drivers: serial: uart_stm32.c: RxDataFlush on async_rx_enable

When enabling async RX the first time after boot, there is an
additional byte received with the first RX_DATA_RDY event,
which seems to be caused by the RX data not being flushed before
enabling the UART RX DMA.

Adding a request to flush the RX data register before enabling
the RX DMA, solves the issue.

Signed-off-by: Bjarki Arge Andreasen <[email protected]>

Author: bjarki-andreasen

drivers/serial/uart_stm32.c

@@ -1579,6 +1579,13 @@ static int uart_stm32_async_rx_enable(const struct device *dev,
 		return -EFAULT;
 	}
 
+	/* Flush RX data buffer */
+#ifdef USART_SR_RXNE
+	LL_USART_ClearFlag_RXNE(config->usart);
+#else
+	LL_USART_RequestRxDataFlush(config->usart);
+#endif /* USART_SR_RXNE */
+
 	/* Enable RX DMA requests */
 	uart_stm32_dma_rx_enable(dev);