linux/Documentation/x86/i386/IO-APIC.rst
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   1.. SPDX-License-Identifier: GPL-2.0
   2
   3=======
   4IO-APIC
   5=======
   6
   7:Author: Ingo Molnar <mingo@kernel.org>
   8
   9Most (all) Intel-MP compliant SMP boards have the so-called 'IO-APIC',
  10which is an enhanced interrupt controller. It enables us to route
  11hardware interrupts to multiple CPUs, or to CPU groups. Without an
  12IO-APIC, interrupts from hardware will be delivered only to the
  13CPU which boots the operating system (usually CPU#0).
  14
  15Linux supports all variants of compliant SMP boards, including ones with
  16multiple IO-APICs. Multiple IO-APICs are used in high-end servers to
  17distribute IRQ load further.
  18
  19There are (a few) known breakages in certain older boards, such bugs are
  20usually worked around by the kernel. If your MP-compliant SMP board does
  21not boot Linux, then consult the linux-smp mailing list archives first.
  22
  23If your box boots fine with enabled IO-APIC IRQs, then your
  24/proc/interrupts will look like this one::
  25
  26  hell:~> cat /proc/interrupts
  27             CPU0
  28    0:    1360293    IO-APIC-edge  timer
  29    1:          4    IO-APIC-edge  keyboard
  30    2:          0          XT-PIC  cascade
  31   13:          1          XT-PIC  fpu
  32   14:       1448    IO-APIC-edge  ide0
  33   16:      28232   IO-APIC-level  Intel EtherExpress Pro 10/100 Ethernet
  34   17:      51304   IO-APIC-level  eth0
  35  NMI:          0
  36  ERR:          0
  37  hell:~>
  38
  39Some interrupts are still listed as 'XT PIC', but this is not a problem;
  40none of those IRQ sources is performance-critical.
  41
  42
  43In the unlikely case that your board does not create a working mp-table,
  44you can use the pirq= boot parameter to 'hand-construct' IRQ entries. This
  45is non-trivial though and cannot be automated. One sample /etc/lilo.conf
  46entry::
  47
  48        append="pirq=15,11,10"
  49
  50The actual numbers depend on your system, on your PCI cards and on their
  51PCI slot position. Usually PCI slots are 'daisy chained' before they are
  52connected to the PCI chipset IRQ routing facility (the incoming PIRQ1-4
  53lines)::
  54
  55               ,-.        ,-.        ,-.        ,-.        ,-.
  56     PIRQ4 ----| |-.    ,-| |-.    ,-| |-.    ,-| |--------| |
  57               |S|  \  /  |S|  \  /  |S|  \  /  |S|        |S|
  58     PIRQ3 ----|l|-. `/---|l|-. `/---|l|-. `/---|l|--------|l|
  59               |o|  \/    |o|  \/    |o|  \/    |o|        |o|
  60     PIRQ2 ----|t|-./`----|t|-./`----|t|-./`----|t|--------|t|
  61               |1| /\     |2| /\     |3| /\     |4|        |5|
  62     PIRQ1 ----| |-  `----| |-  `----| |-  `----| |--------| |
  63               `-'        `-'        `-'        `-'        `-'
  64
  65Every PCI card emits a PCI IRQ, which can be INTA, INTB, INTC or INTD::
  66
  67                               ,-.
  68                         INTD--| |
  69                               |S|
  70                         INTC--|l|
  71                               |o|
  72                         INTB--|t|
  73                               |x|
  74                         INTA--| |
  75                               `-'
  76
  77These INTA-D PCI IRQs are always 'local to the card', their real meaning
  78depends on which slot they are in. If you look at the daisy chaining diagram,
  79a card in slot4, issuing INTA IRQ, it will end up as a signal on PIRQ4 of
  80the PCI chipset. Most cards issue INTA, this creates optimal distribution
  81between the PIRQ lines. (distributing IRQ sources properly is not a
  82necessity, PCI IRQs can be shared at will, but it's a good for performance
  83to have non shared interrupts). Slot5 should be used for videocards, they
  84do not use interrupts normally, thus they are not daisy chained either.
  85
  86so if you have your SCSI card (IRQ11) in Slot1, Tulip card (IRQ9) in
  87Slot2, then you'll have to specify this pirq= line::
  88
  89        append="pirq=11,9"
  90
  91the following script tries to figure out such a default pirq= line from
  92your PCI configuration::
  93
  94        echo -n pirq=; echo `scanpci | grep T_L | cut -c56-` | sed 's/ /,/g'
  95
  96note that this script won't work if you have skipped a few slots or if your
  97board does not do default daisy-chaining. (or the IO-APIC has the PIRQ pins
  98connected in some strange way). E.g. if in the above case you have your SCSI
  99card (IRQ11) in Slot3, and have Slot1 empty::
 100
 101        append="pirq=0,9,11"
 102
 103[value '0' is a generic 'placeholder', reserved for empty (or non-IRQ emitting)
 104slots.]
 105
 106Generally, it's always possible to find out the correct pirq= settings, just
 107permute all IRQ numbers properly ... it will take some time though. An
 108'incorrect' pirq line will cause the booting process to hang, or a device
 109won't function properly (e.g. if it's inserted as a module).
 110
 111If you have 2 PCI buses, then you can use up to 8 pirq values, although such
 112boards tend to have a good configuration.
 113
 114Be prepared that it might happen that you need some strange pirq line::
 115
 116        append="pirq=0,0,0,0,0,0,9,11"
 117
 118Use smart trial-and-error techniques to find out the correct pirq line ...
 119
 120Good luck and mail to linux-smp@vger.kernel.org or
 121linux-kernel@vger.kernel.org if you have any problems that are not covered
 122by this document.
 123
 124