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

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Date:	Thu, 20 Mar 2014 15:15:59 +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 03/20/2014 03:03 PM, Viresh Kumar wrote:
> On 20 March 2014 14:54, Srivatsa S. Bhat
> <srivatsa.bhat@...ux.vnet.ibm.com> wrote:
>> On 03/20/2014 02:07 PM, Viresh Kumar wrote:
>>>     WARN_ON(policy->transition_ongoing);
>>>
>>
>> I guess you meant WARN_ON(!policy->transition_ongoing)
>> perhaps?
> 
> Ooops!!
> 
>> I'm not sure whether its really worth it, because it kinda looks
>> obvious. Not sure what kind of bugs it would catch. I can't think of any
>> such scenario :-(
> 
> Just to catch if somebody is sending a POSTCHANGE one without first
> sending a PRECHANGE one.. Just another check to make sure things are
> in order.
> 

Well, that's unlikely, since they will have to call _end() before
_begin() :-) That's the power of having great function names - they make
it impossible to use them incorrectly ;-) But anyway, I can add the check,
just in case somebody misses even such an obvious cue! :-)

By the way, I'm also thinking of using a spinlock instead of a mutex.
The critical section is tiny and we don't sleep inside the critical
section - sounds like the perfect case for a spinlock.

Regards,
Srivatsa S. Bhat

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