linux-kernel - Re: [PATCH] printk/of_serial: fix serial console cessation part way through boot.

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Message-Id: <20140513220439.5A063E0692@canberra.localdomain>
Date:	Wed, 14 May 2014 08:04:39 +1000
From:	schivers@....com (Stephen Chivers)
To:	geert@...ux-m68k.org, jslaby@...e.cz, gregkh@...uxfoundation.org
Cc:	linuxppc-dev@...ts.ozlabs.org, benh@...nel.crashing.org,
	devicetree@...r.kernel.org, rob+dt@...nel.org,
	grant.likely@...aro.org, linux-kernel@...r.kernel.org,
	cproctor@....com, linux-serial@...r.kernel.org, schivers@....com
Subject: Re: [PATCH] printk/of_serial: fix serial console cessation part way
 through boot.

Commit 5f5c9ae56c38942623f69c3e6dc6ec78e4da2076
"serial_core: Unregister console in uart_remove_one_port()"
fixed a crash where a serial port was removed but
not deregistered as a console.

There is a side effect of that commit for platforms having serial consoles
and of_serial configured (CONFIG_SERIAL_OF_PLATFORM). The serial console
is disabled midway through the boot process.

This cessation of the serial console affects PowerPC computers
such as the MVME5100 and SAM440EP.

The sequence is:

	bootconsole [udbg0] enabled
	....
	serial8250/16550 driver initialises and registers its UARTS,
	one of these is the serial console.
	console [ttyS0] enabled
	....
	of_serial probes "platform" devices, registering them as it goes.
	One of these is the serial console.
	console [ttyS0] disabled.

The disabling of the serial console is due to:

	a.  unregister_console in printk not clearing the
	    CONS_ENABLED bit in the console flags,
	    even though it has announced that the console is disabled; and

	b.  of_platform_serial_probe in of_serial not setting the port type
	    before it registers with serial8250_register_8250_port.

This patch ensures that the serial console is re-enabled when of_serial
registers a serial port that corresponds to the designated console.

Signed-off-by: Stephen Chivers <schivers@....com>
Tested-by: Stephen Chivers <schivers@....com>
Acked-by: Geert Uytterhoeven <geert@...ux-m68k.org> [unregister_console]

===
The above failure was identified in Linux-3.15-rc2.

Tested using MVME5100 and SAM440EP PowerPC computers with
kernels built from Linux-3.15-rc5 and tty-next.

The continued operation of the serial console is vital for computers
such as the MVME5100 as that Single Board Computer does not
have any grapical/display hardware.

---
 drivers/tty/serial/of_serial.c |    1 +
 kernel/printk/printk.c         |    1 +
 2 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/drivers/tty/serial/of_serial.c b/drivers/tty/serial/of_serial.c
index 9924660..27981e2 100644
--- a/drivers/tty/serial/of_serial.c
+++ b/drivers/tty/serial/of_serial.c
@@ -173,6 +173,7 @@ static int of_platform_serial_probe(struct platform_device *ofdev)
 	{
 		struct uart_8250_port port8250;
 		memset(&port8250, 0, sizeof(port8250));
+		port.type = port_type;
 		port8250.port = port;
 
 		if (port.fifosize)
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 7228258..221229c 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -2413,6 +2413,7 @@ int unregister_console(struct console *console)
 	if (console_drivers != NULL && console->flags & CON_CONSDEV)
 		console_drivers->flags |= CON_CONSDEV;
 
+	console->flags &= ~CON_ENABLED;
 	console_unlock();
 	console_sysfs_notify();
 	return res;
--
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