[PATCH v3] printk: add halt_delay parameter for printk delay in halt phase

[Dave Young <hidave.darkstar@gmail.com>]

Add a halt_delay module parameter in printk.c used to read the printk
messages in halt/poweroff/restart phase, delay each printk messages
by halt_delay milliseconds. It is useful for debugging if there's no
other way to dump kernel messages that time.

The halt_delay max value is 65535, default value is 0, change it by:

echo xxx > /sys/module/printk/parameters/halt_delay

Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
---
Documentation/kernel-parameters.txt |    5 +++++
kernel/printk.c                     |   17 +++++++++++++++++
2 files changed, 22 insertions(+)

--- linux-2.6.orig/kernel/printk.c	2009-06-08 13:55:35.000000000 +0800
+++ linux-2.6/kernel/printk.c	2009-06-08 13:56:23.000000000 +0800
@@ -250,6 +250,22 @@ static inline void boot_delay_msec(void)
 }
 #endif

+/* msecs delay after each halt/poweroff/restart phase printk,
+ unsigned short is enough for delay in milliseconds */
+static unsigned short halt_delay;
+
+static inline void halt_delay_msec(void)
+{
+	if (unlikely(halt_delay == 0 || !(system_state == SYSTEM_HALT
+				|| system_state == SYSTEM_POWER_OFF
+				|| system_state == SYSTEM_RESTART)))
+		return;
+
+	mdelay(halt_delay);
+}
+
+module_param(halt_delay, ushort, S_IRUGO | S_IWUSR);
+
 /*
  * Commands to do_syslog:
  *
@@ -649,6 +665,7 @@ asmlinkage int vprintk(const char *fmt,
 	char *p;

 	boot_delay_msec();
+	halt_delay_msec();

 	preempt_disable();
 	/* This stops the holder of console_sem just where we want him */
--- linux-2.6.orig/Documentation/kernel-parameters.txt	2009-05-28
08:40:42.000000000 +0800
+++ linux-2.6/Documentation/kernel-parameters.txt	2009-06-08
14:06:47.000000000 +0800
@@ -1897,6 +1897,11 @@ and is between 256 and 4096 characters.
 	printk.time=	Show timing data prefixed to each printk message line
 			Format: <bool>  (1/Y/y=enable, 0/N/n=disable)

+	printk.halt_delay=
+			Milliseconds to delay each printk during
+			halt/poweroff/restart phase.
+			Format: <ushort>
+
 	processor.max_cstate=	[HW,ACPI]
 			Limit processor to maximum C-state
 			max_cstate=9 overrides any DMI blacklist limit.