bluez5: media-source: add option to control the target latency of the decode-buffer

On production systems, having a constant high latency is favored over
dynamically adjusting it in order to optimize for low latency,
because every time a dynamic adjustment happens, there's a glitch.

This adds an option to let the user specify the exact amount of latency
they want.

doc/dox/config/pipewire-props.7.md

@@ -1196,6 +1196,11 @@ this instance. Available values:
   - input: appear as source node.
 \endparblock
 
+@PAR@ node-prop  bluez5.decode-buffer.latency   # integer
+Applies on media source nodes and defines the target amount
+of samples to be buffered on the output of the decoder.
+Default: 0, which means it is automatically determined.
+
 @PAR@ node-prop  node.latency-offset-msec   # string
 Applies only for BLE MIDI nodes.
 Latency adjustment to apply on the node. Larger values add a

spa/plugins/bluez5/media-source.c

@@ -141,6 +141,8 @@ struct impl {
 	unsigned int is_duplex:1;
 	unsigned int is_internal:1;
 
+	unsigned int decode_buffer_target;
+
 	unsigned int node_latency;
 
 	int fd;
@@ -885,6 +887,8 @@ static int transport_start(struct impl *this)
 			this->quantum_limit, this->quantum_limit)) < 0)
 		return res;
 
+	spa_bt_decode_buffer_set_target_latency(&port->buffer, (int32_t) this->decode_buffer_target);
+
 	if (this->codec->kind == MEDIA_CODEC_HFP) {
 		/* 40 ms max buffer (on top of duration) */
 		spa_bt_decode_buffer_set_max_extra_latency(&port->buffer,
@@ -2057,6 +2061,9 @@ impl_init(const struct spa_handle_factory *factory,
 			this->is_duplex = spa_atob(str);
 		if ((str = spa_dict_lookup(info, "api.bluez5.internal")) != NULL)
 			this->is_internal = spa_atob(str);
+		if ((str = spa_dict_lookup(info, "bluez5.decode-buffer.latency")) != NULL) {
+			spa_atou32(str, &this->decode_buffer_target, 0);
+		}
 	}
 
 	if (this->is_duplex) {