[PATCH v4 1/4] dma: mmp_pdma: only complete one transaction from dma_do_tasklet()

Daniel Mack [off-list ref] writes:

Hi Xiang,

On 20.08.2013 04:27, Xiang Wang wrote:

quoted

I think we would not run into the situation that there is a descriptor 
with ENDIRQEN set in the middle of the running chain.

in mmp_pdma.c:
mmp_pdma_tx_submit() -> append_pending_queue() ->
tail->desc.dcmd &= ~DCMD_ENDIRQEN;

So in the pending list (same for running list), only the last descriptor 
will have ENDIRQEN set.

Right, thanks for noting that.

But is that what we want? I think that clearing of the ENDIRQEN bit
needs to be removed then, because if we really have multiple
transactions running, then we want to get an interrupt whenever _any_ of
it is completed.

At least that's something pxa_camera.c needs, ie. have the dma interrupt handler
called as soon as the _first_ of many transactions queued on the same channel is
finished.

This is necessary to declare the video capture buffer as "done", which is
signaled by a terminated DMA transaction. And of course, in video recording,
another one is beginning on the channel, while the finished video buffer can be
safely unmapped and handed over to userland.

Hence, would you agree that we should remove that "tail->desc.dcmd &=
~DCMD_ENDIRQEN;" modification in order to make the above logic work?

Same question on my side.

My understanding is that the current bebaviour violates the dmaengine
specification (Documentation/dmaengine.txt, chapter 5, "On completion of each
DMA operation, the next in queue is started and a tasklet triggered. The tasklet
will then call the client driver completion callback routine for notification,
if set.)

Cheers.

--
Robert