[PATCH] fix to clock running too fast

From: Akira Tsukamoto
Date: Sun Nov 20 2005 - 09:44:56 EST

This one line patch adds upper bound testing when evaluating irq timer
on boot up.

It fix the machine having problem with clock running too fast.

What this patch do is,
if timer interrupts running too fast through IO-APIC IRQ then false back to
i8259A IRQ.

I really appreciate for the feedback from ATI Xpress 200 chipset user,
It should eliminate the needs of adding no_timer_check on kernel options.

I have NEC laptop using ATI Xpress 200 chipset with Pentium M 1.8GHz and
its clock keep going forward when kernel compiled with local APIC support.
Many machines based on RS200 chipset seem to have the same problem,
including Acer Ferrari 400X AMD notebook or Compaq R4000.

Also I would like to have comments on upper bound limit, 16 ticks, which
I chose in this patch. My laptop always reports around 20, which is double from normal.

--- linux-2.6.14/arch/i386/kernel/io_apic.c 2005-10-28 09:02:08.000000000 +0900
+++ linux-2.6.14-io_apic-atifix/arch/i386/kernel/io_apic.c 2005-11-09 00:31:56.000000000 +0900
@@ -1798,21 +1798,21 @@
/* Let ten ticks pass... */
mdelay((10 * 1000) / HZ);

* Expect a few ticks at least, to be sure some possible
* glue logic does not lock up after one or two first
* ticks in a non-ExtINT mode. Also the local APIC
* might have cached one ExtINT interrupt. Finally, at
* least one tick may be lost due to delays.
- if (jiffies - t1 > 4)
+ if (jiffies - t1 > 4 && jiffies - t1 < 16)
return 1;

return 0;

* In the SMP+IOAPIC case it might happen that there are an unspecified
* number of pending IRQ events unhandled. These cases are very rare,
* so we 'resend' these IRQs via IPIs, to the same CPU. It's much
* better to do it this way as thus we do not have to be aware of

Akira Tsukamoto <akira-t@xxxxxxxxxxxxx> <akira-t@xxxxxxxxxxxxxx>

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