diff --git a/libraries/AP_HAL_ChibiOS/UARTDriver.cpp b/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
index 604f94210d..6f612097f2 100644
--- a/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
+++ b/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
@@ -404,7 +404,7 @@ void UARTDriver::dma_tx_deallocate(Shared_DMA *ctx)
 }
 
 /*
-  DMA transmit complettion interrupt handler
+  DMA transmit completion interrupt handler
  */
 void UARTDriver::tx_complete(void* self, uint32_t flags)
 {
@@ -605,12 +605,19 @@ int16_t UARTDriver::read_locked(uint32_t key)
     return byte;
 }
 
-/* Empty implementations of Print virtual methods */
+/* write one byte to the port */
 size_t UARTDriver::write(uint8_t c)
 {
-    if (lock_write_key != 0 || !_write_mutex.take_nonblocking()) {
+    if (lock_write_key != 0) {
         return 0;
     }
+    if (unbuffered_writes && _blocking_writes) {
+        _write_mutex.take_blocking();
+    } else {
+        if (!_write_mutex.take_nonblocking()) {
+            return 0;
+        }
+    }
 
     if (!_initialised) {
         _write_mutex.give();
@@ -632,6 +639,7 @@ size_t UARTDriver::write(uint8_t c)
     return ret;
 }
 
+/* write a block of bytes to the port */
 size_t UARTDriver::write(const uint8_t *buffer, size_t size)
 {
     if (!_initialised || lock_write_key != 0) {
@@ -771,6 +779,7 @@ void UARTDriver::handle_tx_timeout(void *arg)
  */
 void UARTDriver::write_pending_bytes_DMA(uint32_t n)
 {
+    WITH_SEMAPHORE(_write_mutex);
     check_dma_tx_completion();
 
     if (!tx_bounce_buf_ready) {
@@ -823,6 +832,8 @@ void UARTDriver::write_pending_bytes_DMA(uint32_t n)
  */
 void UARTDriver::write_pending_bytes_NODMA(uint32_t n)
 {
+    WITH_SEMAPHORE(_write_mutex);
+
     ByteBuffer::IoVec vec[2];
     uint16_t nwritten = 0;
 
@@ -937,11 +948,13 @@ void UARTDriver::half_duplex_setup_tx(void)
 #ifdef HAVE_USB_SERIAL
     if (!hd_tx_active) {
         chEvtGetAndClearFlags(&hd_listener);
-        SerialDriver *sd = (SerialDriver*)(sdef.serial);
         hd_tx_active = true;
-        sdStop(sd);
-        sercfg.cr3 &= ~USART_CR3_HDSEL;
-        sdStart(sd, &sercfg);
+        if (_last_options & (OPTION_RXINV | OPTION_TXINV)) {
+            SerialDriver *sd = (SerialDriver*)(sdef.serial);
+            sdStop(sd);
+            sercfg.cr3 &= ~USART_CR3_HDSEL;
+            sdStart(sd, &sercfg);
+        }
     }
 #endif
 }
@@ -962,10 +975,12 @@ void UARTDriver::_timer_tick(void)
           half-duplex transmit has finished. We now re-enable the
           HDSEL bit for receive
          */
-        SerialDriver *sd = (SerialDriver*)(sdef.serial);
-        sdStop(sd);
-        sercfg.cr3 |= USART_CR3_HDSEL;
-        sdStart(sd, &sercfg);
+        if (_last_options & (OPTION_RXINV | OPTION_TXINV)) {
+            SerialDriver *sd = (SerialDriver*)(sdef.serial);
+            sdStop(sd);
+            sercfg.cr3 |= USART_CR3_HDSEL;
+            sdStart(sd, &sercfg);
+        }
         hd_tx_active = false;
     }
 #endif
@@ -1065,7 +1080,7 @@ void UARTDriver::_timer_tick(void)
         // provided by the write() call, but if the write is larger
         // than the DMA buffer size then there can be extra bytes to
         // send here, and they must be sent with the write lock held
-        _write_mutex.take(HAL_SEMAPHORE_BLOCK_FOREVER);
+        _write_mutex.take_blocking();
         write_pending_bytes();
         _write_mutex.give();
     } else {
@@ -1159,16 +1174,8 @@ void UARTDriver::update_rts_line(void)
  */
 bool UARTDriver::set_unbuffered_writes(bool on)
 {
-#ifdef HAL_UART_NODMA
-    return false;
-#else
-    if (on && !tx_dma_enabled) {
-        // we can't implement low latemcy writes safely without TX DMA
-        return false;
-    }
     unbuffered_writes = on;
     return true;
-#endif
 }
 
 /*
diff --git a/libraries/AP_HAL_ChibiOS/UARTDriver.h b/libraries/AP_HAL_ChibiOS/UARTDriver.h
index 2b243cac2f..87efeb5586 100644
--- a/libraries/AP_HAL_ChibiOS/UARTDriver.h
+++ b/libraries/AP_HAL_ChibiOS/UARTDriver.h
@@ -159,7 +159,7 @@ private:
 #endif
     ByteBuffer _readbuf{0};
     ByteBuffer _writebuf{0};
-    Semaphore _write_mutex;
+    HAL_Semaphore_Recursive _write_mutex;
 #ifndef HAL_UART_NODMA
     const stm32_dma_stream_t* rxdma;
     const stm32_dma_stream_t* txdma;