linux-kernel - Re: [PATCH v7] DMA: sun6i: Add driver for the Allwinner A31 DMA controller

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Date:	Fri, 2 May 2014 22:04:29 +0530
From:	Vinod Koul <vinod.koul@...el.com>
To:	Maxime Ripard <maxime.ripard@...e-electrons.com>
Cc:	Dan Williams <dan.j.williams@...el.com>,
	linux-arm-kernel@...ts.infradead.org, dmaengine@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	linux-sunxi@...glegroups.com, kevin.z.m.zh@...il.com,
	sunny@...winnertech.com, shuge@...winnertech.com,
	zhuzhenhua@...winnertech.com, andriy.shevchenko@...el.com,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH v7] DMA: sun6i: Add driver for the Allwinner A31 DMA
 controller

On Wed, Apr 30, 2014 at 02:53:22PM -0700, Maxime Ripard wrote:
> Hi Vinod,
> 
> On Wed, Apr 30, 2014 at 12:34:08PM +0530, Vinod Koul wrote:
> > On Thu, Apr 24, 2014 at 04:22:44PM +0200, Maxime Ripard wrote:
> > > +static inline void sun6i_dma_free(struct sun6i_dma_dev *sdc)
> > > +{
> > > +	int i;
> > > +
> > > +	for (i = 0; i < NR_MAX_VCHANS; i++) {
> > > +		struct sun6i_vchan *vchan = &sdc->vchans[i];
> > > +
> > > +		list_del(&vchan->vc.chan.device_node);
> > > +		tasklet_kill(&vchan->vc.task);
> > > +	}
> > > +
> > > +	tasklet_kill(&sdc->task);
> > This is again not good. see http://lwn.net/Articles/588457/
> > At this point HW can still generate interrupts or you can have irq running!
> 
> I'm not sure to fully understand the issue here, but what is not good?
> the first or the second tasklet_kill calls, or both?
> 
> From what I understood, the issue is only there whenever you are
> calling tasklet_disable without making sure that no one will schedule
> your tasklet before disabling it.
> 
> But the point is I don't actually use either _enable/_disable. I might
> be wrong in not using those functions, but I don't really see how I
> can be impacted.

Well that was one part of it. How do you ensure the tasklet is not scheduled
while and after you are killing it. You need to ensure irq is disabled and pending irqs
have finished processing. I dont see that bit.

-- 
~Vinod


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