To:     Pierre Gondois <pierre.gondois@....com>,
        Zeng Heng <zengheng4@...wei.com>,
        Ionela Voinescu <Ionela.Voinescu@....com>
CC:     <linux-kernel@...r.kernel.org>, <linux-pm@...r.kernel.org>,
        <wangxiongfeng2@...wei.com>, <xiexiuqi@...wei.com>,
        <liwei391@...wei.com>, <linux-acpi@...r.kernel.org>,
        <weiyongjun1@...wei.com>, <lenb@...nel.org>,
        <viresh.kumar@...aro.org>, <rafael@...nel.org>,
        Yang Shi <yang@...amperecomputing.com>
Subject: Re: [PATCH v2 1/2] cpufreq: CPPC: keep target core awake when reading
 its cpufreq rate



On 17/05/23 13:47, Pierre Gondois wrote:
> External email: Use caution opening links or attachments
> 
> 
> +Ionela, Sumit, Yang,
> 
> Hello Zeng,
> 
> I think solutions around related issues were suggested at:
> 
> [1] https://lore.kernel.org/all/20230418113459.12860-7-sumitg@nvidia.com/
> [2] 
> https://lore.kernel.org/all/20230328193846.8757-1-yang@os.amperecomputing.com/
> [3] https://lore.kernel.org/all/ZEl1Fms%2FJmdEZsVn@arm.com/
> 
> About this patch, it seems to mean that CPPC counters of CPUx are always
> accessed from CPUx, even when they are not AMUs. For instance CPPC
> counters could be memory mapped and accessible from any CPU.
> cpu_has_amu_feat() should allow to probe if a CPU uses AMUs or not,
> and [2] had an implementation using it.
> 
> Another comment about PATCH 2/2 is that if the counters are accessed
> through FFH, arm64 version of cpc_read_ffh() is calling
> counters_read_on_cpu(), and a comment in counters_read_on_cpu() seems
> to specify the function must be called with interrupt enabled.
> 
> I think the best solution so far was the one at [3], suggested by Ionela,
> but it doesn't seem to solve your issue. Indeed, it is not checked whether
> the counters are AMU counters and that they must be remotely read (to
> have the CPU awake),
> 
> Regards,
> Pierre
> 

I think the solution in [1] is simple and solves all the three cases.
Also, it provides better accuracy between the set and get frequency as 
compared to [3].

This can be merged and can later still be improved in Upstream.

If OK, I can send new version by changing the patch to apply for all ARM 
SoC's with AMU and not specific to Tegra.

Thank you,
Sumit Gupta

> 
> On 5/16/23 15:32, 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 during calculation.
>>
>> Signed-off-by: Zeng Heng <zengheng4@...wei.com>
>> ---
>>   drivers/cpufreq/cppc_cpufreq.c | 30 +++++++++++++++++++-----------
>>   1 file changed, 19 insertions(+), 11 deletions(-)
>>
>> diff --git a/drivers/cpufreq/cppc_cpufreq.c 
>> b/drivers/cpufreq/cppc_cpufreq.c
>> index 022e3555407c..910167f58bb3 100644
>> --- a/drivers/cpufreq/cppc_cpufreq.c
>> +++ b/drivers/cpufreq/cppc_cpufreq.c
>> @@ -837,9 +837,24 @@ static int cppc_perf_from_fbctrs(struct 
>> cppc_cpudata *cpu_data,
>>       return (reference_perf * delta_delivered) / delta_reference;
>>   }
>>
>> +static int cppc_get_perf_ctrs_smp(void *val)
>> +{
>> +     int cpu = smp_processor_id();
>> +     struct cppc_perf_fb_ctrs *fb_ctrs = val;
>> +     int ret;
>> +
>> +     ret = cppc_get_perf_ctrs(cpu, fb_ctrs);
>> +     if (ret)
>> +             return ret;
>> +
>> +     udelay(2); /* 2usec delay between sampling */
>> +
>> +     return cppc_get_perf_ctrs(cpu, fb_ctrs + 1);
>> +}
>> +
>>   static unsigned int cppc_cpufreq_get_rate(unsigned int cpu)
>>   {
>> -     struct cppc_perf_fb_ctrs fb_ctrs_t0 = {0}, fb_ctrs_t1 = {0};
>> +     struct cppc_perf_fb_ctrs fb_ctrs[2] = {0};
>>       struct cpufreq_policy *policy = cpufreq_cpu_get(cpu);
>>       struct cppc_cpudata *cpu_data = policy->driver_data;
>>       u64 delivered_perf;
>> @@ -847,19 +862,12 @@ 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 ret;
>> -
>> -     udelay(2); /* 2usec delay between sampling */
>> -
>> -     ret = cppc_get_perf_ctrs(cpu, &fb_ctrs_t1);
>> +     ret = smp_call_on_cpu(cpu, cppc_get_perf_ctrs_smp, fb_ctrs, 1);
>>       if (ret)
>>               return ret;
>>
>> -     delivered_perf = cppc_perf_from_fbctrs(cpu_data, &fb_ctrs_t0,
>> -                                            &fb_ctrs_t1);
>> -
>> +     delivered_perf = cppc_perf_from_fbctrs(cpu_data, fb_ctrs,
>> +                                            fb_ctrs + 1);
>>       return cppc_cpufreq_perf_to_khz(cpu_data, delivered_perf);
>>   }
>>