IO-APIC.txt 4.6 KB

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  1. Most (all) Intel-MP compliant SMP boards have the so-called 'IO-APIC',
  2. which is an enhanced interrupt controller, it enables us to route
  3. hardware interrupts to multiple CPUs, or to CPU groups.
  4. Linux supports all variants of compliant SMP boards, including ones with
  5. multiple IO-APICs. (multiple IO-APICs are used in high-end servers to
  6. distribute IRQ load further).
  7. There are (a few) known breakages in certain older boards, which bugs are
  8. usually worked around by the kernel. If your MP-compliant SMP board does
  9. not boot Linux, then consult the linux-smp mailing list archives first.
  10. If your box boots fine with enabled IO-APIC IRQs, then your
  11. /proc/interrupts will look like this one:
  12. ---------------------------->
  13. hell:~> cat /proc/interrupts
  14. CPU0
  15. 0: 1360293 IO-APIC-edge timer
  16. 1: 4 IO-APIC-edge keyboard
  17. 2: 0 XT-PIC cascade
  18. 13: 1 XT-PIC fpu
  19. 14: 1448 IO-APIC-edge ide0
  20. 16: 28232 IO-APIC-level Intel EtherExpress Pro 10/100 Ethernet
  21. 17: 51304 IO-APIC-level eth0
  22. NMI: 0
  23. ERR: 0
  24. hell:~>
  25. <----------------------------
  26. some interrupts are still listed as 'XT PIC', but this is not a problem,
  27. none of those IRQ sources is performance-critical.
  28. in the unlikely case that your board does not create a working mp-table,
  29. you can use the pirq= boot parameter to 'hand-construct' IRQ entries. This
  30. is nontrivial though and cannot be automated. One sample /etc/lilo.conf
  31. entry:
  32. append="pirq=15,11,10"
  33. the actual numbers depend on your system, on your PCI cards and on their
  34. PCI slot position. Usually PCI slots are 'daisy chained' before they are
  35. connected to the PCI chipset IRQ routing facility (the incoming PIRQ1-4
  36. lines):
  37. ,-. ,-. ,-. ,-. ,-.
  38. PIRQ4 ----| |-. ,-| |-. ,-| |-. ,-| |--------| |
  39. |S| \ / |S| \ / |S| \ / |S| |S|
  40. PIRQ3 ----|l|-. `/---|l|-. `/---|l|-. `/---|l|--------|l|
  41. |o| \/ |o| \/ |o| \/ |o| |o|
  42. PIRQ2 ----|t|-./`----|t|-./`----|t|-./`----|t|--------|t|
  43. |1| /\ |2| /\ |3| /\ |4| |5|
  44. PIRQ1 ----| |- `----| |- `----| |- `----| |--------| |
  45. `-' `-' `-' `-' `-'
  46. every PCI card emits a PCI IRQ, which can be INTA,INTB,INTC,INTD:
  47. ,-.
  48. INTD--| |
  49. |S|
  50. INTC--|l|
  51. |o|
  52. INTB--|t|
  53. |x|
  54. INTA--| |
  55. `-'
  56. These INTA-D PCI IRQs are always 'local to the card', their real meaning
  57. depends on which slot they are in. If you look at the daisy chaining diagram,
  58. a card in slot4, issuing INTA IRQ, it will end up as a signal on PIRQ2 of
  59. the PCI chipset. Most cards issue INTA, this creates optimal distribution
  60. between the PIRQ lines. (distributing IRQ sources properly is not a
  61. necessity, PCI IRQs can be shared at will, but it's a good for performance
  62. to have non shared interrupts). Slot5 should be used for videocards, they
  63. do not use interrupts normally, thus they are not daisy chained either.
  64. so if you have your SCSI card (IRQ11) in Slot1, Tulip card (IRQ9) in
  65. Slot2, then you'll have to specify this pirq= line:
  66. append="pirq=11,9"
  67. the following script tries to figure out such a default pirq= line from
  68. your PCI configuration:
  69. echo -n pirq=; echo `scanpci | grep T_L | cut -c56-` | sed 's/ /,/g'
  70. note that this script wont work if you have skipped a few slots or if your
  71. board does not do default daisy-chaining. (or the IO-APIC has the PIRQ pins
  72. connected in some strange way). E.g. if in the above case you have your SCSI
  73. card (IRQ11) in Slot3, and have Slot1 empty:
  74. append="pirq=0,9,11"
  75. [value '0' is a generic 'placeholder', reserved for empty (or non-IRQ emitting)
  76. slots.]
  77. generally, it's always possible to find out the correct pirq= settings, just
  78. permute all IRQ numbers properly ... it will take some time though. An
  79. 'incorrect' pirq line will cause the booting process to hang, or a device
  80. won't function properly (if it's inserted as eg. a module).
  81. If you have 2 PCI buses, then you can use up to 8 pirq values. Although such
  82. boards tend to have a good configuration.
  83. Be prepared that it might happen that you need some strange pirq line:
  84. append="pirq=0,0,0,0,0,0,9,11"
  85. use smart try-and-err techniques to find out the correct pirq line ...
  86. good luck and mail to linux-smp@vger.kernel.org or
  87. linux-kernel@vger.kernel.org if you have any problems that are not covered
  88. by this document.
  89. -- mingo