linux-kernel - Re: [RFC v3] cpufreq: Make sure frequency transitions are serialized

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Message-ID: <5329AE56.8080204@linux.vnet.ibm.com>
Date:	Wed, 19 Mar 2014 20:18:54 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Amit Daniel <amit.daniel@...sung.com>
Subject: Re: [RFC v3] cpufreq: Make sure frequency transitions are serialized

On 03/19/2014 07:05 PM, Viresh Kumar wrote:
> On 19 March 2014 17:45, Srivatsa S. Bhat
> <srivatsa.bhat@...ux.vnet.ibm.com> wrote:
>> diff --git a/include/linux/cpufreq.h b/include/linux/cpufreq.h
>> +       bool                    transition_ongoing; /* Tracks transition status */
>> +       struct mutex            transition_lock;
>> +       wait_queue_head_t       transition_wait;
> 
> Similar to what I have done in my last version, why do you need
> transition_ongoing and transition_wait? Simply work with
> transition_lock? i.e. Acquire it for the complete transition sequence.
> 

We *can't* acquire it for the complete transition sequence
in case of drivers that do asynchronous notification, because
PRECHANGE is done in one thread and POSTCHANGE is done in a
totally different thread! You can't acquire a lock in one
task and release it in a different task. That would be a
fundamental violation of locking.

That's why I introduced the wait queue to help us create
a "flow" which encompasses 2 different, but co-ordinating
tasks. You simply can't do that elegantly by using plain
locks alone.

Regards,
Srivatsa S. Bhat

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