diff --git a/libraries/AP_HAL_ChibiOS/UARTDriver.cpp b/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
index 116d4472b1..d2053bb843 100644
--- a/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
+++ b/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
@@ -80,6 +80,13 @@ static const eventmask_t EVT_TRANSMIT_UNBUFFERED = EVENT_MASK(3);
 #define HAL_UART_RX_STACK_SIZE 768
 #endif
 
+// threshold for disabling TX DMA due to contention
+#if defined(USART_CR1_FIFOEN)
+#define CONTENTION_BAUD_THRESHOLD 460800
+#else
+#define CONTENTION_BAUD_THRESHOLD 115200
+#endif
+
 UARTDriver::UARTDriver(uint8_t _serial_num) :
 serial_num(_serial_num),
 sdef(_serial_tab[_serial_num]),
@@ -306,6 +313,15 @@ void UARTDriver::begin(uint32_t b, uint16_t rxS, uint16_t txS)
         rx_dma_enabled = rx_bounce_buf[0] != nullptr && rx_bounce_buf[1] != nullptr;
         tx_dma_enabled = tx_bounce_buf != nullptr;
     }
+    if (contention_counter > 1000 && _baudrate <= CONTENTION_BAUD_THRESHOLD) {
+        // we've previously disabled TX DMA due to contention, don't
+        // re-enable on a new begin() unless high baudrate
+        tx_dma_enabled = false;
+    }
+    if (_baudrate <= 115200 && sdef.dma_tx && Shared_DMA::is_shared(sdef.dma_tx_stream_id)) {
+        // avoid DMA on shared low-baudrate links
+        tx_dma_enabled = false;
+    }
 #endif
 
 #if defined(USART_CR1_FIFOEN)
@@ -374,17 +390,20 @@ void UARTDriver::begin(uint32_t b, uint16_t rxS, uint16_t txS)
                     dmaSetRequestSource(rxdma, sdef.dma_rx_channel_id);
 #endif
                 }
-                if (tx_dma_enabled) {
-                    // we only allow for sharing of the TX DMA channel, not the RX
-                    // DMA channel, as the RX side is active all the time, so
-                    // cannot be shared
-                    dma_handle = new Shared_DMA(sdef.dma_tx_stream_id,
-                                                SHARED_DMA_NONE,
-                                                FUNCTOR_BIND_MEMBER(&UARTDriver::dma_tx_allocate, void, Shared_DMA *),
-                                                FUNCTOR_BIND_MEMBER(&UARTDriver::dma_tx_deallocate, void, Shared_DMA *));
-                }
                 _device_initialised = true;
             }
+            if (tx_dma_enabled && dma_handle == nullptr) {
+                // we only allow for sharing of the TX DMA channel, not the RX
+                // DMA channel, as the RX side is active all the time, so
+                // cannot be shared
+                dma_handle = new Shared_DMA(sdef.dma_tx_stream_id,
+                                            SHARED_DMA_NONE,
+                                            FUNCTOR_BIND_MEMBER(&UARTDriver::dma_tx_allocate, void, Shared_DMA *),
+                                            FUNCTOR_BIND_MEMBER(&UARTDriver::dma_tx_deallocate, void, Shared_DMA *));
+                if (dma_handle == nullptr) {
+                    tx_dma_enabled = false;
+                }
+            }
 #endif // HAL_UART_NODMA
             sercfg.speed = _baudrate;
 
@@ -935,12 +954,7 @@ void UARTDriver::write_pending_bytes_DMA(uint32_t n)
 
         if (dma_handle->has_contention()) {
             // on boards with a hw fifo we can use a higher threshold for disabling DMA
-#if defined(USART_CR1_FIFOEN)
-            const uint32_t baud_threshold = 460800;
-#else
-            const uint32_t baud_threshold = 115200;
-#endif
-            if (_baudrate <= baud_threshold) {
+            if (_baudrate <= CONTENTION_BAUD_THRESHOLD) {
                 contention_counter += 3;
                 if (contention_counter > 1000) {
                     // more than 25% of attempts to use this DMA
diff --git a/libraries/AP_HAL_ChibiOS/hwdef/scripts/dma_resolver.py b/libraries/AP_HAL_ChibiOS/hwdef/scripts/dma_resolver.py
index 409977b33c..9b67e12050 100755
--- a/libraries/AP_HAL_ChibiOS/hwdef/scripts/dma_resolver.py
+++ b/libraries/AP_HAL_ChibiOS/hwdef/scripts/dma_resolver.py
@@ -438,11 +438,15 @@ def write_dma_header(f, peripheral_list, mcu_type, dma_exclude=[],
     for key in ordered_up_channels:
         ordered_timers.append(key[0:-3])
 
+    shared_set = set()
+
     for key in sorted(curr_dict.keys()):
         stream = curr_dict[key]
         shared = ''
         if len(stream_assign[stream]) > 1:
             shared = ' // shared %s' % ','.join(stream_assign[stream])
+            if stream[0] in [1,2]:
+                shared_set.add("(1U<<STM32_DMA_STREAM_ID(%u,%u))" % (stream[0],stream[1]))
         if curr_dict[key] == "STM32_DMA_STREAM_ID_ANY":
             f.write("#define %-30s STM32_DMA_STREAM_ID_ANY\n" % (chibios_dma_define_name(key)+'STREAM'))
             f.write("#define %-30s %s\n" % (chibios_dma_define_name(key)+'CHAN', dmamux_channel(key)))
@@ -486,6 +490,12 @@ def write_dma_header(f, peripheral_list, mcu_type, dma_exclude=[],
                                 chan.replace('_UP', '_CH{}'.format(ch))))
                 break
 
+    f.write("\n// Mask of DMA streams which are shared\n")
+    if len(shared_set) == 0:
+        f.write("#define SHARED_DMA_MASK 0\n")
+    else:
+        f.write("#define SHARED_DMA_MASK (%s)\n" % '|'.join(list(shared_set)))
+
     # now generate UARTDriver.cpp DMA config lines
     f.write("\n\n// generated UART DMA configuration lines\n")
     for u in range(1, 9):
diff --git a/libraries/AP_HAL_ChibiOS/shared_dma.cpp b/libraries/AP_HAL_ChibiOS/shared_dma.cpp
index 1518a98e34..23dc620790 100644
--- a/libraries/AP_HAL_ChibiOS/shared_dma.cpp
+++ b/libraries/AP_HAL_ChibiOS/shared_dma.cpp
@@ -53,6 +53,14 @@ Shared_DMA::Shared_DMA(uint8_t _stream_id1,
     deallocate = _deallocate;
 }
 
+/*
+  return true if a stream ID is shared between two peripherals
+*/
+bool Shared_DMA::is_shared(uint8_t stream_id)
+{
+    return (stream_id < SHARED_DMA_MAX_STREAM_ID) && ((1U<<stream_id) & SHARED_DMA_MASK) != 0;
+}
+
 //remove any assigned deallocator or allocator
 void Shared_DMA::unregister()
 {
diff --git a/libraries/AP_HAL_ChibiOS/shared_dma.h b/libraries/AP_HAL_ChibiOS/shared_dma.h
index f408cf7b56..7bd0570831 100644
--- a/libraries/AP_HAL_ChibiOS/shared_dma.h
+++ b/libraries/AP_HAL_ChibiOS/shared_dma.h
@@ -67,6 +67,9 @@ public:
     // display dma contention statistics as text buffer for @SYS/dma.txt
     static void dma_info(ExpandingString &str);
 
+    // return true if a stream ID is shared between two peripherals
+    static bool is_shared(uint8_t stream_id);
+
 private:
     dma_allocate_fn_t allocate;
     dma_allocate_fn_t deallocate;