pl2303: Fix mode switching regression

Cleaning out all the incorrect 'no change made' checks for termios
settings showed up a problem with the PL2303. The hardware here seems to
lose sync and bits if you tell it to make no changes. This shows up with
a real world application.

To fix this the driver check for meaningful hardware changes is restored
but doing the tests correctly and as a tty layer function so it doesn't
get duplicated wrongly everywhere if other drivers turn out to need it.

Signed-off-by: Alan Cox <[e-mail]>
Tested-by: Mirko Parthey <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/char/tty_ioctl.c

···
 EXPORT_SYMBOL(tty_termios_copy_hw);
 
 /**
+ *	tty_termios_hw_change	-	check for setting change
+ *	@a: termios
+ *	@b: termios to compare
+ *
+ *	Check if any of the bits that affect a dumb device have changed
+ *	between the two termios structures, or a speed change is needed.
+ */
+
+int tty_termios_hw_change(struct ktermios *a, struct ktermios *b)
+{
+	if (a->c_ispeed != b->c_ispeed || a->c_ospeed != b->c_ospeed)
+		return 1;
+	if ((a->c_cflag ^ b->c_cflag) & ~(HUPCL | CREAD | CLOCAL))
+		return 1;
+	return 0;
+}
+EXPORT_SYMBOL(tty_termios_hw_change);
+
+/**
  *	change_termios		-	update termios values
  *	@tty: tty to update
  *	@new_termios: desired new value

drivers/usb/serial/pl2303.c

···
 	}
 	spin_unlock_irqrestore(&priv->lock, flags);
 
+	/* The PL2303 is reported to lose bytes if you change
+	   serial settings even to the same values as before. Thus
+	   we actually need to filter in this specific case */
+
+	if (!tty_termios_hw_change(port->tty->termios, old_termios))
+		return;
+
 	cflag = port->tty->termios->c_cflag;
 
 	buf = kzalloc(7, GFP_KERNEL);

include/linux/tty.h

···
 extern void tty_termios_encode_baud_rate(struct ktermios *termios, speed_t ibaud, speed_t obaud);
 extern void tty_encode_baud_rate(struct tty_struct *tty, speed_t ibaud, speed_t obaud);
 extern void tty_termios_copy_hw(struct ktermios *new, struct ktermios *old);
+extern int tty_termios_hw_change(struct ktermios *a, struct ktermios *b);
 
 extern struct tty_ldisc *tty_ldisc_ref(struct tty_struct *);
 extern void tty_ldisc_deref(struct tty_ldisc *);