linux-kernel - Re: dmaengine: BUG: Unable to handle kernel paging request

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Date:	Mon, 12 Jan 2009 12:08:38 -0700
From:	Dan Williams <dan.j.williams@...el.com>
To:	Ira Snyder <iws@...o.caltech.edu>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Sosnowski, Maciej" <maciej.sosnowski@...el.com>,
	"leoli@...escale.com" <leoli@...escale.com>,
	"zw@...kernel.org" <zw@...kernel.org>
Subject: Re: dmaengine: BUG: Unable to handle kernel paging request

On Mon, 2009-01-12 at 11:04 -0700, Ira Snyder wrote:
> Hello all,
> 
> I'm working on a driver that uses DMAEngine. With the recent changes,
> the driver doesn't work anymore.
> 
> I believe I have tracked it down to commit
> 41d5e59c1299f27983977bcfe3b360600996051c, but it could be one of the
> other DMAEngine commits.
> 
> The code that crashes is in mm/dmapool.c, line 178:
> if (list_empty(&dev->dma_pools))
> 
> I distilled the crash down to the following simple driver, which should
> just get a reference to dmaengine, in preparation for acquiring a
> channel to use.

I believe the problem is that the driver uses the channel device value
before it is created.  Prior to this patch the following line in
fsl_dma_chan_probe:

new_fsl_chan->dev = &new_fsl_chan->common.dev

...would retrieve a pointer to the uninitialized struct device in
dma_chan.  The later call to dma_async_device_register in
of_fsl_dma_probe fixed up this uninitialized data.

However, the dmaengine sysfs implementation was fixed to support proper
lifetime rules which means that the current:

new_fsl_chan->dev = &new_fsl_chan->common.dev->device;

...retrieves a NULL pointer because new_fsl_chan->common.dev has not
been allocated at this point.

The following may fix this up...

diff --git a/drivers/dma/fsldma.c b/drivers/dma/fsldma.c
index ca70a21..748e140 100644
--- a/drivers/dma/fsldma.c
+++ b/drivers/dma/fsldma.c
@@ -822,7 +822,7 @@ static int __devinit fsl_dma_chan_probe(struct fsl_dma_device *fdev,
 	 */
 	WARN_ON(fdev->feature != new_fsl_chan->feature);
 
-	new_fsl_chan->dev = &new_fsl_chan->common.dev->device;
+	new_fsl_chan->dev = fdev->dev;
 	new_fsl_chan->reg_base = ioremap(new_fsl_chan->reg.start,
 			new_fsl_chan->reg.end - new_fsl_chan->reg.start + 1);
 


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