serial: don't optimise away baud rate changes when BOTHER is used

The uart_set_termios() function will bail out early without bothering to
touch the hardware, if it decides that nothing "relevant" has changed.
Unfortunately, its idea of "relevant" doesn't include c_[io]speed.  So if
the baud rate bits are BOTHER and you just change the speed, the change
gets optimised away.

This patch makes it ignore the old Bfoo bits in c_cflag and just check
whether c_ispeed and c_ospeed have changed.  Those integers are always set
appropriately for us by set_termios().
Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Acked-by: Alan Cox <alan@redhat.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Mariusz Kozlowski <m.kozlowski@tuxland.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/serial/serial_core.c

@@ -1146,11 +1146,14 @@ static void uart_set_termios(struct tty_struct *tty, struct ktermios *old_termio
 
 	/*
 	 * These are the bits that are used to setup various
-	 * flags in the low level driver.
+	 * flags in the low level driver. We can ignore the Bfoo
+	 * bits in c_cflag; c_[io]speed will always be set
+	 * appropriately by set_termios() in tty_ioctl.c
 	 */
 #define RELEVANT_IFLAG(iflag)	((iflag) & (IGNBRK|BRKINT|IGNPAR|PARMRK|INPCK))
-
 	if ((cflag ^ old_termios->c_cflag) == 0 &&
+	    tty->termios->c_ospeed == old_termios->c_ospeed &&
+	    tty->termios->c_ispeed == old_termios->c_ispeed &&
 	    RELEVANT_IFLAG(tty->termios->c_iflag ^ old_termios->c_iflag) == 0)
 		return;