[alsa-devel] Question about SNDRV_PCM_STATE_DRAINING and DMA transfer

[Clemens Ladisch]

Kuninori Morimoto wrote:
> 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invalid (outdated) samples, but gets aborted immediately.


Regards,
Clemens