serial: uartps: Add a timeout to the tx empty wait

In case the cable is not connected then the target gets into
an infinite wait for tx empty.
Add a timeout to the tx empty wait.
Reported-by: Jean-Francois Dagenais <jeff.dagenais@gmail.com>
Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/xilinx_uartps.c

@@ -26,12 +26,14 @@
 #include <linux/of.h>
 #include <linux/module.h>
 #include <linux/pm_runtime.h>
+#include <linux/iopoll.h>
 
 #define CDNS_UART_TTY_NAME	"ttyPS"
 #define CDNS_UART_NAME		"xuartps"
 #define CDNS_UART_MAJOR		0	/* use dynamic node allocation */
 #define CDNS_UART_FIFO_SIZE	64	/* FIFO size */
 #define CDNS_UART_REGISTER_SPACE	0x1000
+#define TX_TIMEOUT		500000
 
 /* Rx Trigger level */
 static int rx_trigger_level = 56;
@@ -684,14 +686,18 @@ static void cdns_uart_set_termios(struct uart_port *port,
 	unsigned int cval = 0;
 	unsigned int baud, minbaud, maxbaud;
 	unsigned long flags;
-	unsigned int ctrl_reg, mode_reg;
+	unsigned int ctrl_reg, mode_reg, val;
+	int err;
 
 	/* Wait for the transmit FIFO to empty before making changes */
 	if (!(readl(port->membase + CDNS_UART_CR) &
 				CDNS_UART_CR_TX_DIS)) {
-		while (!(readl(port->membase + CDNS_UART_SR) &
-				CDNS_UART_SR_TXEMPTY)) {
-			cpu_relax();
+		err = readl_poll_timeout(port->membase + CDNS_UART_SR,
+					 val, (val & CDNS_UART_SR_TXEMPTY),
+					 1000, TX_TIMEOUT);
+		if (err) {
+			dev_err(port->dev, "timed out waiting for tx empty");
+			return;
 		}
 	}
 	spin_lock_irqsave(&port->lock, flags);