1 Most (all) Intel SMP boards have the so-called 'IO-APIC', which is
2 an enhanced interrupt controller, able to route hardware interrupts
3 to multiple CPUs, or to CPU groups.
5 Linux supports the IO-APIC, but unfortunately there are broken boards
6 out there which make it unsafe to enable the IO-APIC unconditionally.
7 The Linux policy thus is to enable the IO-APIC only if it's 100% safe, ie.:
9 - the board is on the 'whitelist'
11 or - the board does not have PCI pins connected to the IO-APIC
13 or - the user has overridden blacklisted settings with the
14 pirq= boot option line.
16 Kernel messages tell you whether the board is 'safe'. If your box
17 boots with enabled IO-APIC IRQs, then you have nothing else to do. Your
18 /proc/interrupts will look like this one:
20 ---------------------------->
21 hell:~> cat /proc/interrupts
23 0: 90782 0 XT PIC timer
24 1: 4135 2375 IO-APIC keyboard
26 3: 851 807 IO-APIC serial
27 9: 6 22 IO-APIC ncr53c8xx
28 11: 307 154 IO-APIC NE2000
30 14: 56000 30610 IO-APIC ide0
33 <----------------------------
35 some interrupts will still be 'XT PIC', but this is not a problem, none
36 of those IRQ sources is 'heavy'.
38 If one of your boot messages says 'unlisted/blacklisted board, DISABLING
39 IO-APIC IRQs', then you should do this to get multi-CPU IO-APIC IRQs
42 A) if your board is unlisted, then mail to linux-smp to get
43 it into either the white or the blacklist
44 B) if your board is blacklisted, then figure out the appropriate
45 pirq= option to get your system to boot
48 pirq= lines look like the following in /etc/lilo.conf:
50 append="pirq=15,11,10"
52 the actual numbers depend on your system, on your PCI cards and on their
53 PCI slot position. Usually PCI slots are 'daisy chained' before they are
54 connected to the PCI chipset IRQ routing facility (the incoming PIRQ1-4
58 PIRQ4 ----| |-. ,-| |-. ,-| |-. ,-| |--------| |
59 |S| \ / |S| \ / |S| \ / |S| |S|
60 PIRQ3 ----|l|-. `/---|l|-. `/---|l|-. `/---|l|--------|l|
61 |o| \/ |o| \/ |o| \/ |o| |o|
62 PIRQ2 ----|t|-./`----|t|-./`----|t|-./`----|t|--------|t|
63 |1| /\ |2| /\ |3| /\ |4| |5|
64 PIRQ1 ----| |- `----| |- `----| |- `----| |--------| |
67 every PCI card emits a PCI IRQ, which can be INTA,INTB,INTC,INTD:
79 These INTA-D PCI IRQs are always 'local to the card', their real meaning
80 depends on which slot they are in. If you look at the daisy chaining diagram,
81 a card in slot4, issuing INTA IRQ, it will end up as a signal on PIRQ2 of
82 the PCI chipset. Most cards issue INTA, this creates optimal distribution
83 between the PIRQ lines. (distributing IRQ sources properly is not a
84 necessity, PCI IRQs can be shared at will, but it's a good for performance
85 to have non shared interrupts). Slot5 should be used for videocards, they
86 do not use interrupts normally, thus they are not daisy chained either.
88 so if you have your SCSI card (IRQ11) in Slot1, Tulip card (IRQ9) in
89 Slot2, then you'll have to specify this pirq= line:
93 the following script tries to figure out such a default pirq= line from
94 your PCI configuration:
96 echo -n pirq=; echo `scanpci | grep T_L | cut -c56-` | sed 's/ /,/g'
98 note that this script wont work if you have skipped a few slots or if your
99 board does not do default daisy-chaining. (or the IO-APIC has the PIRQ pins
100 connected in some strange way). E.g. if in the above case you have your SCSI
101 card (IRQ11) in Slot3, and have Slot1 empty:
105 [value '0' is a generic 'placeholder', reserved for empty (or non-IRQ emitting)
108 generally, it's always possible to find out the correct pirq= settings, just
109 permute all IRQ numbers properly ... it will take some time though. An
110 'incorrect' pirq line will cause the booting process to hang, or a device
111 won't function properly (if it's inserted as eg. a module).
113 If you have 2 PCI buses, then you can use up to 8 pirq values. Although such
114 boards tend to have a good configuration and will be included in the
117 Be prepared that it might happen that you need some strange pirq line:
119 append="pirq=0,0,0,0,0,0,9,11"
121 use smart try-and-err techniques to find out the correct pirq line ...
124 the following pirq line can be used to force a board into the whitelist:
128 [if your system works with no problems after this, then it should be added
129 to the official whitelist, contact us]
131 good luck and mail to linux-smp@vger.rutgers.edu or
132 linux-kernel@vger.rutgers.edu if you have any problems that are not covered