linux-kernel - Re: dmaengine.c: question about device_alloc_chan

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Date:	Wed, 17 Sep 2008 15:36:53 -0700
From:	"Dan Williams" <dan.j.williams@...el.com>
To:	"Timur Tabi" <timur@...escale.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	"Haavard Skinnemoen" <haavard.skinnemoen@...el.com>
Subject: Re: dmaengine.c: question about device_alloc_chan_resources

On Tue, Sep 16, 2008 at 2:30 PM, Timur Tabi <timur@...escale.com> wrote:
> 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:
>

I believe the problem is that dmatest is telling the core to take
multiple references on a channel.  Take a look at
net/core/dev.c:netdev_dma_event and
crypto:asynx_tx/async_tx.c:dma_channel_add_remove both of those check
to see if they already have a refence to the channel.

> 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

^...right here we should be returning DMA_DUP rather than DMA_ACK.

> dmatest_add_channel:382 dtc->chan=df1f2898

--
Dan
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