TTY: fix UV serial console regression

Commit 74c21077 (serial: Use block_til_ready helper) and its fixup
3f582b8c (serial: fix termios settings in open) introduced a
regression on UV systems. The serial eventually freezes while being
used. It's completely unpredictable and sometimes needs a heap of
traffic to happen first.

To reproduce this, yast installation was used as it turned out to be
pretty reliable in reproducing. Especially during installation process
where one doesn't have an SSH daemon running. And no monitor as the HW
is completely headless. So this was fun to find. Given the machine
doesn't boot on vanilla before 2.6.36 final. (And the commits above
are older.)

Unless there is some bad race in the code, the hardware seems to be
pretty broken. Otherwise pure MSR read should not cause such a bug,
or?

So to prevent the bug, revert to the old behavior. I.e. read modem
status only if we really have to -- for non-CLOCAL set serials.
Non-CLOCAL works on this hardware OK, I tried. See? I don't.

And document that shit.
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: stable <stable@vger.kernel.org>
References: https://lkml.org/lkml/2011/12/6/573
References: https://bugzilla.novell.com/show_bug.cgi?id=718518Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/tty/tty_port.c

@@ -227,7 +227,6 @@ int tty_port_block_til_ready(struct tty_port *port,
 	int do_clocal = 0, retval;
 	unsigned long flags;
 	DEFINE_WAIT(wait);
-	int cd;
 
 	/* block if port is in the process of being closed */
 	if (tty_hung_up_p(filp) || port->flags & ASYNC_CLOSING) {
@@ -284,11 +283,14 @@ int tty_port_block_til_ready(struct tty_port *port,
 				retval = -ERESTARTSYS;
 			break;
 		}
-		/* Probe the carrier. For devices with no carrier detect this
-		   will always return true */
-		cd = tty_port_carrier_raised(port);
+		/*
+		 * Probe the carrier. For devices with no carrier detect
+		 * tty_port_carrier_raised will always return true.
+		 * Never ask drivers if CLOCAL is set, this causes troubles
+		 * on some hardware.
+		 */
 		if (!(port->flags & ASYNC_CLOSING) &&
-				(do_clocal || cd))
+				(do_clocal || tty_port_carrier_raised(port)))
 			break;
 		if (signal_pending(current)) {
 			retval = -ERESTARTSYS;