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

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Date:   Thu, 18 May 2023 09:44:48 +0800
From:   Zeng Heng <zengheng4@...wei.com>
To:     Sumit Gupta <sumitg@...dia.com>,
        Pierre Gondois <pierre.gondois@....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


在 2023/5/17 23:03, Sumit Gupta 写道:
>
>
> 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
>
I vote solution [1] and it should be applied to all ARM SoCs with AMU.

Zeng Heng

>>
>> 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);
>>>   }
>>>