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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
+