[alsa-devel] async between dmaengine_pcm_dma_complete and snd_pcm_release

[Lars-Peter Clausen]

[...]
> 
>>>> On the other hand that last part could get tricky as the
>>>> dmaengine_terminate_all() might be call from within the callback.
>>> It's tricky indeed in case xrun happens. we should avoid possible deadlock.
>>
>> I think we'll eventually need to versions of dmaengine_terminate_all(). A
>> sync version which makes sure that the tasklet has finished and a non-sync
>> version that only makes sure that no new callbacks are started. I think the
>> sync version should be the default with an optional async version which must
>> be used, if it can run from within the callback. So we'd call the async
>> version in the pcm_trigger callback and the sync version in the pcm_close
>> callback.
> Yes this can be done. We can name this disable_callback cmd. The cmd will tell
> dma driver to disable all callback on the channel. This can be invoked from the
> TRIGEGR_STOP and then terminate_all in the free
> 

I think we should make it the default behavior of dmaengine_terminate_all()
to wait for the tasklet to finish. Since this is what almost always want,
except in this case where you might end up calling dmaeinge_terminate_all()
from within the callback. Internally this can be implemented as two separate
commands. So leave the DMA_TERMINATE_ALL as it is and add a new
DMA_SYNC_CALLBACKS (or whatever it will be named) command. This command will
internally call tasklet_kill(). Then we have two new functions
dmaengine_terminate_all_async() which will just issue the DMA_TERMINATE_ALL
command, the other function is dmaengine_sync_callbacks() which will issue
the DMA_SYNC_CALLBACKS command. dmaengine_terminate_all() will then first
call dmaengine_terminate_all_async() and then dmaengine_sync_callbacks().
The ALSA code would have to be updated first to call
dmaengine_terminate_all_async() for TRIGGER_STOP and
dmaengine_sync_callbacks() on the pcm_close path.

- Lars