IO-APIC.rst (4925B)
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