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SubjectRe: [PATCH v2 2/2] msm: DMAEngine: Add DMAEngine driver based on old MSM DMA APIs
On Sat, Jan 07, 2012 at 10:54:43AM -0800, David Brown wrote:
> On Fri, Jan 06, 2012 at 05:59:29PM -0800, Daniel Walker wrote:
>
> > > diff --git a/drivers/dma/msm-dma.c b/drivers/dma/msm-dma.c
> > > new file mode 100644
> > > index 0000000..51d9a2b
> > > --- /dev/null
> > > +++ b/drivers/dma/msm-dma.c
> > > ...
> > > +static void msm_chan_desc_cleanup(struct msm_dma_chan *chan)
> > > +{
> > > + struct msm_dma_desc_sw *desc, *_desc;
> > > + unsigned long flags;
> > > +
> > > + dev_dbg(chan->dev, "Cleaning completed descriptor of channel %d\n",
> > > + chan->chan_id);
> > > + spin_lock_irqsave(&chan->lock, flags);
> > > +
> > > + list_for_each_entry_safe(desc, _desc, &chan->active_list, node) {
> > > + dma_async_tx_callback callback;
> > > + void *callback_param;
> > > +
> > > + if (msm_dma_desc_status(chan, desc) == DMA_IN_PROGRESS)
> > > + break;
> > > +
> > > + /* Remove from the list of running transactions */
> > > + list_del(&desc->node);
> > > +
> > > + /* Run the link descriptor callback function */
> > > + callback = desc->async_tx.callback;
> > > + callback_param = desc->async_tx.callback_param;
> > > + if (callback) {
> > > + spin_unlock_irqrestore(&chan->lock, flags);
> > > + callback(callback_param);
> >
> > Are you sure unlocking here is safe? at_hdmac.c holds the lock the
> > entire time, and fsldma.c deletes the entire list, then runs the
> > operations.
>
> Good catch.
>
> According to a comment in at_hdmac.c, it is safe to hold the lock
> while calling the callbacks, so that's probably the easiest solution.
> I suspect that you've got something in another driver expecting the
> lock to be released, and that might have to be changed.

It is _not_ safe to hold the lock while calling callbacks.

Please refer to the DMA engine documentation, found here:

Documentation/dmaengine.txt

section 3:

Note:
Although the async_tx API specifies that completion callback
routines cannot submit any new operations, this is not the
case for slave/cyclic DMA.

For slave DMA, the subsequent transaction may not be available
for submission prior to callback function being invoked, so
slave DMA callbacks are permitted to prepare and submit a new
transaction.

For cyclic DMA, a callback function may wish to terminate the
DMA via dmaengine_terminate_all().

* Therefore, it is important that DMA engine drivers drop any
* locks before calling the callback function which may cause a
* deadlock.

Note that callbacks will always be invoked from the DMA
engines tasklet, never from interrupt context.



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