linux-kernel - Re: [RFC V2] cpufreq: make sure frequency transitions are serialized

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Message-ID: <10055939.1CZiP7vyp7@vostro.rjw.lan>
Date:	Wed, 19 Mar 2014 14:53:59 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	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 V2] cpufreq: make sure frequency transitions are serialized

On Wednesday, March 19, 2014 03:39:16 PM Viresh Kumar wrote:
> On 19 March 2014 15:20, Srivatsa S. Bhat
> <srivatsa.bhat@...ux.vnet.ibm.com> wrote:
> > No, its not about burden. Its about the elegance of the design. We should
> > not be overly "smart" in the cpufreq core. Hiding the synchronization inside
> > the cpufreq core only encourages people to write buggy code in their drivers.
> 
> What kind of buggy code can be there? They are supposed to call notifiers
> in the order mentioned and so it shouldn't be a problem at all.. Don't know..
> 
> > Why don't we go with what Rafael suggested? We can have dedicated
> > begin_transition() and end_transition() calls to demarcate the frequency
> > transitions. That way, it makes it very clear how the synchronization is
> > done. Of course, these functions would be provided (exported) by the cpufreq
> > core, by implementing them using locks/counters/whatever.
> >
> > Basically what I'm arguing against, is the idea of having the cpufreq
> > core figure out what the driver _intended_ to do, from inside the
> > cpufreq_notify_transition() call.
> >
> > What I would prefer instead is to have the cpufreq driver do something
> > like this:
> >
> > cpufreq_freq_transition_begin();
> >
> > cpufreq_notify_transition(CPUFREQ_PRECHANGE);
> 
> Why do we need two routines then? What about doing notification from
> inside cpufreq_freq_transition_begin()?

We can do that in my opinion.

> This is a burden for driver writers, who don't normally understand the
> relevance of these calls in detail and may think, only the first one is
> enough or the second one is..
> 
> Its better if they simply let the core that they are starting to do transitions,
> i.e. cpufreq_freq_transition_begin() and then the core should send
> notifications.
> 
> > //perform the frequency change
> >
> > cpufreq_notify_transition(CPUFREQ_POSTCHANGE);
> >
> > cpufreq_freq_transition_end();
> >
> > [ASYNC_NOTIFICATION drivers will invoke the last two functions in a
> > separate context/thread.]
> 
> Same for the last two routines and yes they would be called from
> separate thread for ASYNC_NOTIFICATION drivers..

That'd be fine by me in principle.

-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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