I found a sequence that reliably locks up my 82093AA ioapic with just
one disable/enable_irq.
I've attached a patch for the tulip driver. (tested with a pnic card)

* focus cpu must be enabled.
* Gerard's mask sequence: disable_irq() only sets IRQ_DISABLED,
mask_and_ack masks the ioapic entry if an interrupt is send to a
disabled handler.
* tulip_open:

disable_irq()
<sleep>
force the irq line active (enable both TPLnkPass and TPLnkFail, one of
them is always active)
<sleep>
enable_irq()

#insmod tulip
#ifup eth1

and now the interrupt is delivered as edge triggered, but the IRR bit is
set --> irq line stuck.

Could someone test the patch with newer boards (i840, via apollo pro,
perhaps i815 if the bios builds the MP table)

I also tried changing the trigger mode bit in
{,un}mask_level_IO_APIC_irq(), but that doesn't prevent the hang.

The patch is against 2.4.1
--
	Manfred