Re: [PATCH] i386: Selectable Frequency of the Timer Interrupt

[Nish Aravamudan]

On 7/16/05, Jesper Juhl <jesper.juhl@xxxxxxxxx> wrote:
> On 7/15/05, Jesper Juhl <jesper.juhl@xxxxxxxxx> wrote:
> > On 7/15/05, Linus Torvalds <torvalds@xxxxxxxx> wrote:
> > >
> > > On Fri, 15 Jul 2005, Jesper Juhl wrote:
> > > >
> > > > It's buggy, that I know. setting kernel_hz (the new boot parameter) to
> > > > 250 causes my system clock to run at something like 4-5 times normal
> > > > speed
> > >
> > > 4 times normal. You don't actually make the timer interrupt happen at
> > > 250Hz, so the timer will be programmed to run at the full 1kHz.
> > >
> > Right, that's the basic problem. I increase jiffies at a higher rate
> > but didn't slow the timer interrupt down at the same time.
> >
> > > You also need to actually change the LATCH define (in
> > > include/linux/jiffies.h) to take this into account (there might be
> > > something else too).
> > >
> > [...]
> > > and you might be getting closer.
> > >
> > > Of course, you need to make sure that LATCH is used only after
> > > jiffies_increment is set up. See "setup_pit_timer(void)" in
> > > arch/i386/kernel/timers/timer_pit.c for more details.
> > >
> >
> > Thank you for all the pointers and hints. This is a new area of code
> > for me, so I'll need some time to poke around - the above helps a lot.
> > Unfortunately I won't have any time to work on this today, but I'll
> > see if I can get a working implementation together tomorrow.
> >
> 
> Ok, I'm afraid I'm going to need another hint or two.
> 
> I've been looking at the timer code and getting thoroughly confused.
> I've tried to find out where we actually program the interrupt
> controller to say "this is the frequency I want you to interrupt me
> at", but I can't seem to find it.
> I'm aware that there are multiple possible time sources, and I've been
> looking at the 8259 code, the ioapic code, the hpet code and various
> other bits in arch/i386/kernel/ and arch/i386/kernel/timers/ , but I
> seem to end up getting confused about all the different defines like
> CLOCK_TICK_RATE, ACTHZ, TICK_NSEC, TICK_USEC, etc...
> 
> Where do we actually program the tick rate we want?

As you've seen, I think it depends on the timesource: for the PIT, it
would be arch/i386/kernel/timers/timer_pit.c::setup_pit_timer(). In
that function, you'll notice we outb() the LATCH value. I think there
are similar functions in the other timesources, e.g. the
arch/i386/kernel/apic.c::setup_APIC_timer().

Does that help some?

Thanks,
Nish
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