serial: imx: always restart DMA if more data is available

Simplify the DMA restart logic to always queue up the next transfer
immediately if there is at least one more byte available in the FIFO,
so that the transfer will finish in a limited time.

This way the driver stops to rely on zero length transfers to signal
transfers ends. Those will go away when the idle detect DMA requests
are disabled.
Signed-off-by: NLucas Stach <l.stach@pengutronix.de>
Acked-by: NJiada Wang <jiada_wang@mentor.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -927,23 +927,20 @@ static void dma_rx_callback(void *data)
 				sport->port.icount.buf_overrun++;
 		}
 		tty_flip_buffer_push(port);
+	}
 
-		start_rx_dma(sport);
-	} 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.
+	 * Restart RX DMA directly if more data is available in order to skip
+	 * the roundtrip through the IRQ handler. If there is some data already
+	 * in the FIFO, DMA needs to be restarted soon anyways.
+	 *
+	 * Otherwise stop the DMA and reactivate FIFO IRQs to restart DMA once
+	 * data starts to arrive again.
 	 */
+	if (readl(sport->port.membase + USR2) & USR2_RDR)
 		start_rx_dma(sport);
-	} else {
-		/*
-		 * stop dma to prevent too many IDLE event trigged if no data
-		 * in RXFIFO
-		 */
+	else
 		imx_rx_dma_done(sport);
-	}
 }
 
 static int start_rx_dma(struct imx_port *sport)