[vlc-devel] [PATCH] pulse: follow PulseAudio write requests

[Rémi Denis-Courmont]

Le mardi 19 mars 2019, 14:59:01 EET Thomas Guillem a écrit :
> Wait from Play() if the server don't ask for more data (like every other
> aout modules).

So far this could not be a problem because PA would always allocate a larger 
buffer than VLC core ever will use. Did something change to break that 
assumption?

> Note: tlength (target length of the buffer) is increased from 120ms to 2s.
> Before this commit, the average audio delay (time_get) was around 500ms
> (file caching and input dependant). If we don't increase it, the delay of
> the stream will be arround 120ms (since we now follow PulseAudio write
> requests).

Are you sure this is safe and minreq is enough to keep low latency cases 
working?

> Setting tlength to 2sec will restore the previous time_get
> behavior: delay of arround 500ms, and always lower than 2sec (when
> file-caching is increased). This fixes potential overflow.
> ---
>  modules/audio_output/pulse.c | 27 ++++++++++++++++++++++++++-
>  1 file changed, 26 insertions(+), 1 deletion(-)
> 
> diff --git a/modules/audio_output/pulse.c b/modules/audio_output/pulse.c
> index cc4af1e8c0..961fa7129a 100644
> --- a/modules/audio_output/pulse.c
> +++ b/modules/audio_output/pulse.c
> @@ -69,6 +69,7 @@ typedef struct
>      pa_time_event *trigger; /**< Deferred stream trigger */
>      pa_cvolume cvolume; /**< actual sink input volume */
>      vlc_tick_t last_date; /**< Play system timestamp of last buffer */
> +    size_t write_request; /**< Write request from PulseAudio */
> 
>      pa_volume_t volume_force; /**< Forced volume (stream must be NULL) */
>      pa_stream_flags_t flags_force; /**< Forced flags (stream must be NULL)
> */ @@ -357,6 +358,15 @@ static void stream_underflow_cb(pa_stream *s, void
> *userdata) (void) s;
>  }
> 
> +static void stream_request_cb(pa_stream *s, size_t nbytes, void *userdata)
> +{
> +    audio_output_t *aout = userdata;
> +    aout_sys_t *sys = aout->sys;
> +
> +    sys->write_request = nbytes;
> +    pa_threaded_mainloop_signal(sys->mainloop, 0);
> +}
> +
>  static int stream_wait(pa_stream *stream, pa_threaded_mainloop *mainloop)
>  {
>      pa_stream_state_t state;
> @@ -491,6 +501,13 @@ static void Play(audio_output_t *aout, block_t *block,
> vlc_tick_t date) void *ptr;
>          size_t len = block->i_buffer;
> 
> +        while (sys->write_request == 0)
> +            pa_threaded_mainloop_wait(sys->mainloop);
> +
> +        if (len > sys->write_request)
> +            len = sys->write_request;
> +        sys->write_request -= len;
> +

Does this not need a loop to operate properly?

>          if (pa_stream_begin_write(s, &ptr, &len))
>              vlc_pa_error(aout, "cannot begin write", sys->context);
> 
> @@ -540,6 +557,8 @@ static void Flush(audio_output_t *aout)
> 
>      pa_threaded_mainloop_lock(sys->mainloop);
> 
> +    sys->write_request = 0;
> +
>      pa_operation *op = pa_stream_flush(s, NULL, NULL);
>      if (op != NULL)
>          pa_operation_unref(op);
> @@ -555,6 +574,9 @@ static void Drain(audio_output_t *aout)
>      pa_stream *s = sys->stream;
> 
>      pa_threaded_mainloop_lock(sys->mainloop);
> +
> +    sys->write_request = 0;
> +
>      pa_operation *op = pa_stream_drain(s, NULL, NULL);
>      if (op != NULL)
>          pa_operation_unref(op);
> @@ -778,7 +800,7 @@ static int Start(audio_output_t *aout,
> audio_sample_format_t *restrict fmt) /* PulseAudio goes berserk if the
> target length (tlength) is not * significantly longer than 2 periods
> (minreq), or when the period length * is unspecified and the target length
> is short. */
> -    attr.tlength = pa_usec_to_bytes(3 * AOUT_MIN_PREPARE_TIME, &ss);
> +    attr.tlength = pa_usec_to_bytes(AOUT_MAX_PREPARE_TIME, &ss);
>      attr.prebuf = 0; /* trigger manually */
>      attr.minreq = pa_usec_to_bytes(AOUT_MIN_PREPARE_TIME, &ss);
>      attr.fragsize = 0; /* not used for output */
> @@ -790,6 +812,7 @@ static int Start(audio_output_t *aout,
> audio_sample_format_t *restrict fmt) pa_cvolume_set(cvolume, ss.channels,
> sys->volume_force);
>      }
> 
> +    sys->write_request = 0;
>      sys->trigger = NULL;
>      pa_cvolume_init(&sys->cvolume);
>      sys->last_date = VLC_TICK_INVALID;
> @@ -919,6 +942,7 @@ static int Start(audio_output_t *aout,
> audio_sample_format_t *restrict fmt) pa_stream_set_started_callback(s,
> stream_started_cb, aout);
>      pa_stream_set_suspended_callback(s, stream_suspended_cb, aout);
>      pa_stream_set_underflow_callback(s, stream_underflow_cb, aout);
> +    pa_stream_set_write_callback(s, stream_request_cb, aout);
> 
>      if (pa_stream_connect_playback(s, sys->sink_force, &attr, flags,
>                                     cvolume, NULL) < 0
> @@ -976,6 +1000,7 @@ static void Stop(audio_output_t *aout)
>      pa_stream_set_started_callback(s, NULL, NULL);
>      pa_stream_set_suspended_callback(s, NULL, NULL);
>      pa_stream_set_underflow_callback(s, NULL, NULL);
> +    pa_stream_set_write_callback(s, NULL, NULL);
> 
>      pa_stream_unref(s);
>      sys->stream = NULL;


-- 
Rémi Denis-Courmont