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Message-ID: <48D02589.8070601@freescale.com>
Date: Tue, 16 Sep 2008 16:30:49 -0500
From: Timur Tabi <timur@...escale.com>
To: lkml <linux-kernel@...r.kernel.org>,
Haavard Skinnemoen <haavard.skinnemoen@...el.com>,
Dan Williams <dan.j.williams@...el.com>
Subject: dmaengine.c: question about device_alloc_chan_resources
Dan, Haavard, et al,
I am making some fixes to the Freescale DMA driver (drivers/dma/fsldma.c), and
I've come across a situation I don't understand. Specifically, I have an issue
with the return values from fsl_dma_alloc_chan_resources(), which is fsldma's
device_alloc_chan_resources() function.
fsldma calls dma_async_device_register() for each DMA controller it finds. The
problem is that when I use the dmatest driver, this results in
device_alloc_chan_resources() being called multiple times for the same DMA
channel. When this happens, fsl_dma_alloc_chan_resources() must return -1
otherwise fsldma will hang during unload if the dmatest driver is also loaded.
The hang occurs in dma_async_device_unregister(), when it calls
wait_for_completion().
Here's what happens:
1) fsldma finds one DMA controller with four DMA channels. It creates all the
channel objects and then calls dma_async_device_register().
2) Via dma_clients_notify_available(), dmatest is notified that there's a new
DMA resource. It stores those four channels in its dmatest_channels object.
3) fsldma fins another DMA controller with four DMA channels. Again, it creates
all the channel objects and then calls dma_async_device_register().
4) dma_clients_notify_available() is called again. This time, however, it tells
dmatest about *eight* channels, not just the four additional ones. It does this
because fsl_dma_alloc_chan_resources() returns 1 every time it's called.
The result is that the some DMA channel will appear twice in dmatest's channel
list. For example, I added a bunch of printks to dma_test_add_channnel:
dmatest: Started 1 threads using dma2chan1
dmatest_add_channel:372 chan=df1f2a50
dmatest_add_channel:382 dtc->chan=df1f2898
dmatest_add_channel:382 dtc->chan=df1f2a50
dmatest_add_channel:382 dtc->chan=df1f2898
dmatest_add_channel:382 dtc->chan=df1f2a50
As you can see, df1f2898 appears twice, as does df1f2a50.
So my fix was to add this code to fsl_dma_alloc_chan_resources():
/* Has this channel already been allocated? */
if (fsl_chan->desc_pool)
return -ENOMEM;
This fixes the hang, but I have a suspicion that it's still wrong. When I look
at the other DMA device drivers, they don't do this. For instance,
ioat_dma_alloc_chan_resources() does this:
/* have we already been set up? */
if (!list_empty(&ioat_chan->free_desc))
return ioat_chan->desccount;
In other words, it returns the resources that have been allocated already. But
if I do that, my driver hangs during an unload (if dmatest is loaded).
I think what's happening is that fsldma is the only DMA driver that calls
dma_async_device_register() more than once, and so it's exposing a bug in
dmaengine.c. Can anyone confirm that?
--
Timur Tabi
Linux kernel developer at Freescale
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