audio: make buffer size configurable

Really only for testing.

DOCS/man/options.rst

@@ -994,6 +994,20 @@ Audio
 
     Do not use.
 
+``--audio-buffer=<seconds>``
+    Set the audio output minimum buffer. The audio device might actually create
+    a larger buffer if it pleases. If the device creates a smaller buffer,
+    additional audio is buffered in an additional software buffer.
+
+    Making this larger will make soft-volume and other filters react slower,
+    introduce additional issues on playback speed change, and block the
+    player on audio format changes. A smaller buffer might lead to audio
+    dropouts.
+
+    This option should be used for testing only. If a non-default value helps
+    significantly, the mpv developers should be contacted.
+
+    Default: 0.2 (200 ms).
 
 Subtitles
 ---------

audio/out/ao.c

@@ -152,6 +152,7 @@ static struct ao *ao_create(bool probing, struct mpv_global *global,
         .channels = channels,
         .format = format,
         .log = mp_log_new(ao, log, name),
+        .def_buffer = global->opts->audio_buffer,
     };
     if (ao->driver->encode != !!ao->encode_lavc_ctx)
         goto error;
@@ -187,7 +188,7 @@ static struct ao *ao_create(bool probing, struct mpv_global *global,
         ao->device_buffer = ao->driver->get_space(ao);
         MP_VERBOSE(ao, "device buffer: %d samples.\n", ao->device_buffer);
     }
-    ao->buffer = MPMAX(ao->device_buffer, MIN_BUFFER * ao->samplerate);
+    ao->buffer = MPMAX(ao->device_buffer, ao->def_buffer * ao->samplerate);
     MP_VERBOSE(ao, "using soft-buffer of %d samples.\n", ao->buffer);
 
     if (ao->api->init(ao) < 0)

audio/out/internal.h

@@ -25,9 +25,6 @@
 #include "audio/chmap.h"
 #include "audio/chmap_sel.h"
 
-// Minimum buffer size in seconds.
-#define MIN_BUFFER 0.2
-
 // If ao_get_delay() reaches this value after ao_play() was called with the
 // AOPLAY_FINAL_CHUNK flag set, the playback core expects that the audio has
 // all been played.
@@ -56,6 +53,7 @@ struct ao {
     struct mp_log *log; // Using e.g. "[ao/coreaudio]" as prefix
 
     int buffer;
+    double def_buffer;
     void *api_priv;
 };
 

audio/out/push.c

@@ -168,11 +168,11 @@ static int unlocked_get_space(struct ao *ao)
     int space = mp_audio_buffer_get_write_available(p->buffer);
     if (ao->driver->get_space) {
         // The following code attempts to keep the total buffered audio to
-        // MIN_BUFFER/2+device_buffer in order to improve latency.
+        // def_buffer/2+device_buffer in order to improve latency.
         int device_space = ao->driver->get_space(ao);
         int device_buffered = ao->device_buffer - device_space;
         int soft_buffered = mp_audio_buffer_samples(p->buffer);
-        int min_buffer = MIN_BUFFER / 2 * ao->samplerate + ao->device_buffer;
+        int min_buffer = ao->def_buffer / 2 * ao->samplerate + ao->device_buffer;
         int total_buffer = device_buffered + soft_buffered;
         int missing = min_buffer - total_buffer;
         space = MPMIN(space, missing);

options/options.c

@@ -371,6 +371,8 @@ const m_option_t mp_opts[] = {
                ({"no", 0},
                 {"yes", 1}, {"", 1},
                 {"weak", -1})),
+    OPT_DOUBLE("audio-buffer", audio_buffer, M_OPT_MIN | M_OPT_MAX,
+               .min = 0, .max = 10),
 
     OPT_GEOMETRY("geometry", vo.geometry, 0),
     OPT_SIZE_BOX("autofit", vo.autofit, 0),
@@ -558,6 +560,7 @@ const struct MPOpts mp_default_opts = {
     .mixer_init_mute = -1,
     .volstep = 3,
     .gapless_audio = -1,
+    .audio_buffer = 0.2,
     .vo = {
         .video_driver_list = NULL,
         .monitor_pixel_aspect = 1.0,

options/options.h

@@ -74,6 +74,7 @@ typedef struct MPOpts {
     int volstep;
     float softvol_max;
     int gapless_audio;
+    double audio_buffer;
 
     mp_vo_opts vo;
     int allow_win_drag;