Improve IridiumSBD TX buffer handling

- Every time data is written to the iridium module all previous untransmitted data is overwritten
- The buffer is reset only on the following conditions
    - A sbd session is successfully completed
    - An incoming mavlink message would result in less than SATCOM_MIN_TX_BUF_SPACE free bytes after it is written
    - Any other incomming data would result in less than SATCOM_MIN_TX_BUF_SPACE free bytes after it is written to the buffer.

src/drivers/telemetry/iridiumsbd/IridiumSBD.cpp

@@ -78,7 +78,7 @@ int IridiumSBD::start(int argc, char *argv[])
 	IridiumSBD::instance = new IridiumSBD();
 
 	IridiumSBD::task_handle = px4_task_spawn_cmd("iridiumsbd", SCHED_DEFAULT,
-				  SCHED_PRIORITY_SLOW_DRIVER, 1200, (main_t)&IridiumSBD::main_loop_helper, argv);
+				  SCHED_PRIORITY_SLOW_DRIVER, 1300, (main_t)&IridiumSBD::main_loop_helper, argv);
 
 	return OK;
 }
@@ -463,6 +463,9 @@ void IridiumSBD::sbdsession_loop(void)
 			rx_read_pending = true;
 		}
 
+		// after a successful session reset the tx buffer
+		tx_buf_write_idx = 0;
+
 		publish_telemetry_status();
 
 		break;
@@ -472,6 +475,9 @@ void IridiumSBD::sbdsession_loop(void)
 		last_heartbeat = hrt_absolute_time();
 		publish_telemetry_status();
 
+		// after a successful session reset the tx buffer
+		tx_buf_write_idx = 0;
+
 		tx_session_pending = false;
 		break;
 
@@ -579,6 +585,22 @@ void IridiumSBD::start_test(void)
 
 ssize_t IridiumSBD::write(struct file *filp, const char *buffer, size_t buflen)
 {
+	// check and reset the remaining buffer space for incoming MavLink messages
+	if (buflen == 6) {
+		if (*buffer == MAVLINK_PACKAGE_START) {
+			if (SATCOM_TX_BUF_LEN - tx_buf_write_idx - SATCOM_MIN_TX_BUF_SPACE - (*(buffer + 1) + 8) < 0) {
+				tx_buf_write_idx = 0;
+				PX4_INFO("Deleting full TX buffer before writing new message");
+			}
+		}
+	}
+
+	// check and reset the remaining buffer space for any non mavlink messages
+	if (SATCOM_TX_BUF_LEN - tx_buf_write_idx - SATCOM_MIN_TX_BUF_SPACE < 0) {
+		tx_buf_write_idx = 0;
+		PX4_INFO("Deleting full TX buffer");
+	}
+
 	VERBOSE_INFO("WRITE: LEN %d, TX WRITTEN: %d", buflen, tx_buf_write_idx);
 
 	if ((ssize_t)buflen > SATCOM_TX_BUF_LEN - tx_buf_write_idx) {
@@ -672,8 +694,6 @@ void IridiumSBD::write_tx_buf()
 
 	VERBOSE_INFO("WRITE SBD: DATA WRITTEN TO MODEM");
 
-	tx_buf_write_idx = 0;
-
 	pthread_mutex_unlock(&tx_buf_mutex);
 
 	tx_session_pending = true;

src/drivers/telemetry/iridiumsbd/IridiumSBD.h

@@ -84,10 +84,21 @@ typedef enum {
 extern "C" __EXPORT int iridiumsbd_main(int argc, char *argv[]);
 
 #define SATCOM_TX_BUF_LEN			340		// TX buffer size - maximum for a SBD MO message is 340, but billed per 50
+#define SATCOM_MIN_TX_BUF_SPACE		50		// Minimum remaining buffer space required, should be equal to the largerst message intended to send
 #define SATCOM_RX_MSG_BUF_LEN			270		// RX buffer size for MT messages
 #define SATCOM_RX_COMMAND_BUF_LEN		50		// RX buffer size for other commands
 #define SATCOM_SIGNAL_REFRESH_DELAY		20000000 // update signal quality every 20s
+#define MAVLINK_PACKAGE_START		254 // The value of the first byte of the mavlink header
 
+
+/**
+ * The driver for the Rockblock 9602 and 9603 RockBlock module for satellite communication over the Iridium satellite system
+ *
+ * TODO:
+ * 	- Improve TX buffer handling:
+ * 		- Do not reset the full TX buffer but delete the oldest HIGH_LATENCY2 message if one is in the buffer or delete the oldest message in general
+ * 	- Keep CDev active even if the driver is stopped to avoid a hard fault caused by MavLink.
+ */
 class IridiumSBD : public device::CDev
 {
 public: