output/alsa: don't generate silence if ALSA-PCM buffer has enough data

If our `ring_buffer` is smaller than the ALSA-PCM buffer (if the latter has more than the 4 periods we allocate), it can happen that the start threshold is crossed and ALSA switches to `SND_PCM_STATE_RUNNING`, but the `ring_buffer` is empty. In this case, MPDD will generate silence, even though the ALSA-PCM buffer has enough data. This causes stuttering (#420). This commit amends an older workaround for a similar problem (commit e08598e7) by adding a snd_pcm_avail() check, and only generate silence if there is less than one period of data in the ALSA-PCM buffer. Fixes #420

output/alsa: don't generate silence if ALSA-PCM buffer has enough data
ccafe3f3 · Max Kellermann · 3830748d · ccafe3f3 · ccafe3f3
Commit ccafe3f3 authored Nov 14, 2018 by Max Kellermann
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NEWS NEWS +1 -0

AlsaOutputPlugin.cxx src/output/plugins/AlsaOutputPlugin.cxx +18 -1

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--- a/NEWS
+++ b/NEWS
 ver 0.21.3 (not yet released)
 * output
  - alsa: fix crash bug
+  - alsa: fix stuttering at start of playback

 ver 0.21.2 (2018/11/12)
 * protocol

--- a/src/output/plugins/AlsaOutputPlugin.cxx
+++ b/src/output/plugins/AlsaOutputPlugin.cxx
@@ -110,6 +110,13 @@ class AlsaOutput final
 	snd_pcm_uframes_t period_frames;

 	/**
+	 * If snd_pcm_avail() goes above this value and no more data
+	 * is available in the #ring_buffer, we need to play some
+	 * silence.
+	 */
+	snd_pcm_sframes_t max_avail_frames;
+
+	/**
 	 * Is this a buggy alsa-lib version, which needs a workaround
 	 * for the snd_pcm_drain() bug always returning -EAGAIN?  See
 	 * alsa-lib commits fdc898d41135 and e4377b16454f for details.
@@ -482,6 +489,10 @@ AlsaOutput::Setup(AudioFormat &audio_format,

 	period_frames = alsa_period_size;

+	/* generate silence if there's less than once period of data
+	   in the ALSA-PCM buffer */
+	max_avail_frames = hw_result.buffer_size - hw_result.period_size;
+
 	silence = new uint8_t[snd_pcm_frames_to_bytes(pcm, alsa_period_size)];
 	snd_pcm_format_set_silence(hw_result.format, silence,
 				   alsa_period_size * audio_format.channels);
@@ -933,7 +944,8 @@ try {
 	CopyRingToPeriodBuffer();

 	if (period_buffer.IsEmpty()) {
-		if (snd_pcm_state(pcm) == SND_PCM_STATE_PREPARED) {
+		if (snd_pcm_state(pcm) == SND_PCM_STATE_PREPARED ||
+		    snd_pcm_avail(pcm) <= max_avail_frames) {
 			/* at SND_PCM_STATE_PREPARED (not yet switched
 			   to SND_PCM_STATE_RUNNING), we have no
 			   pressure to fill the ALSA buffer, because
@@ -943,6 +955,11 @@ try {
 			   monitoring the ALSA file descriptor, and
 			   let it be reactivated by Play()/Activate()
 			   whenever more data arrives */
+			/* the same applies when there is still enough
+			   data in the ALSA-PCM buffer (determined by
+			   snd_pcm_avail()); this can happend at the
+			   start of playback, when our ring_buffer is
+			   smaller than the ALSA-PCM buffer */

 			{
 				const std::lock_guard<Mutex> lock(mutex);