[alsa-devel] Question about SNDRV_PCM_STATE_DRAINING and DMA transfer

[Kuninori Morimoto]

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 invalid (outdated) samples, but gets aborted immediately.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 ?)

Best regards
---
Kuninori Morimoto