stm32h7:Serial refactor out tx dma semaphore

   Fixes is stuttering output.

   The use of the semaphore was causing blocking
   on non blocking callers. This ensured that
   the TX DAM would be restated, but when it
   was switched to trywait in 660ac6, it left
   data in the xmit queue unsent.

   This solution removes the semaphore and restart
   the DMA on completion if there is more data in
   the xmit queue to be sent.

arch/arm/src/stm32h7/stm32_serial.c

@@ -628,7 +628,6 @@ struct up_dev_s
 #ifdef SERIAL_HAVE_TXDMA
   const unsigned int txdma_channel; /* DMA channel assigned */
   DMA_HANDLE        txdma;          /* currently-open trasnmit DMA stream */
-  sem_t             txdmasem;       /* Indicate TX DMA completion */
 #endif
 
   /* RX DMA state */
@@ -3299,8 +3298,9 @@ static bool up_dma_rxavailable(struct uart_dev_s *dev)
  * Name: up_dma_txcallback
  *
  * Description:
- *   This function clears dma buffer at complete of DMA transfer and wakes up
- *   threads waiting for space in buffer.
+ *   This function clears dma buffer at completion of DMA transfer. It wakes
+ *   up threads waiting for space in buffer and restarts the DMA if there is
+ *   more data to send.
  *
  ****************************************************************************/
 
@@ -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 +3342,13 @@ 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);
+
+  /* Send more if availaible */
+
+  up_dma_txavailable(&priv->dev);
 }
 #endif
 
@@ -3369,8 +3367,7 @@ static void up_dma_txavailable(struct uart_dev_s *dev)
 
   /* Only send when the DMA is idle */
 
-  int rv = nxsem_trywait(&priv->txdmasem);
-  if (rv == OK)
+  if (stm32_dmaresidual(priv->txdma) == 0)
     {
       uart_xmitchars_dma(dev);
     }