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>

diff --git a/drivers/serial/dz.c b/drivers/serial/dz.c
index a81d2c2ff8a2d9d1a71e26cbbdc1411668582ade..6042b87797a1b74cc4fa2cddefe91dd2d9d625fd 100644 (file)
--- a/drivers/serial/dz.c
+++ b/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);