[H-GEN] kernel info

[Anthony Towns]

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On Sun, 7 Sep 1997, In memory of Mighty wrote:

> > I have a curious yet unresistable urge to modify my kernel.. more.. to do
> > my current idea, I need to be able to delay for x ms in a kernel c source
> > file.
> > I want to make the kernel beep every second or so for a brief moment after
> > it has been shut down, so I know when to turn the box off when it is
> > headless.
> You could set a flag when it reaches "System Halted" (or whatever,
> my box hasn't stopped running for 41 days now <grin>). Then just add an
> if for that flag in the idle loop, which sets a variable to a large
> negative value, incrementing whenever the idle loop passes, and beeping
> when it gets back to 0....

It might be a nicer move to swap the standard idle task with a beeping
idle task (hmmm, or just have a beeping idle task as standard -- you
know your processor is too fast when it beeps often enough to be
annoying).

Anyway, the applicable bit of sys_reboot() is:

	printk( KERN_EMERG, "System halted\n");
		(print your average halted message)

	syskill(-1,SIGKILL);
		(syskill(-x,SIGKILL) seems to be the Evil Overlord's
		 call -- it ties up process x, makes it watch while
		 everything else is killed, and then, despite any pleas for
		 mercy, goes ahead and kills x anyway. And we thought
		 Linux stood for freedom and justice. BSD forever? It 
		 doesn't kill the current process either, actually)

#if defined(CONFIG_APM) && defined(CONFIG_APM_POWER_OFF)
	apm_set_power_state(APM_STATE_OFF);
		(turn the power off. This would solve all your
		 problems, Byron :)
#endif

	do_exit(0);
		(kill the current process, presumably reboot(1))


My guess is that inserting something akin to the following before the
do_exit(0) (do_exit() doesn't return) might do some good:

	{
		extern struct task_struct my_beeping_function;
		/* fill in the fields "appropriately" in a different 
		 * file, along with the actually beeping function
		 */

		add_to_runqueue( &my_beeping_function );
	}

If you replaced add_to_runqueue() with

		task[0] = &my_beeping_function

you could replace the idle task instead of just adding a task, but I'm
not sure that that would be overly pleasant. Strange things might
happen on SMP machines too (not that they don't already). 

Filling out the task_struct (<linux/sched.h>) is probably easier said
than done, too.

You _might_ be able to just replace do_exit(0) with your busy loop,
but that might equally well do something nasty.

I believe the beeping_function can then be something along the lines
of

void beeping_function() {
	time_t t;
	
	for(;;) {
		t = time(); /* or the appropriate low level equiv */
	
		beep_on(); /* or the appr... */
		while( time() - t < 30 ) ;
		beep_off();

		while( time() - t < 1000 ) ;
	}
}

If you don't try replacing the idle process you can _probably_ call

	sleep( 1 ); 

to wait for a second, and, I suppose sys_nanosleep() for shorter sleep
periods. Otherwise, you'll have to use a proper busy wait, either with
jiffies (kernel/sched.c), which should continue being updated okay as
far as I can tell, or else the sys_time() call in kernel/time.c.

Hmmm... I reckon replacing do_exit(0) with a busy loop, and counting
jiffies (using jiffiestotimespec() and kernel/sched.c's "jiffies"
variable) would be easiest.

Make up a patch when you get it working, Byron. :)

Cheers,
aj

- --
Anthony Towns <aj at humbug.org.au> <http://student.uq.edu.au/~s343676/>
I don't speak for anyone save myself. PGP encrypted mail preferred.

``Like the ski resort of girls looking for husbands and husbands looking
  for girls, the situation is not as symmetrical as it might seem.''

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