MAVLink: Do a better job at receive rate limiting

src/modules/mavlink/mavlink_main.cpp

@@ -698,6 +698,8 @@ int Mavlink::mavlink_open_uart(int baud, const char *uart_name, struct termios *
 
 	} else {
 		_is_usb_uart = true;
+		/* USB has no baudrate, but use a magic number for 'fast' */
+		_baudrate = 2000000;
 	}
 
 #if defined (__PX4_LINUX) || defined (__PX4_DARWIN)

src/modules/mavlink/mavlink_main.h

@@ -288,6 +288,8 @@ public:
 
 	float			get_rate_mult();
 
+	float			get_baudrate() { return _baudrate; }
+
 	/* Functions for waiting to start transmission until message received. */
 	void			set_has_received_messages(bool received_messages) { _received_messages = received_messages; }
 	bool			get_has_received_messages() { return _received_messages; }

src/modules/mavlink/mavlink_receiver.cpp

@@ -1757,7 +1757,7 @@ MavlinkReceiver::receive_thread(void *arg)
 	uint8_t buf[1600];
 #else
 	/* the serial port buffers internally as well, we just need to fit a small chunk */
-	uint8_t buf[32];
+	uint8_t buf[64];
 #endif
 	mavlink_message_t msg;
 
@@ -1799,8 +1799,13 @@ MavlinkReceiver::receive_thread(void *arg)
 			if (_mavlink->get_protocol() == SERIAL) {
 				/* non-blocking read. read may return negative values */
 				if ((nread = ::read(uart_fd, buf, sizeof(buf))) < (ssize_t)sizeof(buf)) {
-					/* to avoid reading very small chunks wait for data before reading */
-					usleep(1000);
+					/*
+					 * to avoid reading very small chunks wait for data before reading
+					 * this is designed to target ~30 bytes at a time
+					 */
+					const unsigned character_count = 20;
+					unsigned sleeptime = (1.0f / (_mavlink->get_baudrate() / 10)) * character_count * 1000000;
+					usleep(sleeptime);
 				}
 			}
 #ifdef __PX4_POSIX