PX4: moved UART handling to its own thread

this prevents slow writes to the SD card from interfering with MAVLink
or GPS serial communication

libraries/AP_HAL_PX4/Scheduler.cpp

@@ -50,6 +50,16 @@ void PX4Scheduler::init(void *unused)
 
 	pthread_create(&_timer_thread_ctx, &thread_attr, (pthread_startroutine_t)&PX4::PX4Scheduler::_timer_thread, this);
 
+    // the UART thread runs at a medium priority
+	pthread_attr_init(&thread_attr);
+	pthread_attr_setstacksize(&thread_attr, 2048);
+
+	param.sched_priority = APM_UART_PRIORITY;
+	(void)pthread_attr_setschedparam(&thread_attr, &param);
+    pthread_attr_setschedpolicy(&thread_attr, SCHED_FIFO);
+
+	pthread_create(&_uart_thread_ctx, &thread_attr, (pthread_startroutine_t)&PX4::PX4Scheduler::_uart_thread, this);
+
     // the IO thread runs at lower priority
 	pthread_attr_init(&thread_attr);
 	pthread_attr_setstacksize(&thread_attr, 2048);
@@ -239,7 +249,7 @@ void PX4Scheduler::_run_io(void)
     _in_io_proc = false;
 }
 
-void *PX4Scheduler::_io_thread(void)
+void *PX4Scheduler::_uart_thread(void)
 {
     while (!_px4_thread_should_exit) {
         poll(NULL, 0, 1);
@@ -248,6 +258,14 @@ void *PX4Scheduler::_io_thread(void)
         ((PX4UARTDriver *)hal.uartA)->_timer_tick();
         ((PX4UARTDriver *)hal.uartB)->_timer_tick();
         ((PX4UARTDriver *)hal.uartC)->_timer_tick();
+    }
+    return NULL;
+}
+
+void *PX4Scheduler::_io_thread(void)
+{
+    while (!_px4_thread_should_exit) {
+        poll(NULL, 0, 1);
 
         // process any pending storage writes
         ((PX4Storage *)hal.storage)->_timer_tick();

libraries/AP_HAL_PX4/Scheduler.h

@@ -14,7 +14,8 @@
 
 #define APM_MAIN_PRIORITY    180
 #define APM_TIMER_PRIORITY   181
-#define APM_IO_PRIORITY       60
+#define APM_UART_PRIORITY     60
+#define APM_IO_PRIORITY       59
 #define APM_OVERTIME_PRIORITY 10
 #define APM_STARTUP_PRIORITY  10
 
@@ -64,9 +65,11 @@ private:
 
     pthread_t _timer_thread_ctx;
     pthread_t _io_thread_ctx;
+    pthread_t _uart_thread_ctx;
 
     void *_timer_thread(void);
     void *_io_thread(void);
+    void *_uart_thread(void);
 
     void _run_timers(bool called_from_timer_thread);
     void _run_io(void);