1 .. SPDX-License-Identifier: GPL-2.0 2 3 ======= 4 IO-APIC 5 ======= 6 7 :Author: Ingo Molnar <mingo@kernel.org> 8 9 Most (all) Intel-MP compliant SMP boards have the so-called 'IO-APIC', 10 which is an enhanced interrupt controller. It enables us to route 11 hardware interrupts to multiple CPUs, or to CPU groups. Without an 12 IO-APIC, interrupts from hardware will be delivered only to the 13 CPU which boots the operating system (usually CPU#0). 14 15 Linux supports all variants of compliant SMP boards, including ones with 16 multiple IO-APICs. Multiple IO-APICs are used in high-end servers to 17 distribute IRQ load further. 18 19 There are (a few) known breakages in certain older boards, such bugs are 20 usually worked around by the kernel. If your MP-compliant SMP board does 21 not boot Linux, then consult the linux-smp mailing list archives first. 22 23 If 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 39 Some interrupts are still listed as 'XT PIC', but this is not a problem; 40 none of those IRQ sources is performance-critical. 41 42 43 In the unlikely case that your board does not create a working mp-table, 44 you can use the pirq= boot parameter to 'hand-construct' IRQ entries. This 45 is non-trivial though and cannot be automated. One sample /etc/lilo.conf 46 entry:: 47 48 append="pirq=15,11,10" 49 50 The actual numbers depend on your system, on your PCI cards and on their 51 PCI slot position. Usually PCI slots are 'daisy chained' before they are 52 connected to the PCI chipset IRQ routing facility (the incoming PIRQ1-4 53 lines):: 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 65 Every 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 77 These INTA-D PCI IRQs are always 'local to the card', their real meaning 78 depends on which slot they are in. If you look at the daisy chaining diagram, 79 a card in slot4, issuing INTA IRQ, it will end up as a signal on PIRQ4 of 80 the PCI chipset. Most cards issue INTA, this creates optimal distribution 81 between the PIRQ lines. (distributing IRQ sources properly is not a 82 necessity, PCI IRQs can be shared at will, but it's a good for performance 83 to have non shared interrupts). Slot5 should be used for videocards, they 84 do not use interrupts normally, thus they are not daisy chained either. 85 86 so if you have your SCSI card (IRQ11) in Slot1, Tulip card (IRQ9) in 87 Slot2, then you'll have to specify this pirq= line:: 88 89 append="pirq=11,9" 90 91 the following script tries to figure out such a default pirq= line from 92 your PCI configuration:: 93 94 echo -n pirq=; echo `scanpci | grep T_L | cut -c56-` | sed 's/ /,/g' 95 96 note that this script won't work if you have skipped a few slots or if your 97 board does not do default daisy-chaining. (or the IO-APIC has the PIRQ pins 98 connected in some strange way). E.g. if in the above case you have your SCSI 99 card (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) 104 slots.] 105 106 Generally, it's always possible to find out the correct pirq= settings, just 107 permute 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 109 won't function properly (e.g. if it's inserted as a module). 110 111 If you have 2 PCI buses, then you can use up to 8 pirq values, although such 112 boards tend to have a good configuration. 113 114 Be prepared that it might happen that you need some strange pirq line:: 115 116 append="pirq=0,0,0,0,0,0,9,11" 117 118 Use smart trial-and-error techniques to find out the correct pirq line ... 119 120 Good luck and mail to linux-smp@vger.kernel.org or 121 linux-kernel@vger.kernel.org if you have any problems that are not covered 122 by this document. 123
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