usage of disarm_decr[]

usage of disarm_decr[]

[Xiaogeng (Shawn) Jin]

disarm_decr[] is defined as an unsigned long array but never get
initialized in arch/ppc/kernel/time.c.

What's the purpose of using disarm_decr? I read the timer_interrupt()
code and found the only place where it's referenced.

int timer_interrupt(struct pt_regs * regs)
{
...
	if ( !disarm_decr[smp_processor_id()] )
		set_dec(next_dec);
	last_jiffy_stamp(cpu) = jiffy_stamp;
...
}

So in this case, the value of disarm_decr[] is always zero, right? Then
the decrementer get set each time when the timer interrupt is invoked.

--
Shawn Jin
RedSwitch Inc.


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Re: usage of disarm_decr[]

[Paul Mackerras]

Xiaogeng (Shawn) Jin writes:

> What's the purpose of using disarm_decr? I read the timer_interrupt()
> code and found the only place where it's referenced.

RT-Linux (www.fsmlabs.com) uses it.  RT-Linux takes over all interrupt
handling, including decrementer interrupts, and the disarm_decr thing
is there to give RT-Linux a way to take control of the decrementer
register.

Paul.

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Re: usage of disarm_decr[]

[Xiaogeng (Shawn) Jin]

Thank you. I have some further questions regarding to timer_interrupt().

How often is timer_interrupt() invoked? Every 1/HZ seconds? This is
controlled by the decrementer, right? Initially the value of the
decrementer is 'tb_ticks_per_jiffy'. Then it's updated in
timer_interrupt(). Why isn't 'next_dec' the initial value
'tb_ticks_per_jiffy'? It's due to the time drift caused by the accuracy
of external clock (oscillator) or by the precision of software?

	next_dec = tb_ticks_per_jiffy - tb_delta(&jiffy_stamp)

The timebase register is initialized to zero and then increments
periodically. How does it increment? Each clock (TMBCLK) the increment
is 1? by itself, I mean, hardware itself, or by software?

>>What's the purpose of using disarm_decr? I read the timer_interrupt()
>>code and found the only place where it's referenced.
>
> RT-Linux (www.fsmlabs.com) uses it.  RT-Linux takes over all interrupt
> handling, including decrementer interrupts, and the disarm_decr thing
> is there to give RT-Linux a way to take control of the decrementer
> register.

Thanks.

- Shawn.


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