uart_get_baud_rate: stop mangling termios

Russell King noticed this one: We have to avoid replacing B0 when we pick a
baud rate for a "hung up" port.  Ugly but the proper fix is in the tty layer
and means changing the tty<->serial interfaces so we will defer that for now.

[akpm@linux-foundation.org: fix uninitialised var]
Signed-off-by: Alan Cox <alan@redhat.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/serial/serial_core.c

@@ -329,13 +329,15 @@ EXPORT_SYMBOL(uart_update_timeout);
  *	If it's still invalid, we try 9600 baud.
  *
  *	Update the @termios structure to reflect the baud rate
- *	we're actually going to be using.
+ *	we're actually going to be using. Don't do this for the case
+ *	where B0 is requested ("hang up").
  */
 unsigned int
 uart_get_baud_rate(struct uart_port *port, struct ktermios *termios,
 		   struct ktermios *old, unsigned int min, unsigned int max)
 {
 	unsigned int try, baud, altbaud = 38400;
+	int hung_up = 0;
 	upf_t flags = port->flags & UPF_SPD_MASK;
 
 	if (flags == UPF_SPD_HI)
@@ -360,8 +362,10 @@ uart_get_baud_rate(struct uart_port *port, struct ktermios *termios,
 		/*
 		 * Special case: B0 rate.
 		 */
-		if (baud == 0)
+		if (baud == 0) {
+			hung_up = 1;
 			baud = 9600;
+		}
 
 		if (baud >= min && baud <= max)
 			return baud;
@@ -373,7 +377,9 @@ uart_get_baud_rate(struct uart_port *port, struct ktermios *termios,
 		termios->c_cflag &= ~CBAUD;
 		if (old) {
 			baud = tty_termios_baud_rate(old);
-			tty_termios_encode_baud_rate(termios, baud, baud);
+			if (!hung_up)
+				tty_termios_encode_baud_rate(termios,
+								baud, baud);
 			old = NULL;
 			continue;
 		}
@@ -382,6 +388,7 @@ uart_get_baud_rate(struct uart_port *port, struct ktermios *termios,
 		 * As a last resort, if the quotient is zero,
 		 * default to 9600 bps
 		 */
+		if (!hung_up)
 			tty_termios_encode_baud_rate(termios, 9600, 9600);
 	}