linux-kernel - Re: [PATCH v4 6/7] cpufreq: Support for fast frequency switching

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Date:	Wed, 16 Mar 2016 17:58:18 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Juri Lelli <juri.lelli@....com>,
	Steve Muckle <steve.muckle@...aro.org>,
	ACPI Devel Maling List <linux-acpi@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Michael Turquette <mturquette@...libre.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH v4 6/7] cpufreq: Support for fast frequency switching

On Wed, Mar 16, 2016 at 4:43 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, Mar 16, 2016 at 03:52:28PM +0100, Rafael J. Wysocki wrote:
>> +void cpufreq_enable_fast_switch(struct cpufreq_policy *policy)
>> +{
>> +     mutex_lock(&cpufreq_fast_switch_lock);
>> +     if (policy->fast_switch_possible && cpufreq_fast_switch_count >= 0) {
>> +             cpufreq_fast_switch_count++;
>> +             policy->fast_switch_enabled = true;
>> +     } else {
>> +             pr_warn("cpufreq: CPU%u: Fast freqnency switching not enabled\n",
>> +                     policy->cpu);
>
> This happens because there's transition notifiers, right? Would it make
> sense to iterate the notifier here and print the notifier function
> symbol for each? That way we've got a clue as to where to start looking
> when this happens.

OK

>> +     }
>> +     mutex_unlock(&cpufreq_fast_switch_lock);
>> +}
>
>> @@ -1653,8 +1703,18 @@ int cpufreq_register_notifier(struct not
>>
>>       switch (list) {
>>       case CPUFREQ_TRANSITION_NOTIFIER:
>> +             mutex_lock(&cpufreq_fast_switch_lock);
>> +
>> +             if (cpufreq_fast_switch_count > 0) {
>> +                     mutex_unlock(&cpufreq_fast_switch_lock);
>
> So while theoretically (it has a return code)
> cpufreq_register_notifier() could fail, it never actually did. Now we
> do. Do we want to add a WARN here?

Like if (WARN_ON(cpufreq_fast_switch_count > 0)) {

That can be done. :-)