Why does my 'Caps Lock' diode blink when my system freezes?

Question

I noticed a peculiar behavior of my Ubuntu 13.04 (although possibly it's also present in previous versions): when my system freezes completely (I do not ask for troubleshooting this) so that even X does not respond to Ctrl+Alt+FX keys, the CapsLock toggle diode on my keyboard blinks (with frequency of ~1Hz), although the system is completely unresponsive.

• What is the cause of this phenomena? Is it caused by some internal kernel mechanisms? Is this specific to Ubuntu (is it caused some custom Ubuntu additional software)? Does it has anything to do with HAL?
• What is the purpose of it?
• Can I customize this behavior on Ubuntu?

Solution

This feature is implemented in the kernel. It is not specific to Ubuntu, and has been present since kernel 2.4.19 (what, 2002?). I don't remember what kernel is in raring, but for a modern implementation, see for example https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/input/serio/i8042.c?id=refs/tags/v3.9-rc1#n1032 .

The rationale given in that file:

 * i8042_panic_blink() will turn the keyboard LEDs on or off and is called
 * when kernel panics. Flashing LEDs is useful for users running X who may
 * not see the console and will help distingushing panics from "real"
 * lockups.

Comments

[daltonfury42]

What does it signify? Kernel Panic?

[taneli]

Like the comment says, it "is called when kernel panics".