Re: [PATCH 5/6] cpufreq: Support for fast frequency switching

From: "Rafael J. Wysocki" <rafael@kernel.org>
To: Viresh Kumar <viresh.kumar@linaro.org>
Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>,
	Linux PM list <linux-pm@vger.kernel.org>,
	Juri Lelli <juri.lelli@arm.com>,
	Steve Muckle <steve.muckle@linaro.org>,
	ACPI Devel Maling List <linux-acpi@vger.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Michael Turquette <mturquette@baylibre.com>
Subject: Re: [PATCH 5/6] cpufreq: Support for fast frequency switching
Date: Fri, 4 Mar 2016 03:15:59 +0100	[thread overview]
Message-ID: <CAJZ5v0inN24WujzEps+oe0gDcdAP8RjyB1Y2+DbBP044RKpOhA@mail.gmail.com> (raw)
In-Reply-To: <20160303060003.GJ2891@vireshk-i7>

On Thu, Mar 3, 2016 at 7:00 AM, Viresh Kumar <viresh.kumar@linaro.org> wrote:
> On 02-03-16, 03:12, Rafael J. Wysocki wrote:
>> Index: linux-pm/drivers/cpufreq/cpufreq.c
>> ===================================================================
>> --- linux-pm.orig/drivers/cpufreq/cpufreq.c
>> +++ linux-pm/drivers/cpufreq/cpufreq.c
>> @@ -1772,6 +1772,39 @@ EXPORT_SYMBOL(cpufreq_unregister_notifie
>>   *                              GOVERNORS                            *
>>   *********************************************************************/
>>
>> +/**
>> + * cpufreq_driver_fast_switch - Carry out a fast CPU frequency switch.
>> + * @policy: cpufreq policy to switch the frequency for.
>> + * @target_freq: New frequency to set (may be approximate).
>> + * @relation: Relation to use for frequency selection.
>> + *
>> + * Carry out a fast frequency switch from interrupt context.
>> + *
>> + * This function must not be called if policy->fast_switch_possible is unset.
>> + *
>> + * Governors calling this function must guarantee that it will never be invoked
>> + * twice in parallel for the same policy and that it will never be called in
>> + * parallel with either ->target() or ->target_index() for the same policy.
>> + *
>> + * If CPUFREQ_ENTRY_INVALID is returned by the driver's ->fast_switch()
>> + * callback, the hardware configuration must be preserved.
>> + */
>> +void cpufreq_driver_fast_switch(struct cpufreq_policy *policy,
>> +                             unsigned int target_freq, unsigned int relation)
>> +{
>> +     unsigned int freq;
>> +
>> +     if (target_freq == policy->cur)
>
> Maybe an unlikely() here ?
>
>> +             return;
>> +
>> +     freq = cpufreq_driver->fast_switch(policy, target_freq, relation);
>> +     if (freq != CPUFREQ_ENTRY_INVALID) {
>> +             policy->cur = freq;
>
> Hmm.. What will happen to the code relying on the cpufreq-notifiers
> now ?

It will have a problem.

For that code it's like the CPU changing the frequency and not telling
it (which is not unusual for that matter).

Thanks,
Rafael