IO-APIC.rst

  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