x86: docs fixes to Documentation/i386/IO-APIC.txt

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.

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).

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, which bugs are

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.

  hell:~>

  <----------------------------

some interrupts are still listed as 'XT PIC', but this is not a problem,

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,

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 nontrivial though and cannot be automated. One sample /etc/lilo.conf

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

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):

     PIRQ1 ----| |-  `----| |-  `----| |-  `----| |--------| |

               `-'        `-'        `-'        `-'        `-'

every PCI card emits a PCI IRQ, which can be INTA,INTB,INTC,INTD:

Every PCI card emits a PCI IRQ, which can be INTA, INTB, INTC or INTD:

                               ,-.

                         INTD--| |

[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

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 (if it's inserted as eg. a module).

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

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 try-and-err techniques to find out the correct pirq line ...

Use smart trial-and-error techniques to find out the correct pirq line ...

good luck and mail to linux-smp@vger.kernel.org or

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.