input/plugins/Alsa: limit ALSA buffer time to 2 seconds maximum

Some ALSA capture devices can have very large buffers, holding 10 seconds or more audio. Using the maximum buffer size with such devices leads to unacceptably large, and unnecessary, latency. Also, some ALSA drivers (e.g. HDA Intel PCH) report an invalid maximum period size, and the period size that mpd calculates from the maximum buffer size results in "Invalid argument" error when applying the hw_params. Note that the "default" capture device on many cards includes the "dsnoop" plugin which imposes a buffer size of 16384 frames, so that "alsa://" works OK but "alsa://plughw" or "alsa://hw" both fail. Limit the maximum buffer time for ALSA input devices to a more useable 2 seconds, thereby avoiding both the above problems.

input/plugins/Alsa: limit ALSA buffer time to 2 seconds maximum
17d944f6 · borine · Max Kellermann · 0f82f186 · 17d944f6 · 17d944f6
Commit 17d944f6 authored May 06, 2023 by borine Committed by Max Kellermann Dec 20, 2023
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Showing with 11 additions and 4 deletions

NEWS NEWS +2 -0

AlsaInputPlugin.cxx src/input/plugins/AlsaInputPlugin.cxx +9 -4

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--- a/NEWS
+++ b/NEWS
 ver 0.23.15 (not yet released)
 * decoder
  - ffmpeg: fix build failure with FFmpeg 6.1
+* output
+  - alsa: limit buffer time to 2 seconds

 ver 0.23.14 (2023/10/08)
 * decoder

--- a/src/input/plugins/AlsaInputPlugin.cxx
+++ b/src/input/plugins/AlsaInputPlugin.cxx
@@ -369,9 +369,14 @@ AlsaInputStream::ConfigureCapture(AudioFormat audio_format)
 		 period_size_min, period_size_max,
 		 period_time_min, period_time_max);

-	/* choose the maximum possible buffer_size ... */
-	snd_pcm_hw_params_set_buffer_size(capture_handle, hw_params,
-					  buffer_size_max);
+	/* choose the maximum buffer_time up to limit of 2 seconds ... */
+	unsigned buffer_time = buffer_time_max;
+	if (buffer_time > 2000000U)
+		buffer_time = 2000000U;
+	int direction = -1;
+	if ((err = snd_pcm_hw_params_set_buffer_time_near(capture_handle,
+				hw_params, &buffer_time, &direction)) < 0)
+		throw Alsa::MakeError(err, "Cannot set buffer time");

 	/* ... and calculate the period_size to have four periods in
 	   one buffer; this way, we get woken up often enough to avoid
@@ -379,7 +384,7 @@ AlsaInputStream::ConfigureCapture(AudioFormat audio_format)
 	snd_pcm_uframes_t buffer_size;
 	if (snd_pcm_hw_params_get_buffer_size(hw_params, &buffer_size) == 0) {
 		snd_pcm_uframes_t period_size = buffer_size / 4;
-		int direction = -1;
+		direction = -1;
 		if ((err = snd_pcm_hw_params_set_period_size_near(capture_handle,
 		                             hw_params, &period_size, &direction)) < 0)
 			throw Alsa::MakeError(err, "Cannot set period size");