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    SubjectRe: [RFC PATCH] ALSA: compress: Fix gapless playback state machine
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    Dne 11. 06. 20 v 11:44 Vinod Koul napsal(a):
    > 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):
    >>>>>> 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.
    >>
    >> Yes. I agree. It seems that the acknowledge for the partial drain should be
    >> handled differently.
    >
    > 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;
    > }

    I see a race possibility when the last track is too small and the driver
    signals the end-of-track twice. In this case the partial drain should not end
    with the running state. It would be probably better to separate partial / last
    track acknowledgements.

    Jaroslav

    --
    Jaroslav Kysela <perex@perex.cz>
    Linux Sound Maintainer; ALSA Project; Red Hat, Inc.

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