USB: serial: pl2303: add support for tx xon/xoff flow control

Support hardware-level Xon/Xoff flow control in transmit direction with
pl2303.

I only know how to get the hardware to do IXON/!IXANY with ^S/^Q as control
characters, so I preserve the old behaviour for all other cases.
Signed-off-by: Florian Zumbiehl <florz@florz.de>
[ johan: rewrite logic using pl2303_termios_change() helper ]
Signed-off-by: Johan Hovold <johan@kernel.org>

drivers/usb/serial/pl2303.c

@@ -533,6 +533,17 @@ static int pl2303_set_line_request(struct usb_serial_port *port,
 	return 0;
 }
 
+static bool pl2303_termios_change(const struct ktermios *a, const struct ktermios *b)
+{
+	bool ixon_change;
+
+	ixon_change = ((a->c_iflag ^ b->c_iflag) & (IXON | IXANY)) ||
+			a->c_cc[VSTART] != b->c_cc[VSTART] ||
+			a->c_cc[VSTOP] != b->c_cc[VSTOP];
+
+	return tty_termios_hw_change(a, b) || ixon_change;
+}
+
 static void pl2303_set_termios(struct tty_struct *tty,
 		struct usb_serial_port *port, struct ktermios *old_termios)
 {
@@ -544,7 +555,7 @@ static void pl2303_set_termios(struct tty_struct *tty,
 	int ret;
 	u8 control;
 
-	if (old_termios && !tty_termios_hw_change(&tty->termios, old_termios))
+	if (old_termios && !pl2303_termios_change(&tty->termios, old_termios))
 		return;
 
 	buf = kzalloc(7, GFP_KERNEL);
@@ -662,6 +673,9 @@ static void pl2303_set_termios(struct tty_struct *tty,
 			pl2303_vendor_write(serial, 0x0, 0x41);
 		else
 			pl2303_vendor_write(serial, 0x0, 0x61);
+	} else if (I_IXON(tty) && !I_IXANY(tty) && START_CHAR(tty) == 0x11 &&
+			STOP_CHAR(tty) == 0x13) {
+		pl2303_vendor_write(serial, 0x0, 0xc0);
 	} else {
 		pl2303_vendor_write(serial, 0x0, 0x0);
 	}