serial: 8250_port: when using DMA do not split writes in TTY

By default tty_io write() splits large writes into 2K chunks. This
results to a single DMA transfer to split into 2 seperate transfers
which are not combined on the scatter gather list. As a result, under
heavy load or PM conditions an interchar gap occurs which can lead
to errors in the receiver.

Signed-off-by: Ferry Toth <ftoth@exalondelft.nl>

drivers/tty/serial/8250/8250_port.c

@@ -2418,6 +2418,13 @@ int serial8250_do_startup(struct uart_port *port)
 		if (msg) {
 			dev_warn_ratelimited(port->dev, "%s\n", msg);
 			up->dma = NULL;
+		} else {
+			/*
+			 * change tty_io write() to not split
+			 * large writes into 2K chunks
+			 */
+			set_bit(TTY_NO_WRITE_SPLIT,
+				&port->state->port.tty->flags);
 		}
 	}