bluetooth: allow increasing output bitrate

Bluetooth thread may ask encoder to reduce bitrate if writing is not keeping up
with inputs or writing to bluetooth socket takes too much time.

Assuming conditions leading to reduced bitrate are intermittent, allow periodic
attempts to increase encoder bitrate, by default at most twice per second.

Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/481>

src/modules/bluetooth/a2dp-codec-api.h

@@ -88,6 +88,11 @@ typedef struct pa_a2dp_codec {
      * enough */
     size_t (*reduce_encoder_bitrate)(void *codec_info, size_t write_link_mtu);
 
+    /* Increase encoder bitrate for codec, returns new write block size or zero
+     * if not changed, called periodically when socket is keeping up with
+     * encoded data */
+    size_t (*increase_encoder_bitrate)(void *codec_info, size_t write_link_mtu);
+
     /* Encode input_buffer of input_size to output_buffer of output_size,
      * returns size of filled ouput_buffer and set processed to size of
      * processed input_buffer */

src/modules/bluetooth/bluez5-util.h

@@ -110,6 +110,7 @@ struct pa_bluetooth_device {
     bool valid;
     bool autodetect_mtu;
     bool codec_switching_in_progress;
+    uint32_t output_rate_refresh_interval_ms;
 
     /* Device information */
     char *path;

src/modules/bluetooth/module-bluez5-device.c

@@ -66,6 +66,8 @@ PA_MODULE_USAGE("path=<device object path>"
 
 #define HSP_MAX_GAIN 15
 
+#define DEFAULT_OUTPUT_RATE_REFRESH_INTERVAL_MS 500
+
 static const char* const valid_modargs[] = {
     "path",
     "autodetect_mtu",
@@ -1362,6 +1364,8 @@ static void thread_func(void *userdata) {
     struct userdata *u = userdata;
     unsigned blocks_to_write = 0;
     unsigned bytes_to_write = 0;
+    struct timeval tv_last_output_rate_change;
+    pa_usec_t ts_elapsed;
 
     pa_assert(u);
     pa_assert(u->transport);
@@ -1377,6 +1381,8 @@ static void thread_func(void *userdata) {
     if (u->transport_acquired)
         setup_stream(u);
 
+    pa_gettimeofday(&tv_last_output_rate_change);
+
     for (;;) {
         struct pollfd *pollfd;
         int ret;
@@ -1525,6 +1531,7 @@ static void thread_func(void *userdata) {
                                     u->write_block_size = new_write_block_size;
                                     handle_sink_block_size_change(u);
                                 }
+                                pa_gettimeofday(&tv_last_output_rate_change);
                             }
                         }
 
@@ -1557,6 +1564,19 @@ static void thread_func(void *userdata) {
                             next_write_at = pa_bytes_to_usec(u->write_index, &u->encoder_sample_spec);
                             sleep_for = time_passed < next_write_at ? next_write_at - time_passed : 0;
                             /* pa_log("Sleeping for %lu; time passed %lu, next write at %lu", (unsigned long) sleep_for, (unsigned long) time_passed, (unsigned long)next_write_at); */
+
+                            if (u->profile == PA_BLUETOOTH_PROFILE_A2DP_SINK && u->write_memchunk.memblock == NULL) {
+                                /* write_block() is keeping up with input, try increasing bitrate */
+                                if (u->a2dp_codec->increase_encoder_bitrate
+                                    && pa_timeval_age(&tv_last_output_rate_change) >= u->device->output_rate_refresh_interval_ms * PA_USEC_PER_MSEC) {
+                                    size_t new_write_block_size = u->a2dp_codec->increase_encoder_bitrate(u->encoder_info, u->write_link_mtu);
+                                    if (new_write_block_size) {
+                                        u->write_block_size = new_write_block_size;
+                                        handle_sink_block_size_change(u);
+                                    }
+                                    pa_gettimeofday(&tv_last_output_rate_change);
+                                }
+                            }
                         } else
                             /* We could not write because the stream was not ready. Let's try
                              * again in 500 ms and drop audio if we still can't write. The
@@ -2488,6 +2508,7 @@ int pa__init(pa_module* m) {
     pa_modargs *ma;
     bool autodetect_mtu;
     char *message_handler_path;
+    uint32_t output_rate_refresh_interval_ms = DEFAULT_OUTPUT_RATE_REFRESH_INTERVAL_MS;
 
     pa_assert(m);
 
@@ -2526,6 +2547,13 @@ int pa__init(pa_module* m) {
 
     u->device->autodetect_mtu = autodetect_mtu;
 
+    if (pa_modargs_get_value_u32(ma, "output-rate-refresh-interval-ms", &output_rate_refresh_interval_ms) < 0) {
+        pa_log("Invalid output_rate_refresh_interval.");
+        goto fail_free_modargs;
+    }
+
+    u->device->output_rate_refresh_interval_ms = output_rate_refresh_interval_ms;
+
     pa_modargs_free(ma);
 
     u->device_connection_changed_slot =