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

[Charles Keepax]

On Thu, Jun 11, 2020 at 03:14:23PM +0530, Vinod Koul wrote:
> On 11-06-20, 11:09, Jaroslav Kysela wrote:
> > Dne 11. 06. 20 v 10:46 Charles Keepax napsal(a):
> > > On Wed, Jun 10, 2020 at 04:28:20PM +0530, Vinod Koul wrote:
> > > > On 10-06-20, 12:40, Jaroslav Kysela wrote:
> > > > > Dne 10. 06. 20 v 12:07 Srinivas Kandagatla napsal(a):
> > > > > > 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
> Yeah sorry I overlooked that case and was thinking of it being invoked
> from driver!
> 
> Yes this would make the snd_compr_stop() behave incorrectly.. so this
> cant be done as proposed.
> 
> But we still need to set the draining stream state properly and I am
> thinking below now:
> 
> diff --git a/sound/core/compress_offload.c b/sound/core/compress_offload.c
> index 509290f2efa8..9aba851732d7 100644
> --- a/sound/core/compress_offload.c
> +++ b/sound/core/compress_offload.c
> @@ -929,7 +929,9 @@ static int snd_compr_partial_drain(struct snd_compr_stream *stream)
>         }
>  
>         stream->next_track = false;
> -       return snd_compress_wait_for_drain(stream);
> +       retval = snd_compress_wait_for_drain(stream);
> +       stream->runtime->state = SNDRV_PCM_STATE_RUNNING;
> +       return retval;

This is looking better, I think you probably need to make the
switch to RUNNING conditional on snd_compress_wait_for_drain
succeeding, as the state should remain in DRAINING if we are
interrupted or some such.

I also have a slight concern that since
snd_compress_wait_for_drain, releases the lock the set_next_track
could come in before the state is moved to RUNNING, but I guess
from user-spaces perspective that is the same as it coming in
whilst the state is still DRAINING, so should be handled fine?

Thanks,
Charles