AP_HAL: added enable_flow_control() option in AP_HAL

libraries/AP_HAL/UARTDriver.h

@@ -38,6 +38,7 @@ public:
     virtual bool is_initialized() = 0;
     virtual void set_blocking_writes(bool blocking) = 0;
     virtual bool tx_pending() = 0;
+    virtual void enable_flow_control(bool enable) {};
 
     /* Implementations of BetterStream virtual methods. These are
      * provided by AP_HAL to ensure consistency between ports to

libraries/AP_HAL_PX4/UARTDriver.cpp

@@ -117,6 +117,9 @@ void PX4UARTDriver::begin(uint32_t b, uint16_t rxS, uint16_t txS)
 		cfsetspeed(&t, _baudrate);
 		// disable LF -> CR/LF
 		t.c_oflag &= ~ONLCR;
+        // always enable input flow control (notifying devices that we
+        // have space). This is harmless even if the pin is not connected
+        t.c_cflag |= CRTS_IFLOW;
 		tcsetattr(_fd, TCSANOW, &t);
 	}
 
@@ -129,6 +132,22 @@ void PX4UARTDriver::begin(uint32_t b, uint16_t rxS, uint16_t txS)
     }
 }
 
+void PX4UARTDriver::enable_flow_control(bool enable)
+{
+	if (_fd == -1) {
+        return;
+    }
+    struct termios t;
+    tcgetattr(_fd, &t);
+    // we already enabled CRTS_IFLOW above, just enable output flow control
+    if (enable) {
+        t.c_cflag |= CRTSCTS;
+    } else {
+        t.c_cflag &= ~CRTSCTS;
+    }
+    tcsetattr(_fd, TCSANOW, &t);
+}
+
 void PX4UARTDriver::begin(uint32_t b) 
 {
 	begin(b, 0, 0);

libraries/AP_HAL_PX4/UARTDriver.h

@@ -36,6 +36,8 @@ public:
 	    return _fd;
     }
 
+    void enable_flow_control(bool enable);
+
 private:
     const char *_devpath;
     int _fd;