[tip:core/printk] printk: Add halt_delay=<msecs> parameter for printk delay in halt phase

[tip-bot for Dave Young <hidave.darkstar@gmail.com>]

Commit-ID:  f43a6b1611aff688e21f323b64b98b57f8b7f721
Gitweb:     http://git.kernel.org/tip/f43a6b1611aff688e21f323b64b98b57f8b7f721
Author:     Dave Young <hidave.darkstar@gmail.com>
AuthorDate: Mon, 8 Jun 2009 15:40:07 +0800
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Mon, 8 Jun 2009 10:10:34 +0200

printk: Add halt_delay=<msecs> parameter for printk delay in halt phase

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 at that time. (if a system has no serial port - and if
netconsole has already gone down at that stage where the messages
are printed.)

The halt_delay max value is 65535, default value is 0. To add
a halt-day of 10 seconds, change it by:

    echo 10000 > /sys/module/printk/parameters/halt_delay

Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
LKML-Reference: <a8e1da0906080040k7852f770v1e2aa2ebb0f52369@mail.gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>


---
 Documentation/kernel-parameters.txt |    5 +++++
 kernel/printk.c                     |   17 +++++++++++++++++
 2 files changed, 22 insertions(+), 0 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 6172e43..0630428 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1847,6 +1847,11 @@ and is between 256 and 4096 characters. It is defined in the file
 	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.
diff --git a/kernel/printk.c b/kernel/printk.c
index 5052b54..2114231 100644
--- a/kernel/printk.c
+++ b/kernel/printk.c
@@ -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, va_list args)
 	char *p;
 
 	boot_delay_msec();
+	halt_delay_msec();
 
 	preempt_disable();
 	/* This stops the holder of console_sem just where we want him */