linux-kernel - Re: [TEGRA194_CPUFREQ Patch 2/3] cpufreq: Add Tegra194 cpufreq driver

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Date:   Mon, 6 Apr 2020 08:25:49 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     sumitg <sumitg@...dia.com>
Cc:     rjw@...ysocki.net, catalin.marinas@....com, will@...nel.org,
        thierry.reding@...il.com, jonathanh@...dia.com, talho@...dia.com,
        linux-pm@...r.kernel.org, linux-tegra@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        bbasu@...dia.com, mperttunen@...dia.com
Subject: Re: [TEGRA194_CPUFREQ Patch 2/3] cpufreq: Add Tegra194 cpufreq driver

On 05-04-20, 00:08, sumitg wrote:
> 
> 
> On 26/03/20 5:20 PM, Viresh Kumar wrote:
> > On 03-12-19, 23:02, Sumit Gupta wrote:
> > > diff --git a/drivers/cpufreq/tegra194-cpufreq.c b/drivers/cpufreq/tegra194-cpufreq.c
> > > +enum cluster {
> > > +     CLUSTER0,
> > > +     CLUSTER1,
> > > +     CLUSTER2,
> > > +     CLUSTER3,
> > 
> > All these have same CPUs ? Or big little kind of stuff ? How come they
> > have different frequency tables ?
> > 
> T194 SOC has homogeneous architecture where each cluster has two symmetric
> Carmel cores and and not big little. LUT's are per cluster and all LUT's
> have same values currently. Future SOC's may have different LUT values per
> cluster.

LUT ?

> > > +     MAX_CLUSTERS,
> > > +};

> > > +static unsigned int tegra194_get_speed_common(u32 cpu, u32 delay)
> > > +{
> > > +     struct read_counters_work read_counters_work;
> > > +     struct tegra_cpu_ctr c;
> > > +     u32 delta_refcnt;
> > > +     u32 delta_ccnt;
> > > +     u32 rate_mhz;
> > > +
> > > +     read_counters_work.c.cpu = cpu;
> > > +     read_counters_work.c.delay = delay;
> > > +     INIT_WORK_ONSTACK(&read_counters_work.work, tegra_read_counters);
> > > +     queue_work_on(cpu, read_counters_wq, &read_counters_work.work);
> > > +     flush_work(&read_counters_work.work);
> > 
> > Why can't this be done in current context ?
> > 
> We used work queue instead of smp_call_function_single() to have long delay.

Please explain completely, you have raised more questions than you
answered :)

Why do you want to have long delays ?

> > > +static int tegra194_cpufreq_init(struct cpufreq_policy *policy)
> > > +{
> > > +     struct tegra194_cpufreq_data *data = cpufreq_get_driver_data();
> > > +     int cluster = get_cpu_cluster(policy->cpu);
> > > +
> > > +     if (cluster >= data->num_clusters)
> > > +             return -EINVAL;
> > > +
> > > +     policy->cur = tegra194_fast_get_speed(policy->cpu); /* boot freq */
> > > +
> > > +     /* set same policy for all cpus */
> > > +     cpumask_copy(policy->cpus, cpu_possible_mask);
> > 
> > You are copying cpu_possible_mask mask here, and so this routine will
> > get called only once.
> > 
> > I still don't understand the logic behind clusters and frequency
> > tables.
> > 
> Currently, we use same policy for all CPU's to maximize throughput. Will add
> separate patch later to set policy as per cluster. But we are not using that
> in T194 due to perf reasons.

You can't misrepresent the hardware this way, sorry.

Again few questions, I understand that you have multiple clusters
here:

- All clusters will always have the frequency table ?
- All clusters are capable of having a different frequency at any
  point of time ? Or they will always run at same freq ?

> > > +     freqs.old = policy->cur;
> > > +     freqs.new = tbl->frequency;
> > > +
> > > +     cpufreq_freq_transition_begin(policy, &freqs);
> > > +     on_each_cpu_mask(policy->cpus, set_cpu_ndiv, tbl, true);
> > 
> > When CPUs share clock line, why is this required for every CPU ?
> > Per core NVFREQ_REQ system register is written to make frequency
> requests for the core. Cluster h/w then forwards the max(core0, core1)
> request to cluster NAFLL.

You mean that each cluster can set frequency independently ?

If all the clusters can run at independent frequencies but you still
want them to run at same frequency, then that can be done with a
single set of governor tunables, which is the default anyway. So this
should just work for you.

I will not be reviewing the v2 version you sent for now as that is
most likely wrong anyway.

-- 
viresh