linux-kernel - Re: [PATCH v11 0/2] Add support for the Allwinner A31 DMA Controller

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Date:	Mon, 28 Jul 2014 11:41:48 +0200
From:	Maxime Ripard <maxime.ripard@...e-electrons.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Vinod Koul <vinod.koul@...el.com>, andriy.shevchenko@...el.com,
	Arnd Bergmann <arnd@...db.de>, linux-kernel@...r.kernel.org,
	zhuzhenhua@...winnertech.com, dmaengine@...r.kernel.org,
	linux-sunxi@...glegroups.com, kevin.z.m.zh@...il.com,
	sunny@...winnertech.com, shuge@...winnertech.com,
	Dan Williams <dan.j.williams@...el.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v11 0/2] Add support for the Allwinner A31 DMA Controller

Hi Russell,

On Fri, Jul 25, 2014 at 05:45:19PM +0100, Russell King - ARM Linux wrote:
> On Fri, Jul 25, 2014 at 06:37:46PM +0200, Maxime Ripard wrote:
> > Hi Vinod,
> > 
> > On Fri, Jul 25, 2014 at 06:42:17PM +0530, Vinod Koul wrote:
> > > - don't use devm_request_irq(). You have irq enabled and you have killed
> > >   tasklet. This is too racy. You need to ensure no irqs can be generated before killing
> > >   tasklets.
> > 
> > Ok, would calling disable_irq before killing the tasklet an option for
> > you ? that would allow to keep the devm_request_irq.
> 
> That's not really an acceptable approach if you can use shared
> interrupts.

We don't, but yes, I see your point.

> A better alternative would be devm_free_irq() to give a definite point
> that the interrupt is unregistered in the driver remove sequence.  That
> allows you to keep the advantage of devm_request_irq() to clean up during
> the initialisation side.

Ah, right, thanks.

> An alternative approach would be to ensure that the hardware is quiesced,
> and interrupts are disabled.  Then call synchronize_irq() on it, and at
> that point, you should be certain that your interrupt handler should not
> process any further interrupts for your device (though, in a shared
> interrupt environment, it would still be called should a different device
> on the shared line raise its interrupt.)

Actually, unless I'm missing something, that's pretty much what we're
doing here.

I disable all interrupts in the DMA controller, I call
synchronize_irq, and then kill the tasklet. The only interrupts I
could get are spurious, and we made sure such kind of interrupts
couldn't schedule the tasklet either.

Maxime

-- 
Maxime Ripard, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com

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