1Most (all) Intel-MP compliant SMP boards have the so-called 'IO-APIC',
   2which is an enhanced interrupt controller. It enables us to route
   3hardware interrupts to multiple CPUs, or to CPU groups. Without an
   4IO-APIC, interrupts from hardware will be delivered only to the
   5CPU which boots the operating system (usually CPU#0).
   7Linux supports all variants of compliant SMP boards, including ones with
   8multiple IO-APICs. Multiple IO-APICs are used in high-end servers to
   9distribute IRQ load further.
  11There are (a few) known breakages in certain older boards, such bugs are
  12usually worked around by the kernel. If your MP-compliant SMP board does
  13not boot Linux, then consult the linux-smp mailing list archives first.
  15If your box boots fine with enabled IO-APIC IRQs, then your
  16/proc/interrupts will look like this one:
  18   ---------------------------->
  19  hell:~> cat /proc/interrupts
  20             CPU0
  21    0:    1360293    IO-APIC-edge  timer
  22    1:          4    IO-APIC-edge  keyboard
  23    2:          0          XT-PIC  cascade
  24   13:          1          XT-PIC  fpu
  25   14:       1448    IO-APIC-edge  ide0
  26   16:      28232   IO-APIC-level  Intel EtherExpress Pro 10/100 Ethernet
  27   17:      51304   IO-APIC-level  eth0
  28  NMI:          0
  29  ERR:          0
  30  hell:~>
  31  <----------------------------
  33Some interrupts are still listed as 'XT PIC', but this is not a problem;
  34none of those IRQ sources is performance-critical.
  37In the unlikely case that your board does not create a working mp-table,
  38you can use the pirq= boot parameter to 'hand-construct' IRQ entries. This
  39is non-trivial though and cannot be automated. One sample /etc/lilo.conf
  42        append="pirq=15,11,10"
  44The actual numbers depend on your system, on your PCI cards and on their
  45PCI slot position. Usually PCI slots are 'daisy chained' before they are
  46connected to the PCI chipset IRQ routing facility (the incoming PIRQ1-4
  49               ,-.        ,-.        ,-.        ,-.        ,-.
  50     PIRQ4 ----| |-.    ,-| |-.    ,-| |-.    ,-| |--------| |
  51               |S|  \  /  |S|  \  /  |S|  \  /  |S|        |S|
  52     PIRQ3 ----|l|-. `/---|l|-. `/---|l|-. `/---|l|--------|l|
  53               |o|  \/    |o|  \/    |o|  \/    |o|        |o|
  54     PIRQ2 ----|t|-./`----|t|-./`----|t|-./`----|t|--------|t|
  55               |1| /\     |2| /\     |3| /\     |4|        |5|
  56     PIRQ1 ----| |-  `----| |-  `----| |-  `----| |--------| |
  57               `-'        `-'        `-'        `-'        `-'
  59Every PCI card emits a PCI IRQ, which can be INTA, INTB, INTC or INTD:
  61                               ,-.
  62                         INTD--| |
  63                               |S|
  64                         INTC--|l|
  65                               |o|
  66                         INTB--|t|
  67                               |x|
  68                         INTA--| |
  69                               `-'
  71These INTA-D PCI IRQs are always 'local to the card', their real meaning
  72depends on which slot they are in. If you look at the daisy chaining diagram,
  73a card in slot4, issuing INTA IRQ, it will end up as a signal on PIRQ4 of
  74the PCI chipset. Most cards issue INTA, this creates optimal distribution
  75between the PIRQ lines. (distributing IRQ sources properly is not a
  76necessity, PCI IRQs can be shared at will, but it's a good for performance
  77to have non shared interrupts). Slot5 should be used for videocards, they
  78do not use interrupts normally, thus they are not daisy chained either.
  80so if you have your SCSI card (IRQ11) in Slot1, Tulip card (IRQ9) in
  81Slot2, then you'll have to specify this pirq= line:
  83        append="pirq=11,9"
  85the following script tries to figure out such a default pirq= line from
  86your PCI configuration:
  88        echo -n pirq=; echo `scanpci | grep T_L | cut -c56-` | sed 's/ /,/g'
  90note that this script won't work if you have skipped a few slots or if your
  91board does not do default daisy-chaining. (or the IO-APIC has the PIRQ pins
  92connected in some strange way). E.g. if in the above case you have your SCSI
  93card (IRQ11) in Slot3, and have Slot1 empty:
  95        append="pirq=0,9,11"
  97[value '0' is a generic 'placeholder', reserved for empty (or non-IRQ emitting)
 100Generally, it's always possible to find out the correct pirq= settings, just
 101permute all IRQ numbers properly ... it will take some time though. An
 102'incorrect' pirq line will cause the booting process to hang, or a device
 103won't function properly (e.g. if it's inserted as a module).
 105If you have 2 PCI buses, then you can use up to 8 pirq values, although such
 106boards tend to have a good configuration.
 108Be prepared that it might happen that you need some strange pirq line:
 110        append="pirq=0,0,0,0,0,0,9,11"
 112Use smart trial-and-error techniques to find out the correct pirq line ...
 114Good luck and mail to or if you have any problems that are not covered
 116by this document.
 118-- mingo
 120 kindly hosted by Redpill Linpro AS, provider of Linux consulting and operations services since 1995.