[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 invalid (outdated) samples, but gets aborted immediately.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 ?)
> 
> Yes, the function is snd_pcm_drop() .

Thank you !

Best regards
---
Kuninori Morimoto