linux-kernel - Re: [PATCH v2 1/2] cpufreq: tegra194: get consistent cpuinfo_cur

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Message-ID: <20018a6a-e230-c39c-e801-5a3d0dd4c0ec@nvidia.com>
Date:   Mon, 12 Oct 2020 22:04:50 +0530
From:   Sumit Gupta <sumitg@...dia.com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
CC:     <rjw@...ysocki.net>, <sudeep.holla@....com>,
        <thierry.reding@...il.com>, <jonathanh@...dia.com>,
        <linux-pm@...r.kernel.org>, <linux-tegra@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>, <ksitaraman@...dia.com>,
        <bbasu@...dia.com>, Sumit Gupta <sumitg@...dia.com>
Subject: Re: [PATCH v2 1/2] cpufreq: tegra194: get consistent cpuinfo_cur_freq

>> Frequency returned by 'cpuinfo_cur_freq' using counters is not fixed
>> and keeps changing slightly. This change returns a consistent value
>> from freq_table. If the reconstructed frequency has acceptable delta
>> from the last written value, then return the frequency corresponding
>> to the last written ndiv value from freq_table. Otherwise, print a
>> warning and return the reconstructed freq.
>>
>> Signed-off-by: Sumit Gupta <sumitg@...dia.com>
>> ---
>>   drivers/cpufreq/tegra194-cpufreq.c | 71 +++++++++++++++++++++++++++++++++-----
>>   1 file changed, 62 insertions(+), 9 deletions(-)
>>
>> diff --git a/drivers/cpufreq/tegra194-cpufreq.c b/drivers/cpufreq/tegra194-cpufreq.c
>> index e1d931c..d250e49 100644
>> --- a/drivers/cpufreq/tegra194-cpufreq.c
>> +++ b/drivers/cpufreq/tegra194-cpufreq.c
>> @@ -180,9 +180,70 @@ static unsigned int tegra194_get_speed_common(u32 cpu, u32 delay)
>>        return (rate_mhz * KHZ); /* in KHz */
>>   }
>>
>> +static void get_cpu_ndiv(void *ndiv)
>> +{
>> +     u64 ndiv_val;
>> +
>> +     asm volatile("mrs %0, s3_0_c15_c0_4" : "=r" (ndiv_val) : );
>> +
>> +     *(u64 *)ndiv = ndiv_val;
>> +}
>> +
>> +static void set_cpu_ndiv(void *data)
> 
> You weren't required to do this unnecessary change.
> 
ya, moved the function up to keep both {get_|set_} calls together.

>> +{
>> +     struct cpufreq_frequency_table *tbl = data;
>> +     u64 ndiv_val = (u64)tbl->driver_data;
>> +
>> +     asm volatile("msr s3_0_c15_c0_4, %0" : : "r" (ndiv_val));
>> +}
>> +
>>   static unsigned int tegra194_get_speed(u32 cpu)
>>   {
>> -     return tegra194_get_speed_common(cpu, US_DELAY);
>> +     struct tegra194_cpufreq_data *data = cpufreq_get_driver_data();
>> +     struct cpufreq_frequency_table *pos;
>> +     unsigned int rate;
>> +     u64 ndiv;
>> +     int ret;
>> +     u32 cl;
>> +
>> +     if (!cpu_online(cpu))
> 
> This isn't required. The CPU is guaranteed to be online here.
> 
OK, will remove this in next version.

>> +             return -EINVAL;
>> +
>> +     smp_call_function_single(cpu, get_cpu_cluster, &cl, true);
>> +
>> +     if (cl >= data->num_clusters)
> 
> Is it really possible here ? I meant you must have already checked
> this at cpufreq-init level already. Else mark it unlikely at least.
> 
Ya, will remove the check here.

>> +             return -EINVAL;
>> +
>> +     /* reconstruct actual cpu freq using counters */
>> +     rate = tegra194_get_speed_common(cpu, US_DELAY);
>> +
>> +     /* get last written ndiv value */
>> +     ret = smp_call_function_single(cpu, get_cpu_ndiv, &ndiv, true);
>> +     if (ret) {
> 
> What exactly can fail here ? get_cpu_ndiv() can't fail. Do we really
> need this check ? What about WARN_ON_ONCE() ?
> 
OK.

>> +             pr_err("cpufreq: Failed to get ndiv for CPU%d, ret:%d\n",
>> +                    cpu, ret);
>> +             return rate;
>> +     }
>> +
>> +     /*
>> +      * If the reconstructed frequency has acceptable delta from
>> +      * the last written value, then return freq corresponding
>> +      * to the last written ndiv value from freq_table. This is
>> +      * done to return consistent value.
>> +      */
>> +     cpufreq_for_each_valid_entry(pos, data->tables[cl]) {
>> +             if (pos->driver_data != ndiv)
>> +                     continue;
>> +
>> +             if (abs(pos->frequency - rate) > 115200) {
> 
> where does this 115200 comes from ? Strange that it matches tty's baud
> rate :)
>The value is equal to one freq step size.

> This is 115 MHz, right ? Isn't that too big of a delta ?
> 
The is the acceptable delta used during our testing keeping some margin.

>> +                     pr_warn("cpufreq: cpu%d,cur:%u,set:%u,set ndiv:%llu\n",
>> +                             cpu, rate, pos->frequency, ndiv);
>> +             } else {
>> +                     rate = pos->frequency;
>> +             }
>> +             break;
>> +     }
>> +     return rate;
>>   }
> 
> --
> viresh
>