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

[Charles Keepax]

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):
> > > 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
> > > 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
> > 
> > 
> > The snd_compr_drain_notify() is called only from snd_compr_stop(). Something
> > is missing in this sequence?
> 
> It is supposed to be invoked by driver when partial drain is complete..
> both intel and sprd driver are calling this. snd_compr_stop is stop
> while draining case so legit
> 

Not sure I follow this statement, could you elaborate a bit?
snd_compr_stop putting the state to RUNNING seems fundamentally
broken to me, the whole point of snd_compr_stop is to take the
state out of RUNNING.

Thanks,
Charles