slow down printk during boot

Optionally add a boot delay after each kernel printk() call, crudely
measured in milliseconds, with a maximum delay of 10 seconds per printk.

Enable CONFIG_BOOT_PRINTK_DELAY=y and then add (e.g.):
"lpj=loops_per_jiffy boot_delay=100"
to the kernel command line.

It has been useful in cases like "during boot, my machine just reboots or the
screen goes black" by slowing down printk, (and adding initcall_debug), we can
usually see the last thing that happened before the lights went out which is
usually a valuable clue.

[[email protected]: not all architectures implement CONFIG_HZ]
[[email protected]: fix lots of stuff]
[[email protected]: kernel/printk.c: make 2 variables static]
[[email protected]: fix slow down printk on boot compile error]
Signed-off-by: Randy Dunlap <[email protected]>
Signed-off-by: Dave Jones <[email protected]>
Signed-off-by: Adrian Bunk <[email protected]>
Signed-off-by: Heiko Carstens <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

1 files changed, 2 insertions, 0 deletions

diff --git a/include/linux/jiffies.h b/include/linux/jiffies.h
index e757a74b9d17..8b080024bbc1 100644
--- a/include/linux/jiffies.h
+++ b/include/linux/jiffies.h
@@ -148,6 +148,8 @@ static inline u64 get_jiffies_64(void)
  */
 #define MAX_JIFFY_OFFSET ((LONG_MAX >> 1)-1)
 
+extern unsigned long preset_lpj;
+
 /*
  * We want to do realistic conversions of time so we need to use the same
  * values the update wall clock code uses as the jiffies size.  This value