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+Most (all) Intel-MP compliant SMP boards have the so-called 'IO-APIC',
+which is an enhanced interrupt controller, it enables us to route
+hardware interrupts to multiple CPUs, or to CPU groups.
+Linux supports all variants of compliant SMP boards, including ones with
+multiple IO-APICs. (multiple IO-APICs are used in high-end servers to
+distribute IRQ load further).
+There are (a few) known breakages in certain older boards, which bugs are
+usually worked around by the kernel. If your MP-compliant SMP board does
+not boot Linux, then consult the linux-smp mailing list archives first.
+If your box boots fine with enabled IO-APIC IRQs, then your
+/proc/interrupts will look like this one:
+ hell:~> cat /proc/interrupts
+ 0: 1360293 IO-APIC-edge timer
+ 1: 4 IO-APIC-edge keyboard
+ 2: 0 XT-PIC cascade
+ 13: 1 XT-PIC fpu
+ 14: 1448 IO-APIC-edge ide0
+ 16: 28232 IO-APIC-level Intel EtherExpress Pro 10/100 Ethernet
+ 17: 51304 IO-APIC-level eth0
+ NMI: 0
+ ERR: 0
+some interrupts are still listed as 'XT PIC', but this is not a problem,
+none of those IRQ sources is performance-critical.
+in the unlikely case that your board does not create a working mp-table,
+you can use the pirq= boot parameter to 'hand-construct' IRQ entries. This
+is nontrivial though and cannot be automated. One sample /etc/lilo.conf
+the actual numbers depend on your system, on your PCI cards and on their
+PCI slot position. Usually PCI slots are 'daisy chained' before they are
+connected to the PCI chipset IRQ routing facility (the incoming PIRQ1-4
+ ,-. ,-. ,-. ,-. ,-.
+ PIRQ4 ----| |-. ,-| |-. ,-| |-. ,-| |--------| |
+ |S| \ / |S| \ / |S| \ / |S| |S|
+ PIRQ3 ----|l|-. `/---|l|-. `/---|l|-. `/---|l|--------|l|
+ |o| \/ |o| \/ |o| \/ |o| |o|
+ PIRQ2 ----|t|-./`----|t|-./`----|t|-./`----|t|--------|t|
+ |1| /\ |2| /\ |3| /\ |4| |5|
+ PIRQ1 ----| |- `----| |- `----| |- `----| |--------| |
+ `-' `-' `-' `-' `-'
+every PCI card emits a PCI IRQ, which can be INTA,INTB,INTC,INTD:
+ INTD--| |
+ INTA--| |
+These INTA-D PCI IRQs are always 'local to the card', their real meaning
+depends on which slot they are in. If you look at the daisy chaining diagram,
+a card in slot4, issuing INTA IRQ, it will end up as a signal on PIRQ2 of
+the PCI chipset. Most cards issue INTA, this creates optimal distribution
+between the PIRQ lines. (distributing IRQ sources properly is not a
+necessity, PCI IRQs can be shared at will, but it's a good for performance
+to have non shared interrupts). Slot5 should be used for videocards, they
+do not use interrupts normally, thus they are not daisy chained either.
+so if you have your SCSI card (IRQ11) in Slot1, Tulip card (IRQ9) in
+Slot2, then you'll have to specify this pirq= line:
+the following script tries to figure out such a default pirq= line from
+your PCI configuration:
+ echo -n pirq=; echo `scanpci | grep T_L | cut -c56-` | sed 's/ /,/g'
+note that this script wont work if you have skipped a few slots or if your
+board does not do default daisy-chaining. (or the IO-APIC has the PIRQ pins
+connected in some strange way). E.g. if in the above case you have your SCSI
+card (IRQ11) in Slot3, and have Slot1 empty:
+[value '0' is a generic 'placeholder', reserved for empty (or non-IRQ emitting)
+generally, it's always possible to find out the correct pirq= settings, just
+permute all IRQ numbers properly ... it will take some time though. An
+'incorrect' pirq line will cause the booting process to hang, or a device
+won't function properly (if it's inserted as eg. a module).
+If you have 2 PCI buses, then you can use up to 8 pirq values. Although such
+boards tend to have a good configuration.
+Be prepared that it might happen that you need some strange pirq line:
+use smart try-and-err techniques to find out the correct pirq line ...
+good luck and mail to email@example.com or
+firstname.lastname@example.org if you have any problems that are not covered
+by this document.