linux-kernel - Re: [RFC PATCH] ALSA: compress: Fix gapless playback state machine

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Date:   Thu, 11 Jun 2020 12:40:46 +0200
From:   Jaroslav Kysela <perex@...ex.cz>
To:     Vinod Koul <vkoul@...nel.org>
Cc:     Charles Keepax <ckeepax@...nsource.cirrus.com>,
        alsa-devel@...a-project.org, broonie@...nel.org,
        Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        tiwai@...e.com, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] ALSA: compress: Fix gapless playback state machine

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@...ex.cz>
Linux Sound Maintainer; ALSA Project; Red Hat, Inc.