Messages in this thread | | | Date | Sat, 7 Jan 2012 19:21:12 +0000 | From | Russell King - ARM Linux <> | Subject | Re: [PATCH v2 2/2] msm: DMAEngine: Add DMAEngine driver based on old MSM DMA APIs |
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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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