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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
e08598e7e2) 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
This commit is contained in:
Max Kellermann 2018-11-14 11:06:59 +01:00
parent 3830748de5
commit ccafe3f3cf
2 changed files with 19 additions and 1 deletions
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1
NEWS
View File

@ -1,6 +1,7 @@
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

19
src/output/plugins/AlsaOutputPlugin.cxx
View File

@ -109,6 +109,13 @@ 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
@ -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);