Use MAVLink v1 only as opt-in (#25583)

* Remove support for MAVLink 1

* Add back support for MAVLink 1 but don't default to it

* Update src/modules/mavlink/mavlink_params.c

Co-authored-by: Hamish Willee <hamishwillee@gmail.com>

---------

Co-authored-by: Jacob Dahl <37091262+dakejahl@users.noreply.github.com>
Co-authored-by: Hamish Willee <hamishwillee@gmail.com>

src/modules/mavlink/mavlink_main.cpp

@@ -187,12 +187,7 @@ Mavlink::mavlink_update_parameters()
 {
 	updateParams();
 
-	int32_t proto = _param_mav_proto_ver.get();
-
-	if (_protocol_version_switch != proto) {
-		_protocol_version_switch = proto;
-		set_proto_version(proto);
-	}
+	set_protocol_version(_param_mav_proto_ver.get());
 
 	if (_param_mav_type.get() < 0 || _param_mav_type.get() >= MAV_TYPE_ENUM_END) {
 		_param_mav_type.set(0);
@@ -277,16 +272,13 @@ Mavlink::set_instance_id()
 	return false;
 }
 
-void
-Mavlink::set_proto_version(unsigned version)
+void Mavlink::set_protocol_version(unsigned version)
 {
-	if ((version == 1 || version == 0) &&
-	    ((_protocol_version_switch == 0) || (_protocol_version_switch == 1))) {
+	if (version == 1) {
 		get_status()->flags |= MAVLINK_STATUS_FLAG_OUT_MAVLINK1;
 		_protocol_version = 1;
 
-	} else if (version == 2 &&
-		   ((_protocol_version_switch == 0) || (_protocol_version_switch == 2))) {
+	} else {
 		get_status()->flags &= ~(MAVLINK_STATUS_FLAG_OUT_MAVLINK1);
 		_protocol_version = 2;
 	}
@@ -1156,13 +1148,7 @@ Mavlink::send_protocol_version()
 	//memcpy(&msg.spec_version_hash, &mavlink_spec_git_version_binary, sizeof(msg.spec_version_hash));
 	memcpy(&msg.library_version_hash, &mavlink_lib_git_version_binary, sizeof(msg.library_version_hash));
 
-	// Switch to MAVLink 2
-	int curr_proto_ver = _protocol_version;
-	set_proto_version(2);
-	// Send response - if it passes through the link its fine to use MAVLink 2
 	mavlink_msg_protocol_version_send_struct(get_channel(), &msg);
-	// Reset to previous value
-	set_proto_version(curr_proto_ver);
 }
 
 int

src/modules/mavlink/mavlink_main.h

@@ -157,14 +157,7 @@ public:
 
 	mavlink_status_t 	*get_status() { return &_mavlink_status; }
 
-	/**
-	 * Set the MAVLink version
-	 *
-	 * Currently supporting v1 and v2
-	 *
-	 * @param version MAVLink version
-	 */
-	void			set_proto_version(unsigned version);
+	void			set_protocol_version(unsigned version);
 
 	static int		destroy_all_instances();
 
@@ -620,8 +613,7 @@ private:
 	uint64_t		_last_write_success_time{0};
 	uint64_t		_last_write_try_time{0};
 	uint64_t		_mavlink_start_time{0};
-	int32_t			_protocol_version_switch{-1};
-	int32_t			_protocol_version{0};
+	int32_t _protocol_version = 0; ///< after initialization the only values are 1 and 2
 
 	unsigned		_bytes_tx{0};
 	unsigned		_bytes_txerr{0};

src/modules/mavlink/mavlink_params.c

@@ -52,11 +52,10 @@ PARAM_DEFINE_INT32(MAV_COMP_ID, 1);
 /**
  * MAVLink protocol version
  * @group MAVLink
- * @value 0 Default to 1, switch to 2 if GCS sends version 2
- * @value 1 Always use version 1
- * @value 2 Always use version 2
+ * @value 1 Version 1 with auto-upgrade to v2 if detected
+ * @value 2 Version 2
  */
-PARAM_DEFINE_INT32(MAV_PROTO_VER, 0);
+PARAM_DEFINE_INT32(MAV_PROTO_VER, 2);
 
 /**
  * MAVLink SiK Radio ID

src/modules/mavlink/mavlink_receiver.cpp

@@ -3228,10 +3228,10 @@ MavlinkReceiver::run()
 				for (ssize_t i = 0; i < nread; i++) {
 					if (mavlink_parse_char(_mavlink.get_channel(), buf[i], &msg, &_status)) {
 
-						/* check if we received version 2 and request a switch. */
+						// If we receive a complete MAVLink 2 packet, also switch the outgoing protocol version
 						if (!(_mavlink.get_status()->flags & MAVLINK_STATUS_FLAG_IN_MAVLINK1)) {
-							/* this will only switch to proto version 2 if allowed in settings */
-							_mavlink.set_proto_version(2);
+							PX4_INFO("Upgrade to MAVLink v2 because of incoming packet");
+							_mavlink.set_protocol_version(2);
 						}
 
 						switch (_mavlink.get_mode()) {