| .. SPDX-License-Identifier: GPL-2.0 |
| |
| ======= |
| IO-APIC |
| ======= |
| |
| :Author: Ingo Molnar <mingo@kernel.org> |
| |
| 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. 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. |
| |
| 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. |
| |
| If your box boots fine with enabled IO-APIC IRQs, then your |
| /proc/interrupts will look like this one:: |
| |
| hell:~> cat /proc/interrupts |
| CPU0 |
| 0: 1360293 IO-APIC-edge timer |
| 1: 4 IO-APIC-edge keyboard |
| 2: 0 XT-PIC cascade |
| 13: 1 XT-PIC fpu |
| 14: 1448 IO-APIC-edge ide0 |
| 16: 28232 IO-APIC-level Intel EtherExpress Pro 10/100 Ethernet |
| 17: 51304 IO-APIC-level eth0 |
| NMI: 0 |
| ERR: 0 |
| hell:~> |
| |
| 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, |
| you can use the pirq= boot parameter to 'hand-construct' IRQ entries. This |
| 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 |
| 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):: |
| |
| ,-. ,-. ,-. ,-. ,-. |
| PIRQ4 ----| |-. ,-| |-. ,-| |-. ,-| |--------| | |
| |S| \ / |S| \ / |S| \ / |S| |S| |
| PIRQ3 ----|l|-. `/---|l|-. `/---|l|-. `/---|l|--------|l| |
| |o| \/ |o| \/ |o| \/ |o| |o| |
| PIRQ2 ----|t|-./`----|t|-./`----|t|-./`----|t|--------|t| |
| |1| /\ |2| /\ |3| /\ |4| |5| |
| PIRQ1 ----| |- `----| |- `----| |- `----| |--------| | |
| `-' `-' `-' `-' `-' |
| |
| Every PCI card emits a PCI IRQ, which can be INTA, INTB, INTC or INTD:: |
| |
| ,-. |
| INTD--| | |
| |S| |
| INTC--|l| |
| |o| |
| INTB--|t| |
| |x| |
| INTA--| | |
| `-' |
| |
| These INTA-D PCI IRQs are always 'local to the card', their real meaning |
| depends on which slot they are in. If you look at the daisy chaining diagram, |
| a card in slot4, issuing INTA IRQ, it will end up as a signal on PIRQ4 of |
| the PCI chipset. Most cards issue INTA, this creates optimal distribution |
| between the PIRQ lines. (distributing IRQ sources properly is not a |
| necessity, PCI IRQs can be shared at will, but it's a good for performance |
| to have non shared interrupts). Slot5 should be used for videocards, they |
| do not use interrupts normally, thus they are not daisy chained either. |
| |
| so if you have your SCSI card (IRQ11) in Slot1, Tulip card (IRQ9) in |
| Slot2, then you'll have to specify this pirq= line:: |
| |
| append="pirq=11,9" |
| |
| the following script tries to figure out such a default pirq= line from |
| your PCI configuration:: |
| |
| echo -n pirq=; echo `scanpci | grep T_L | cut -c56-` | sed 's/ /,/g' |
| |
| note that this script won't work if you have skipped a few slots or if your |
| board does not do default daisy-chaining. (or the IO-APIC has the PIRQ pins |
| connected in some strange way). E.g. if in the above case you have your SCSI |
| card (IRQ11) in Slot3, and have Slot1 empty:: |
| |
| append="pirq=0,9,11" |
| |
| [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 |
| 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 (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 |
| 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 trial-and-error techniques to find out the correct pirq line ... |
| |
| 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. |
| |
