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[PATCH] serial: DZ11: Avoid a hang at console switch-over

To: Andrew Morton <akpm@linux-foundation.org>
Subject: [PATCH] serial: DZ11: Avoid a hang at console switch-over
From: "Maciej W. Rozycki" <macro@linux-mips.org>
Date: Thu, 12 Jun 2008 23:43:47 +0100 (BST)
Cc: linux-serial@vger.kernel.org, linux-mips@linux-mips.org, linux-kernel@vger.kernel.org
Original-recipient: rfc822;linux-mips@linux-mips.org
Sender: linux-mips-bounce@linux-mips.org
 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>
---
patch-mips-2.6.23-rc5-20070904-dz-pm-1
diff -up --recursive --new-file 
linux-mips-2.6.23-rc5-20070904.macro/drivers/serial/dz.c 
linux-mips-2.6.23-rc5-20070904/drivers/serial/dz.c
--- linux-mips-2.6.23-rc5-20070904.macro/drivers/serial/dz.c    2007-10-14 
00:49:58.000000000 +0000
+++ linux-mips-2.6.23-rc5-20070904/drivers/serial/dz.c  2007-10-14 
01:46:41.000000000 +0000
@@ -637,6 +637,26 @@ static void dz_set_termios(struct uart_p
        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";
@@ -733,6 +753,7 @@ 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,
@@ -856,7 +877,10 @@ static int __init dz_console_setup(struc
        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);

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