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

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Date:	Wed, 19 Mar 2014 14:50:37 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
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 V2] cpufreq: make sure frequency transitions are serialized

On 19 March 2014 14:47, Srivatsa S. Bhat
<srivatsa.bhat@...ux.vnet.ibm.com> wrote:
> Wait, I think I remember. The problem was about dealing with drivers that
> do asynchronous notification (those that have the ASYNC_NOTIFICATION flag
> set). In particular, exynos-5440 driver sends out the POSTCHANGE notification
> from a workqueue worker, much later than sending the PRECHANGE notification.
>
> From what I saw, this is how the exynos-5440 driver works:
>
> 1. ->target() is invoked, and the driver writes to a register and returns
>    to its caller.
>
> 2. An interrupt occurs that indicates that the frequency was changed.
>
> 3. The interrupt handler kicks off a worker thread which then sends out
>    the POSTCHANGE notification.

Correct!!

> So the important question here is, how does the exynos-5440 driver
> protect itself from say 2 ->target() calls which occur in close sequence
> (before allowing the entire chain for the first call to complete)?
>
> As far as I can see there is no such synchronization in the driver at
> the moment. Adding Amit to CC for his comments.

Yes, and that's what my patch is trying to fix. Where is the confusion?
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