HAL_ChibiOS: replace volatile bools with mutexes

this replaces the two booleans used to mediate TX and RX buffer
protection with mutexes.

The booleans were a hangover from the very early HAL_ChibiOS code, and
can lead to a deadlock. The sequence is as follows:

 - a very high CAN bus bandwidth usage, triggered by MissionPlanner
   requesting CAN_FORWARD on a CAN serial port. That causes a
   "infinite" number of CAN_FRAME messages which saturates the bus,
   and leads to the DroneCAN thread looping with no pause

 - a serial port configured as GPS type AUTO, auto-probing for a GPS
   that isn't there. This calls begin() periodically

 - the UART TX thread assocated with that UART not making progress as
   the TX thread priority is below the DroneCAN thread priority

 - this causes the begin() in main thread waiting for _in_tx_timer to
   loop forever, which triggers a watchdog

libraries/AP_HAL_ChibiOS/UARTDriver.cpp

@@ -295,9 +295,7 @@ void UARTDriver::_begin(uint32_t b, uint16_t rxS, uint16_t txS)
       thrashing of the heap once we are up. The ttyACM0 driver may not
       connect for some time after boot
      */
-    while (_in_rx_timer) {
-        hal.scheduler->delay(1);
-    }
+    WITH_SEMAPHORE(rx_sem);
     if (rxS != _readbuf.get_size()) {
         _rx_initialised = false;
         _readbuf.set_size_best(rxS);
@@ -359,9 +357,7 @@ void UARTDriver::_begin(uint32_t b, uint16_t rxS, uint16_t txS)
     /*
       allocate the write buffer
      */
-    while (_in_tx_timer) {
-        hal.scheduler->delay(1);
-    }
+    WITH_SEMAPHORE(tx_sem);
     if (txS != _writebuf.get_size()) {
         _tx_initialised = false;
         _writebuf.set_size_best(txS);
@@ -640,9 +636,9 @@ __RAMFUNC__ void UARTDriver::rxbuff_full_irq(void* self, uint32_t flags)
 
 void UARTDriver::_end()
 {
-    while (_in_rx_timer) hal.scheduler->delay(1);
+    WITH_SEMAPHORE(rx_sem);
+    WITH_SEMAPHORE(tx_sem);
     _rx_initialised = false;
-    while (_in_tx_timer) hal.scheduler->delay(1);
     _tx_initialised = false;
 
     if (sdef.is_usb) {
@@ -1112,7 +1108,7 @@ void UARTDriver::_rx_timer_tick(void)
         return;
     }
 
-    _in_rx_timer = true;
+    WITH_SEMAPHORE(rx_sem);
 
 #if HAL_UART_STATS_ENABLED && CH_CFG_USE_EVENTS == TRUE
     if (!sdef.is_usb) {
@@ -1165,7 +1161,6 @@ void UARTDriver::_rx_timer_tick(void)
     if (sdef.is_usb) {
 #ifdef HAVE_USB_SERIAL
         if (((SerialUSBDriver*)sdef.serial)->config->usbp->state != USB_ACTIVE) {
-            _in_rx_timer = false;
             return;
         }
 #endif
@@ -1189,7 +1184,6 @@ void UARTDriver::_rx_timer_tick(void)
         fwd_otg2_serial();
     }
 #endif
-    _in_rx_timer = false;
 }
 
 // forward data from a serial port to the USB
@@ -1308,14 +1302,13 @@ void UARTDriver::_tx_timer_tick(void)
         return;
     }
 
-    _in_tx_timer = true;
-
     if (hd_tx_active) {
+        WITH_SEMAPHORE(tx_sem);
         hd_tx_active &= ~chEvtGetAndClearFlags(&hd_listener);
         if (!hd_tx_active) {
             /*
-                half-duplex transmit has finished. We now re-enable the
-                HDSEL bit for receive
+              half-duplex transmit has finished. We now re-enable the
+              HDSEL bit for receive
             */
             SerialDriver *sd = (SerialDriver*)(sdef.serial);
             sdStop(sd);
@@ -1328,7 +1321,6 @@ void UARTDriver::_tx_timer_tick(void)
     if (sdef.is_usb) {
 #ifdef HAVE_USB_SERIAL
         if (((SerialUSBDriver*)sdef.serial)->config->usbp->state != USB_ACTIVE) {
-            _in_tx_timer = false;
             return;
         }
 #endif
@@ -1341,18 +1333,16 @@ void UARTDriver::_tx_timer_tick(void)
 
     // half duplex we do reads in the write thread
     if (half_duplex) {
-        _in_rx_timer = true;
+        WITH_SEMAPHORE(rx_sem);
         read_bytes_NODMA();
         if (_wait.thread_ctx && _readbuf.available() >= _wait.n) {
             chEvtSignal(_wait.thread_ctx, EVT_DATA);
         }
-        _in_rx_timer = false;
     }
 
     // now do the write
+    WITH_SEMAPHORE(tx_sem);
     write_pending_bytes();
-
-    _in_tx_timer = false;
 }
 
 /*

libraries/AP_HAL_ChibiOS/UARTDriver.h

@@ -196,8 +196,8 @@ class ChibiOS::UARTDriver : public AP_HAL::UARTDriver {
     const stm32_dma_stream_t* rxdma;
     const stm32_dma_stream_t* txdma;
 #endif
-    volatile bool _in_rx_timer;
-    volatile bool _in_tx_timer;
+    HAL_Semaphore tx_sem;
+    HAL_Semaphore rx_sem;
     volatile bool _rx_initialised;
     volatile bool _tx_initialised;
     volatile bool _device_initialised;