Change arming transfer to only set the register if the local configuration changed. Move its write operation to the fast rate so that arming / disarming is instantaneous

src/drivers/px4io/px4io.cpp

@@ -273,6 +273,7 @@ private:
 	/* cached IO state */
 	uint16_t		_status;		///< Various IO status flags
 	uint16_t		_alarms;		///< Various IO alarms
+	uint16_t		_last_written_arming;	///< the last written arming state reg
 
 	/* subscribed topics */
 	int			_t_actuator_controls_0;	///< actuator controls group 0 topic
@@ -509,6 +510,7 @@ PX4IO::PX4IO(device::Device *interface) :
 	_perf_sample_latency(perf_alloc(PC_ELAPSED, "io latency")),
 	_status(0),
 	_alarms(0),
+	_last_written_arming(0),
 	_t_actuator_controls_0(-1),
 	_t_actuator_controls_1(-1),
 	_t_actuator_controls_2(-1),
@@ -977,7 +979,7 @@ PX4IO::task_main()
 		}
 
 		if (now >= poll_last + IO_POLL_INTERVAL) {
-			/* run at 50Hz */
+			/* run at 50-250Hz */
 			poll_last = now;
 
 			/* pull status and alarms from IO */
@@ -988,16 +990,6 @@ PX4IO::task_main()
 
 			/* fetch PWM outputs from IO */
 			io_publish_pwm_outputs();
-		}
-
-		if (now >= orb_check_last + ORB_CHECK_INTERVAL) {
-			/* run at 5Hz */
-			orb_check_last = now;
-
-			/* try to claim the MAVLink log FD */
-			if (_mavlink_fd < 0) {
-				_mavlink_fd = ::open(MAVLINK_LOG_DEVICE, 0);
-			}
 
 			/* check updates on uORB topics and handle it */
 			bool updated = false;
@@ -1012,6 +1004,19 @@ PX4IO::task_main()
 			if (updated) {
 				io_set_arming_state();
 			}
+		}
+
+		if (now >= orb_check_last + ORB_CHECK_INTERVAL) {
+			/* run at 5Hz */
+			orb_check_last = now;
+
+			/* try to claim the MAVLink log FD */
+			if (_mavlink_fd < 0) {
+				_mavlink_fd = ::open(MAVLINK_LOG_DEVICE, 0);
+			}
+
+			/* check updates on uORB topics and handle it */
+			bool updated = false;
 
 			/* vehicle command */
 			orb_check(_t_vehicle_command, &updated);
@@ -1350,7 +1355,15 @@ PX4IO::io_set_arming_state()
 		}
 	}
 
-	return io_reg_modify(PX4IO_PAGE_SETUP, PX4IO_P_SETUP_ARMING, clear, set);
+	uint16_t new_arming_state = set;
+	new_arming_state &= ~(clear);
+
+	if (_last_written_arming != new_arming_state) {
+		_last_written_arming = new_arming_state;
+		return io_reg_modify(PX4IO_PAGE_SETUP, PX4IO_P_SETUP_ARMING, clear, set);
+	}
+
+	return 0;
 }
 
 int