[PATCH] serial_core: fix uart PORT_UNKNOWN handling

[Thomas Pfaff <tpfaff@pcs.com>]

From: "Thomas Pfaff" <tpfaff@pcs.com>

While porting a RS485 driver from 2.6.29 to 3.14, i noticed that the serial tty 
driver could break it by using uart ports that it does not own :

1. uart_change_pm ist called during uart_open and calls the uart pm function
   without checking for PORT_UNKNOWN.
2. uart_shutdown is called from uart_set_info and does not check it either.
3. The return code from the uart request_port call in uart_set_info is not
   handled properly, leading to the situation that the serial driver also
   thinks it owns the uart ports.
   This can triggered by doing following actions :

   setserial /dev/ttyS0 uart none    # release the uart ports
   modprobe lirc-serial              # or any other device that uses the uart
   setserial /dev/ttyS0 uart 16550   # gives no error and the uart tty driver
                                     # can use the ports as well

Signed-off-by: Thomas Pfaff <tpfaff@pcs.com>
---
diff -urp linux-3.14.org/drivers/tty/serial/serial_core.c linux-3.14/drivers/tty/serial/serial_core.c
--- linux-3.14.org/drivers/tty/serial/serial_core.c	2014-03-31 05:40:15.000000000 +0200
+++ linux-3.14/drivers/tty/serial/serial_core.c	2014-04-08 16:51:28.176459065 +0200
@@ -225,6 +225,9 @@ static void uart_shutdown(struct tty_str
 	struct uart_port *uport = state->uart_port;
 	struct tty_port *port = &state->port;
 
+	if (uport->type == PORT_UNKNOWN)
+		return;
+
 	/*
 	 * Set the TTY IO error marker
 	 */
@@ -826,25 +829,29 @@ static int uart_set_info(struct tty_stru
 		 * If we fail to request resources for the
 		 * new port, try to restore the old settings.
 		 */
-		if (retval && old_type != PORT_UNKNOWN) {
+		if (retval) {
 			uport->iobase = old_iobase;
 			uport->type = old_type;
 			uport->hub6 = old_hub6;
 			uport->iotype = old_iotype;
 			uport->regshift = old_shift;
 			uport->mapbase = old_mapbase;
-			retval = uport->ops->request_port(uport);
-			/*
-			 * If we failed to restore the old settings,
-			 * we fail like this.
-			 */
-			if (retval)
-				uport->type = PORT_UNKNOWN;
 
-			/*
-			 * We failed anyway.
-			 */
-			retval = -EBUSY;
+			if (old_type != PORT_UNKNOWN) {
+				retval = uport->ops->request_port(uport);
+				/*
+				 * If we failed to restore the old settings,
+				 * we fail like this.
+				 */
+				if (retval)
+					uport->type = PORT_UNKNOWN;
+
+				/*
+				 * We failed anyway.
+				 */
+				retval = -EBUSY;
+			}
+
 			/* Added to return the correct error -Ram Gupta */
 			goto exit;
 		}
@@ -1900,6 +1907,9 @@ static void uart_change_pm(struct uart_s
 {
 	struct uart_port *port = state->uart_port;
 
+	if (port->type == PORT_UNKNOWN)
+		return;
+
 	if (state->pm_state != pm_state) {
 		if (port->ops->pm)
 			port->ops->pm(port, pm_state, state->pm_state);