serial: assert DTR for serial console devices

Some RS-232 devices require DTR to be asserted before they can be used.  DTR
is normally asserted in uart_startup() when the port is opened.  But we don't
actually open serial console ports, so assert DTR when the port is added.

BTW:
earlyprintk and early_uart are hard coded to set DTR/RTS.

rmk says

  The only issue I can think of is the possibility for an attached modem to
  auto-answer or maybe even auto-dial before the system is ready for it to do
  so.  Might have an undesirable cost implication for some running with such a
  setup.

  Apart from that, I can't think of any other side effect of this specific
  patch.

Signed-off-by: Yinghai Lu <yinghai.lu@sun.com>
Acked-by: Russell King <rmk@arm.linux.org.uk>
Cc: Andi Kleen <ak@suse.de>
Cc: Bjorn Helgaas <bjorn.helgaas@hp.com>
Cc: Gerd Hoffmann <kraxel@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/serial/serial_core.c

@@ -1910,6 +1910,12 @@ uart_set_options(struct uart_port *port, struct console *co,
 	if (flow == 'r')
 		termios.c_cflag |= CRTSCTS;
 
+	/*
+	 * some uarts on other side don't support no flow control.
+	 * So we set * DTR in host uart to make them happy
+	 */
+	port->mctrl |= TIOCM_DTR;
+
 	port->ops->set_termios(port, &termios, NULL);
 	co->cflag = termios.c_cflag;