serial: DZ11: avoid a hang at console switch-over

Changes to the generic console support code that happened a while ago
introduced a scenario where the initial console is used in parallel with
the final console during a brief period when switching between the two is
in progress.  During that time a message about the switch-over is printed.

With some combinations of chips, firmware and drivers, such as the DEC
DZ11 clone used with the DECstation, a hang may happen because the
firmware used for the initial console may not expect the state of the chip
after it has been initialised by the driver.

This is a workaround for the DZ11 which reuses the power-management
callback to keep the transmitter of the line associated with the console
enabled.  It reflects the consensus reached in a discussion a while ago.
Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: Jiri Slaby <jirislaby@gmail.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/serial/dz.c

@@ -642,6 +642,26 @@ static void dz_set_termios(struct uart_port *uport, struct ktermios *termios,
 	spin_unlock_irqrestore(&dport->port.lock, flags);
 }
 
+/*
+ * Hack alert!
+ * Required solely so that the initial PROM-based console
+ * works undisturbed in parallel with this one.
+ */
+static void dz_pm(struct uart_port *uport, unsigned int state,
+		  unsigned int oldstate)
+{
+	struct dz_port *dport = to_dport(uport);
+	unsigned long flags;
+
+	spin_lock_irqsave(&dport->port.lock, flags);
+	if (state < 3)
+		dz_start_tx(&dport->port);
+	else
+		dz_stop_tx(&dport->port);
+	spin_unlock_irqrestore(&dport->port.lock, flags);
+}
+
+
 static const char *dz_type(struct uart_port *uport)
 {
 	return "DZ";
@@ -738,6 +758,7 @@ static struct uart_ops dz_ops = {
 	.startup	= dz_startup,
 	.shutdown	= dz_shutdown,
 	.set_termios	= dz_set_termios,
+	.pm		= dz_pm,
 	.type		= dz_type,
 	.release_port	= dz_release_port,
 	.request_port	= dz_request_port,
@@ -861,7 +882,10 @@ static int __init dz_console_setup(struct console *co, char *options)
 	if (ret)
 		return ret;
 
+	spin_lock_init(&dport->port.lock);	/* For dz_pm().  */
+
 	dz_reset(dport);
+	dz_pm(uport, 0, -1);
 
 	if (options)
 		uart_parse_options(options, &baud, &parity, &bits, &flow);