[alsa-devel] Question about SNDRV_PCM_STATE_DRAINING and DMA transfer

[Jaroslav Kysela]

Dne 26.9.2014 v 10:45 Kuninori Morimoto napsal(a):
> 
> Hi Jaroslav
> 
>>>>> 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 invalid (outdated) samples, but gets aborted immediately.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...
> 
> Thank you about this help.
> Can you show me how to do this in app side ?
> (Calling some alsa-lib function ?)

Yes, the function is snd_pcm_drop() .

				Jaroslav

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