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

From: Viresh Kumar <viresh.kumar@linaro.org>
To: sumitg <sumitg@nvidia.com>
Cc: bbasu@nvidia.com, linux-pm@vger.kernel.org,
	catalin.marinas@arm.com, rjw@rjwysocki.net,
	linux-kernel@vger.kernel.org, jonathanh@nvidia.com,
	talho@nvidia.com, thierry.reding@gmail.com,
	linux-tegra@vger.kernel.org, mperttunen@nvidia.com,
	will@kernel.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [TEGRA194_CPUFREQ Patch 2/3] cpufreq: Add Tegra194 cpufreq driver
Date: Thu, 9 Apr 2020 13:14:15 +0530	[thread overview]
Message-ID: <20200409074415.twpzu2n4frqlde7b@vireshk-i7> (raw)
In-Reply-To: <08307e54-0e14-14a3-7d6a-d59e1e04a683@nvidia.com>

On 08-04-20, 16:54, sumitg wrote:
> 
> 
> On 08/04/20 11:23 AM, Viresh Kumar wrote:
> > External email: Use caution opening links or attachments
> > 
> > 
> > On 07-04-20, 23:48, sumitg wrote:
> > > On 06/04/20 8:25 AM, Viresh Kumar wrote:
> > > > 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
> > > > > > > +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);

Initialize the work only once from init routine.

> > > > > > > +     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 ?
> > > > 
> > > Long delay value is used to have the observation window long enough for
> > > correctly reconstructing the CPU frequency considering noise.
> > > In next patch version, changed delay value to 500us which in our tests is
> > > considered reliable.
> > 
> > I understand that you need to put a udelay() while reading the freq from
> > hardware, that is fine, but why do you need a workqueue for that? Why can't you
> > just read the values directly from the same context ?
> > 
> The register to read frequency is per core and not accessible to other
> cores. So, we have to execute the function remotely as the target core to
> read frequency might be different from current.
> The functions for that are smp_call_function_single or queue_work_on.
> We used queue_work_on() to avoid long delay inside ipi interrupt context
> with interrupts disabled.

Okay, I understand this now, finally :)

But if the interrupts are disabled during some call, won't workqueues face the
same problem ?

-- 
viresh

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