Re: [alsa-devel] Question about SNDRV_PCM_STATE_DRAINING and DMA transfer

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In other words, the last actual DMA transfer is likely to contain invalid (outdated) samples, but gets aborted immediately.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