[alsa-devel] Question about SNDRV_PCM_STATE_DRAINING and DMA transfer

[Jaroslav Kysela]

Dne 26.9.2014 v 10:03 Kuninori Morimoto napsal(a):
> 
> Hi Clemens
> 
> Thank you for your explain
> 
>>> We noticed that DMA seems transfered +1 time when Ctrl-C happen.
>>> But, is this correct ? is this our driver bug ?
>>> [...]
>>> 7. DMA transfer interrupt happen
>>>
>>>    It calls snd_pcm_period_elapsed() and try to transfer next 2048 byte
>>>    snd_pcm_playback_avail() in snd_pcm_update_state() return 8192 this time.
>>>    then, it calls snd_pcm_drain_done()
>>>
>>> 9. snd_soc_dai_ops :: trigger called with SNDRV_PCM_TRIGGER_STOP
>>>
>>>    driver stops DMA transfer
>>
>> There is no strong synchronization between snd_pcm_drain() and the rest
>> of the system; snd_pcm_drain() just waits for an underrun to happen.
>>
>> In other words, the last actual DMA transfer is likely to contain
>> invalid (outdated) samples, but gets aborted immediately.
> 
> I wonder why we need drain ?
> This "likely to contain invalid samples" will be solved if
> we can skip "complete drain" ?

The user app should do the flush instead the drain in this case...

					Jaroslav

-- 
Jaroslav Kysela <perex at perex.cz>
Linux Kernel Sound Maintainer
ALSA Project; Red Hat, Inc.