serial: imx: start rx_dma once RXFIFO is not empty

Start rx_dma once RXFIFO is not empty that can avoid dma request lost
and causes data delay issue.
Signed-off-by: Robin Gong <b38343@freescale.com>
Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -946,8 +946,21 @@ static void dma_rx_callback(void *data)
 		tty_flip_buffer_push(port);
 
 		start_rx_dma(sport);
-	} else
+	} else if (readl(sport->port.membase + USR2) & USR2_RDR) {
+		/*
+		 * start rx_dma directly once data in RXFIFO, more efficient
+		 * than before:
+		 *	1. call imx_rx_dma_done to stop dma if no data received
+		 *	2. wait next  RDR interrupt to start dma transfer.
+		 */
+		start_rx_dma(sport);
+	} else {
+		/*
+		 * stop dma to prevent too many IDLE event trigged if no data
+		 * in RXFIFO
+		 */
 		imx_rx_dma_done(sport);
+	}
 }
 
 static int start_rx_dma(struct imx_port *sport)