serial: imx: call imx_dma_tx() again in dma_tx_callback

Currently in dma_tx_callback(), no matter if there is still
remaining data pending in circle buffer or not, DMA transmit
will be terminated.

This will result in some data never get transmitted.
In order to fix this issue, call imx_dma_tx() again in
dma_tx_callback, when there is pending data and uart hasn't
been stopped.
Signed-off-by: Jiada Wang <jiada_wang@mentor.com>
Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -495,6 +495,7 @@ static inline void imx_transmit_buffer(struct imx_port *sport)
 		imx_stop_tx(&sport->port);
 }
 
+static void imx_dma_tx(struct imx_port *sport);
 static void dma_tx_callback(void *data)
 {
 	struct imx_port *sport = data;
@@ -524,6 +525,11 @@ static void dma_tx_callback(void *data)
 		dev_dbg(sport->port.dev, "exit in %s.\n", __func__);
 		return;
 	}
+
+	spin_lock_irqsave(&sport->port.lock, flags);
+	if (!uart_circ_empty(xmit) && !uart_tx_stopped(&sport->port))
+		imx_dma_tx(sport);
+	spin_unlock_irqrestore(&sport->port.lock, flags);
 }
 
 static void imx_dma_tx(struct imx_port *sport)