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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. Without an
-IO-APIC, interrupts from hardware will be delivered only to the
-CPU which boots the operating system (usually CPU#0).
-
-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, such 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
- CPU0
- 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
- hell:~>
- <----------------------------
-
-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 non-trivial though and cannot be automated. One sample /etc/lilo.conf
-entry:
-
- append="pirq=15,11,10"
-
-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
-lines):
-
- ,-. ,-. ,-. ,-. ,-.
- 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 or INTD:
-
- ,-.
- INTD--| |
- |S|
- INTC--|l|
- |o|
- INTB--|t|
- |x|
- 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 PIRQ4 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:
-
- append="pirq=11,9"
-
-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:
-
- append="pirq=0,9,11"
-
-[value '0' is a generic 'placeholder', reserved for empty (or non-IRQ emitting)
-slots.]
-
-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 (e.g. if it's inserted as 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:
-
- append="pirq=0,0,0,0,0,0,9,11"
-
-Use smart trial-and-error techniques to find out the correct pirq line ...
-
-Good luck and mail to linux-smp@vger.kernel.org or
-linux-kernel@vger.kernel.org if you have any problems that are not covered
-by this document.
-
--- mingo
-