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Subject[RFC][PATCH] irq0 affinity broke on some i386 boxes
Hi everyone,
The following patch fixes a bug that prevents a write to
/proc/irq/0/smp_affinity from actually changing the cpu affinity
of IRQ #0, on all the (Dell server) SMP machines I have access to.

Given the wide variety of IO APIC and legacy PIC usage on various SMP
motherboards, and the nascent state of my APIC understanding, it is
quite likely that this fix is not universal.

I would like expand this patch so that IRQ0 affinity assignment works
properly on as many i386 SMP motherboards as possible. If you have
such a motherboard, please first 1) verify that assignments to
/proc/irq/0/smp_affinity NOP for you, and 2) if it does NOP, that
this patch does or does not fix the problem on your system.

To verify that your system has the problem or not:

in one window, run `watch -n1 cat /proc/interrupts'.

in another window, assign some affinity value to irq0. In the
following example, cpu #0 (in a 4-cpu system) is to no longer get
irq0 interrupts:

echo e >/proc/irq/0/smp_affinity

If your system is working properly, the watch-window should no
longer show increments for the irq0 value for cpu0.

This patch is against 2.4.18-rc4


--- linux/arch/i386/kernel/io_apic.c.orig Tue Nov 13 20:28:41 2001
+++ linux/arch/i386/kernel/io_apic.c Mon Feb 25 13:17:13 2002
@@ -1537,6 +1537,7 @@
+ add_pin_to_irq(0, 0, pin2);
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