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
[off-list ref] wrote:

quoted

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..

quoted

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.

quoted

//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.