stm32h7/serial: fix TX DMA

arch/arm/src/stm32h7/stm32_serial.c

@@ -3314,11 +3314,7 @@ static void up_dma_txcallback(DMA_HANDLE handle, uint8_t status, void *arg)
  * This is important to free TX buffer space by 'uart_xmitchars_done'.
  */
 
- if (status & DMA_SCR_HTIE)
- {
- priv->dev.dmatx.nbytes += priv->dev.dmatx.length / 2;
- }
- else if (status & DMA_SCR_TCIE)
+ if (status & DMA_SCR_TCIE)
  {
  priv->dev.dmatx.nbytes += priv->dev.dmatx.length;
  if (priv->dev.dmatx.nlength)
@@ -3346,11 +3342,20 @@ static void up_dma_txcallback(DMA_HANDLE handle, uint8_t status, void *arg)
  }
  }
 
- nxsem_post(&priv->txdmasem);
-
  /* Adjust the pointers */
 
  uart_xmitchars_done(&priv->dev);
+
+ /* Initiate another transmit if data is ready */
+
+ if (priv->dev.xmit.tail != priv->dev.xmit.head)
+ {
+ uart_xmitchars_dma(&priv->dev);
+ }
+ else
+ {
+ nxsem_post(&priv->txdmasem);
+ }
 }
 #endif
 
@@ -3372,6 +3377,14 @@ static void up_dma_txavailable(struct uart_dev_s *dev)
  int rv = nxsem_trywait(&priv->txdmasem);
  if (rv == OK)
  {
+ if (dev->xmit.head == dev->xmit.tail)
+ {
+ /* No data to transfer. Release semaphore. */
+
+ nxsem_post(&priv->txdmasem);
+ return;
+ }
+
  uart_xmitchars_dma(dev);
  }
 }