audioconvert: add monitor.passthrough option

Add a monitor.passthrough option. This will pass all latency information
directly between the port and its monitor ports.

This is interesting when the adapter (and audioconvert) is used with a
null-audio-sink that simply forwards the data to a real sink/souce. In
that case, we want the sink/source latency to be passed unmodified.

Set the monitor.passthrough on the pulseaudio null-sink because
a passthrough virtual sink is the most likely use case for this.

Add some monitor.passthrough default config where it makes sense.

Fixes #3888

spa/plugins/audioconvert/audioconvert.c

@@ -238,6 +238,7 @@ struct impl {
 	unsigned int drained:1;
 	unsigned int rate_adjust:1;
 	unsigned int port_ignore_latency:1;
+	unsigned int monitor_passthrough:1;
 
 	uint32_t scratch_size;
 	uint32_t scratch_ports;
@@ -2191,8 +2192,6 @@ impl_node_port_enum_params(void *object, int seq,
 		case 0: case 1:
 		{
 			uint32_t idx = result.index;
-			if (port->is_monitor)
-				idx = idx ^ 1;
 			param = spa_latency_build(&b, id, &port->latency[idx]);
 			break;
 		}
@@ -2258,9 +2257,6 @@ static int port_set_latency(void *object,
 			this, direction, port_id, latency);
 
 	port = GET_PORT(this, direction, port_id);
-	if (port->is_monitor)
-		return 0;
-
 	if (latency == NULL) {
 		info = SPA_LATENCY_INFO(other);
 		have_latency = false;
@@ -2282,10 +2278,26 @@ static int port_set_latency(void *object,
 			info.min_rate, info.max_rate,
 			info.min_ns, info.max_ns);
 
+	if (this->monitor_passthrough) {
+		if (port->is_monitor)
+			oport = GET_PORT(this, other, port_id-1);
+		else if (this->monitor && direction == SPA_DIRECTION_INPUT)
+			oport = GET_PORT(this, other, port_id+1);
+		else
+			return 0;
+
+		if (oport != NULL &&
+		    spa_latency_info_compare(&info, &oport->latency[other]) != 0) {
+			oport->latency[other] = info;
+			oport->info.change_mask |= SPA_PORT_CHANGE_MASK_PARAMS;
+			oport->params[IDX_Latency].user++;
+			emit_port_info(this, oport, false);
+		}
+	} else {
 		spa_latency_info_combine_start(&info, other);
 		for (i = 0; i < this->dir[direction].n_ports; i++) {
 			oport = GET_PORT(this, direction, i);
-		if (oport->is_monitor || !oport->have_latency)
+			if ((oport->is_monitor) || !oport->have_latency)
 				continue;
 			spa_log_debug(this->log, "%p: combine %d", this, i);
 			spa_latency_info_combine(&info, &oport->latency[other]);
@@ -2300,7 +2312,8 @@ static int port_set_latency(void *object,
 
 		for (i = 0; i < this->dir[other].n_ports; i++) {
 			oport = GET_PORT(this, other, i);
-
+			if (oport->is_monitor)
+				continue;
 			spa_log_debug(this->log, "%p: change %d", this, i);
 			if (spa_latency_info_compare(&info, &oport->latency[other]) != 0) {
 				oport->latency[other] = info;
@@ -2309,6 +2322,7 @@ static int port_set_latency(void *object,
 				emit_port_info(this, oport, false);
 			}
 		}
+	}
 	if (emit) {
 		port->info.change_mask |= SPA_PORT_CHANGE_MASK_PARAMS;
 		port->params[IDX_Latency].user++;
@@ -2332,7 +2346,7 @@ static int port_set_tag(void *object,
 			this, direction, port_id, tag);
 
 	port = GET_PORT(this, direction, port_id);
-	if (port->is_monitor)
+	if (port->is_monitor && !this->monitor_passthrough)
 		return 0;
 
 	if (tag != NULL) {
@@ -3428,6 +3442,8 @@ impl_init(const struct spa_handle_factory *factory,
 		}
 		else if (spa_streq(k, SPA_KEY_PORT_IGNORE_LATENCY))
 			this->port_ignore_latency = spa_atob(s);
+		else if (spa_streq(k, "monitor.passthrough"))
+			this->monitor_passthrough = spa_atob(s);
 		else
 			audioconvert_set_param(this, k, s);
 	}

src/daemon/minimal.conf.in

@@ -319,6 +319,7 @@ context.objects = [
     #        node.description = "Microphone"
     #        media.class      = "Audio/Source/Virtual"
     #        audio.position   = "FL,FR"
+    #        monitor.passthrough = true
     #        adapter.auto-port-config = {
     #            mode = dsp
     #            monitor = true

src/daemon/pipewire.conf.in

@@ -255,6 +255,7 @@ context.objects = [
     #        node.description = "Microphone"
     #        media.class      = "Audio/Source/Virtual"
     #        audio.position   = "FL,FR"
+    #        monitor.passthrough = true
     #    }
     #}
 

src/modules/module-protocol-pulse/modules/module-null-sink.c

@@ -196,6 +196,8 @@ static int module_null_sink_prepare(struct module * const module)
 
 	if (pw_properties_get(props, "monitor.channel-volumes") == NULL)
 		pw_properties_set(props, "monitor.channel-volumes", "true");
+	if (pw_properties_get(props, "monitor.passthrough") == NULL)
+		pw_properties_set(props, "monitor.passthrough", "true");
 
 	return 0;
 }