diff --git a/libraries/AP_HAL_ChibiOS/RCOutput.cpp b/libraries/AP_HAL_ChibiOS/RCOutput.cpp
index c0f96c2b58..c5d3d98ce5 100644
--- a/libraries/AP_HAL_ChibiOS/RCOutput.cpp
+++ b/libraries/AP_HAL_ChibiOS/RCOutput.cpp
@@ -955,15 +955,15 @@ void RCOutput::send_pulses_DMAR(pwm_group &group, uint32_t buffer_length)
 void RCOutput::dma_irq_callback(void *p, uint32_t flags)
 {
     pwm_group *group = (pwm_group *)p;
+    chSysLockFromISR();
     dmaStreamDisable(group->dma);
     if (group->in_serial_dma && irq.waiter) {
         // tell the waiting process we've done the DMA
-        chSysLockFromISR();
         chEvtSignalI(irq.waiter, serial_event_mask);
-        chSysUnlockFromISR();
     } else {
         group->dma_handle->unlock_from_IRQ();
     }
+    chSysUnlockFromISR();
 }
 
 /*
diff --git a/libraries/AP_HAL_ChibiOS/UARTDriver.cpp b/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
index 5167d1a79d..4b278363ab 100644
--- a/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
+++ b/libraries/AP_HAL_ChibiOS/UARTDriver.cpp
@@ -324,22 +324,20 @@ void UARTDriver::dma_tx_deallocate(Shared_DMA *ctx)
 void UARTDriver::tx_complete(void* self, uint32_t flags)
 {
     UARTDriver* uart_drv = (UARTDriver*)self;
+    chSysLockFromISR();
     if (!uart_drv->tx_bounce_buf_ready) {
         // reset timeout 
-        chSysLockFromISR();
         chVTResetI(&uart_drv->tx_timeout);
-        chSysUnlockFromISR();
         
         uart_drv->_last_write_completed_us = AP_HAL::micros();
         uart_drv->tx_bounce_buf_ready = true;
         if (uart_drv->unbuffered_writes && uart_drv->_writebuf.available()) {
             // trigger a rapid send of next bytes
-            chSysLockFromISR();
             chEvtSignalI(uart_thread_ctx, EVENT_MASK(uart_drv->serial_num));
-            chSysUnlockFromISR();
         }
         uart_drv->dma_handle->unlock_from_IRQ();
     }
+    chSysUnlockFromISR();
 }
 
 
@@ -375,7 +373,7 @@ void UARTDriver::rxbuff_full_irq(void* self, uint32_t flags)
     if (!uart_drv->sdef.dma_rx) {
         return;
     }
-    uint8_t len = RX_BOUNCE_BUFSIZE - uart_drv->rxdma->stream->NDTR;
+    uint8_t len = RX_BOUNCE_BUFSIZE - dmaStreamGetTransactionSize(uart_drv->rxdma);
     if (len == 0) {
         return;
     }
@@ -605,7 +603,7 @@ void UARTDriver::check_dma_tx_completion(void)
     chSysLock();
     if (!tx_bounce_buf_ready) {
         if (!(txdma->stream->CR & STM32_DMA_CR_EN)) {
-            if (txdma->stream->NDTR == 0) {
+            if (dmaStreamGetTransactionSize(txdma) == 0) {
                 tx_bounce_buf_ready = true;
                 _last_write_completed_us = AP_HAL::micros();
                 chVTResetI(&tx_timeout);
@@ -623,12 +621,14 @@ void UARTDriver::check_dma_tx_completion(void)
 void UARTDriver::handle_tx_timeout(void *arg)
 {
     UARTDriver* uart_drv = (UARTDriver*)arg;
+    chSysLockFromISR();
     if (!uart_drv->tx_bounce_buf_ready) {
-        uart_drv->tx_len = 0; // fix for n sent
         dmaStreamDisable(uart_drv->txdma);
+        uart_drv->tx_len -= dmaStreamGetTransactionSize(uart_drv->txdma);
         uart_drv->tx_bounce_buf_ready = true;
         uart_drv->dma_handle->unlock_from_IRQ();
     }
+    chSysUnlockFromISR();
 }
 
 /*
@@ -767,7 +767,7 @@ void UARTDriver::_timer_tick(void)
         //if not, it might be because the DMA interrupt was silenced
         //let's handle that here so that we can continue receiving
         if (!(rxdma->stream->CR & STM32_DMA_CR_EN)) {
-            uint8_t len = RX_BOUNCE_BUFSIZE - rxdma->stream->NDTR;
+            uint8_t len = RX_BOUNCE_BUFSIZE - dmaStreamGetTransactionSize(rxdma);
             if (len != 0) {
                 _readbuf.write(rx_bounce_buf, len);
 
diff --git a/libraries/AP_HAL_ChibiOS/shared_dma.cpp b/libraries/AP_HAL_ChibiOS/shared_dma.cpp
index e349085fec..59669a0755 100644
--- a/libraries/AP_HAL_ChibiOS/shared_dma.cpp
+++ b/libraries/AP_HAL_ChibiOS/shared_dma.cpp
@@ -164,7 +164,6 @@ void Shared_DMA::unlock_from_lockzone(void)
 void Shared_DMA::unlock_from_IRQ(void)
 {
     osalDbgAssert(have_lock, "must have lock");
-    chSysLockFromISR();
     if (stream_id2 != SHARED_DMA_NONE) {
         chBSemSignalI(&locks[stream_id2].semaphore);        
     }
@@ -172,7 +171,6 @@ void Shared_DMA::unlock_from_IRQ(void)
         chBSemSignalI(&locks[stream_id1].semaphore);
     }
     have_lock = false;
-    chSysUnlockFromISR();
 }
 
 /*