[RFC PATCH] ALSA: compress: Fix gapless playback state machine

[Pierre-Louis Bossart]

On 6/10/20 5:07 AM, Srinivas Kandagatla wrote:
> For gapless playback call to snd_compr_drain_notify() after
> partial drain should put the state to SNDRV_PCM_STATE_RUNNING
> rather than SNDRV_PCM_STATE_SETUP as the driver is ready to
> process the buffers for new track.
> 
> With existing code, if we are playing 3 tracks in gapless, after

The gapless playback only deals with transitions between two tracks of 
identical format. I am not sure what the reference to 3 tracks means.

> partial drain finished on previous track 1 the state is set to
> SNDRV_PCM_STATE_SETUP which is then moved to SNDRV_PCM_STATE_PREPARED
> after data write. With this state calls to snd_compr_next_track() and
> few other calls will fail as they expect the state to be in
> SNDRV_PCM_STATE_RUNNING.
> 
> Here is the sequence of events and state transitions:
> 
> 1. set_params (Track 1), state =  SNDRV_PCM_STATE_SETUP
> 2. set_metadata (Track 1), no state change, state = SNDRV_PCM_STATE_SETUP
> 3. fill and trigger start (Track 1), state = SNDRV_PCM_STATE_RUNNING
> 4. set_next_track (Track 2), state = SNDRV_PCM_STATE_RUNNING
> 5. partial_drain (Track 1), state = SNDRV_PCM_STATE_SETUP
> 6  snd_compr_drain_notify (Track 1), state = SNDRV_PCM_STATE_SETUP
> 7. fill data (Track 2), state = SNDRV_PCM_STATE_PREPARED
> 8. set_metadata (Track 3), no state change, state = SNDRV_PCM_STATE_PREPARED
> 9. set_next_track (Track 3), !! FAILURE as state != SNDRV_PCM_STATE_RUNNING
> 
> Fixes: f44f2a5417b2 ("ALSA: compress: fix drain calls blocking other compress functions (v6)")
> Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla at linaro.org>
> ---
> 
> I wonder who did gapless work on upstream so far?

IIRC the only users of gapless playback are Android platforms, where the 
HAL deals with the transitions. I am not aware of any plain vanilla 
Linux solution.

> 
>   include/sound/compress_driver.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/sound/compress_driver.h b/include/sound/compress_driver.h
> index 6ce8effa0b12..eabac33864c2 100644
> --- a/include/sound/compress_driver.h
> +++ b/include/sound/compress_driver.h
> @@ -182,7 +182,7 @@ static inline void snd_compr_drain_notify(struct snd_compr_stream *stream)
>   	if (snd_BUG_ON(!stream))
>   		return;
>   
> -	stream->runtime->state = SNDRV_PCM_STATE_SETUP;
> +	stream->runtime->state = SNDRV_PCM_STATE_RUNNING;
>   
>   	wake_up(&stream->runtime->sleep);
>   }
>