ao: document some functions

audio/out/ao.c

@@ -209,12 +209,21 @@ done:
     return ao;
 }
 
+// Uninitialize and destroy the AO.
+//  cut_audio: if false, block until all remaining audio was played.
 void ao_uninit(struct ao *ao, bool cut_audio)
 {
     ao->driver->uninit(ao, cut_audio);
     talloc_free(ao);
 }
 
+// Queue the given audio data. Start playback if it hasn't started yet. Return
+// the number of samples that was accepted (the core will try to queue the rest
+// again later). Should never block.
+//  data: start pointer for each plane. If the audio data is packed, only
+//        data[0] is valid, otherwise there is a plane for each channel.
+//  samples: size of the audio data (see ao->sstride)
+//  flags: currently AOPLAY_FINAL_CHUNK can be set
 int ao_play(struct ao *ao, void **data, int samples, int flags)
 {
     return ao->driver->play(ao, data, samples, flags);
@@ -227,6 +236,11 @@ int ao_control(struct ao *ao, enum aocontrol cmd, void *arg)
     return CONTROL_UNKNOWN;
 }
 
+// Return size of the buffered data in seconds. Can include the device latency.
+// Basically, this returns how much data there is still to play, and how long
+// it takes until the last sample in the buffer reaches the speakers. This is
+// used for audio/video synchronization, so it's very important to implement
+// this correctly.
 double ao_get_delay(struct ao *ao)
 {
     if (!ao->driver->get_delay) {
@@ -236,23 +250,32 @@ double ao_get_delay(struct ao *ao)
     return ao->driver->get_delay(ao);
 }
 
+// Return free size of the internal audio buffer. This controls how much audio
+// the core should decode and try to queue with ao_play().
 int ao_get_space(struct ao *ao)
 {
     return ao->driver->get_space(ao);
 }
 
+// Stop playback and empty buffers. Essentially go back to the state after
+// ao->init().
 void ao_reset(struct ao *ao)
 {
     if (ao->driver->reset)
         ao->driver->reset(ao);
 }
 
+// Pause playback. Keep the current buffer. ao_get_delay() must return the
+// same value as before pausing.
 void ao_pause(struct ao *ao)
 {
     if (ao->driver->pause)
         ao->driver->pause(ao);
 }
 
+// Resume playback. Play the remaining buffer. If the driver doesn't support
+// pausing, it has to work around this and e.g. use ao_play_silence() to fill
+// the lost audio.
 void ao_resume(struct ao *ao)
 {
     if (ao->driver->resume)

audio/out/ao.h

@@ -38,6 +38,8 @@ enum aocontrol {
     AOCONTROL_UPDATE_STREAM_TITLE,
 };
 
+// If set, then the queued audio data is the last. Note that after a while, new
+// data might be written again, instead of closing the AO.
 #define AOPLAY_FINAL_CHUNK 1
 
 typedef struct ao_control_vol {
@@ -48,11 +50,18 @@ typedef struct ao_control_vol {
 struct ao;
 
 struct ao_driver {
+    // If true, use with encoding only.
     bool encode;
+    // Name used for --ao.
     const char *name;
+    // Description shown with --ao=help.
     const char *description;
-    int (*control)(struct ao *ao, enum aocontrol cmd, void *arg);
+    // Init the device using ao->format/ao->channels/ao->samplerate. If the
+    // device doesn't accept these parameters, you can attempt to negotiate
+    // fallback parameters, and set the ao format fields accordingly.
     int (*init)(struct ao *ao);
+    // See ao_control() etc. in ao.c
+    int (*control)(struct ao *ao, enum aocontrol cmd, void *arg);
     void (*uninit)(struct ao *ao, bool cut_audio);
     void (*reset)(struct ao*ao);
     int (*get_space)(struct ao *ao);
@@ -71,7 +80,7 @@ struct ao_driver {
 struct ao {
     int samplerate;
     struct mp_chmap channels;
-    int format;
+    int format;                 // one of AF_FORMAT_...
     int bps;                    // bytes per second
     int sstride;                // size of a sample on each plane
                                 // (format_size*num_channels/num_planes)
@@ -80,7 +89,7 @@ struct ao {
     int buffer_playable_samples;// part of the part of the buffer the AO hasn't
                                 // accepted yet with play()
     bool probing;               // if true, don't fail loudly on init
-    bool untimed;
+    bool untimed;               // don't assume realtime playback
     bool no_persistent_volume;  // the AO does the equivalent of af_volume
     bool per_application_mixer; // like above, but volume persists (per app)
     const struct ao_driver *driver;