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Alsa: limit buffer size, so shairport can start in time #345

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Alsa: limit buffer size, so shairport can start in time #345

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52 changes: 47 additions & 5 deletions audio_alsa.c
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Original file line number Diff line number Diff line change
Expand Up @@ -155,7 +155,7 @@ static void start(int sample_rate) {
die("Unexpected sample rate!");

int ret, dir = 0;
snd_pcm_uframes_t frames = 64;
snd_pcm_uframes_t period_size, buffer_size;
ret = snd_pcm_open(&alsa_handle, alsa_out_dev, SND_PCM_STREAM_PLAYBACK, 0);
if (ret < 0)
die("Alsa initialization failed: unable to open pcm device: %s\n", snd_strerror(ret));
Expand All @@ -166,10 +166,52 @@ static void start(int sample_rate) {
snd_pcm_hw_params_set_format(alsa_handle, alsa_params, SND_PCM_FORMAT_S16);
snd_pcm_hw_params_set_channels(alsa_handle, alsa_params, 2);
snd_pcm_hw_params_set_rate_near(alsa_handle, alsa_params, (unsigned int *)&sample_rate, &dir);
snd_pcm_hw_params_set_period_size_near(alsa_handle, alsa_params, &frames, &dir);
ret = snd_pcm_hw_params(alsa_handle, alsa_params);
if (ret < 0)
die("unable to set hw parameters: %s\n", snd_strerror(ret));

// setting period and buffer is a simplified version of what XBMC does
snd_pcm_hw_params_get_period_size_max(alsa_params, &period_size, NULL);
snd_pcm_hw_params_get_buffer_size_max(alsa_params, &buffer_size);
debug(1, "Hardware supports period_size_max: %d, buffer_size_max: %d\n", period_size, buffer_size);

// we want about 333 ms of buffer, and 50ms period
// buffer might still need some tweaking to get reliable operation on RPi + USB DAC...
// make sure we do not exceed what HW supports
period_size = (period_size < sample_rate / 20 ? period_size : sample_rate / 20);
buffer_size = (buffer_size < sample_rate / 3 ? buffer_size : sample_rate / 3);

// make sure buffer size is at least 4 times period size
period_size = (period_size < buffer_size / 4 ? period_size : buffer_size / 4);
debug(1, "Trying to set period_size: %d, buffer_size: %d\n", period_size, buffer_size);

// we keep the originals and try setting period and buffer using copies
snd_pcm_uframes_t period_temp, buffer_temp;
period_temp = period_size;
buffer_temp = buffer_size;
snd_pcm_hw_params_t *alsa_params_copy;
snd_pcm_hw_params_alloca(&alsa_params_copy);
snd_pcm_hw_params_copy(alsa_params_copy, alsa_params);

// some HW seems to be picky about the order period and buffer are set, so try both ways
// first try with buffer_size, period_size
if (snd_pcm_hw_params_set_buffer_size_near(alsa_handle, alsa_params_copy, &buffer_temp) != 0
|| snd_pcm_hw_params_set_period_size_near(alsa_handle, alsa_params_copy, &period_temp, NULL) != 0
|| snd_pcm_hw_params(alsa_handle, alsa_params_copy) != 0) {
period_temp = period_size;
buffer_temp = buffer_size;
snd_pcm_hw_params_copy(alsa_params_copy, alsa_params);
// retry with period_size, buffer_size
if (snd_pcm_hw_params_set_period_size_near(alsa_handle, alsa_params_copy, &period_temp, NULL) != 0
|| snd_pcm_hw_params_set_buffer_size_near(alsa_handle, alsa_params_copy, &buffer_temp) != 0
|| snd_pcm_hw_params(alsa_handle, alsa_params_copy) != 0) {
// set what alsa would have
warn("Setting period and buffer failed, going with the defaults\n");
ret = snd_pcm_hw_params(alsa_handle, alsa_params);
if (ret < 0)
die("unable to set hw parameters: %s\n", snd_strerror(ret));
// using alsa defaults, so see what they are
snd_pcm_get_params(alsa_handle, &buffer_size, &period_size);
debug(1, "Defaults are period_size: %d, buffer_size: %d\n", period_size, buffer_size);
}
}
}

static void play(short buf[], int samples) {
Expand Down