124 lines
4.8 KiB
ReStructuredText
124 lines
4.8 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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=======
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IO-APIC
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=======
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:Author: Ingo Molnar <mingo@kernel.org>
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Most (all) Intel-MP compliant SMP boards have the so-called 'IO-APIC',
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which is an enhanced interrupt controller. It enables us to route
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hardware interrupts to multiple CPUs, or to CPU groups. Without an
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IO-APIC, interrupts from hardware will be delivered only to the
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CPU which boots the operating system (usually CPU#0).
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Linux supports all variants of compliant SMP boards, including ones with
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multiple IO-APICs. Multiple IO-APICs are used in high-end servers to
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distribute IRQ load further.
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There are (a few) known breakages in certain older boards, such bugs are
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usually worked around by the kernel. If your MP-compliant SMP board does
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not boot Linux, then consult the linux-smp mailing list archives first.
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If your box boots fine with enabled IO-APIC IRQs, then your
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/proc/interrupts will look like this one::
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hell:~> cat /proc/interrupts
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CPU0
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0: 1360293 IO-APIC-edge timer
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1: 4 IO-APIC-edge keyboard
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2: 0 XT-PIC cascade
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13: 1 XT-PIC fpu
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14: 1448 IO-APIC-edge ide0
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16: 28232 IO-APIC-level Intel EtherExpress Pro 10/100 Ethernet
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17: 51304 IO-APIC-level eth0
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NMI: 0
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ERR: 0
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hell:~>
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Some interrupts are still listed as 'XT PIC', but this is not a problem;
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none of those IRQ sources is performance-critical.
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In the unlikely case that your board does not create a working mp-table,
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you can use the pirq= boot parameter to 'hand-construct' IRQ entries. This
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is non-trivial though and cannot be automated. One sample /etc/lilo.conf
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entry::
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append="pirq=15,11,10"
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The actual numbers depend on your system, on your PCI cards and on their
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PCI slot position. Usually PCI slots are 'daisy chained' before they are
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connected to the PCI chipset IRQ routing facility (the incoming PIRQ1-4
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lines)::
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,-. ,-. ,-. ,-. ,-.
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PIRQ4 ----| |-. ,-| |-. ,-| |-. ,-| |--------| |
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|S| \ / |S| \ / |S| \ / |S| |S|
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PIRQ3 ----|l|-. `/---|l|-. `/---|l|-. `/---|l|--------|l|
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|o| \/ |o| \/ |o| \/ |o| |o|
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PIRQ2 ----|t|-./`----|t|-./`----|t|-./`----|t|--------|t|
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|1| /\ |2| /\ |3| /\ |4| |5|
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PIRQ1 ----| |- `----| |- `----| |- `----| |--------| |
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`-' `-' `-' `-' `-'
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Every PCI card emits a PCI IRQ, which can be INTA, INTB, INTC or INTD::
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,-.
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INTD--| |
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|S|
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INTC--|l|
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|o|
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INTB--|t|
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|x|
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INTA--| |
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`-'
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These INTA-D PCI IRQs are always 'local to the card', their real meaning
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depends on which slot they are in. If you look at the daisy chaining diagram,
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a card in slot4, issuing INTA IRQ, it will end up as a signal on PIRQ4 of
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the PCI chipset. Most cards issue INTA, this creates optimal distribution
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between the PIRQ lines. (distributing IRQ sources properly is not a
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necessity, PCI IRQs can be shared at will, but it's a good for performance
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to have non shared interrupts). Slot5 should be used for videocards, they
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do not use interrupts normally, thus they are not daisy chained either.
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so if you have your SCSI card (IRQ11) in Slot1, Tulip card (IRQ9) in
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Slot2, then you'll have to specify this pirq= line::
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append="pirq=11,9"
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the following script tries to figure out such a default pirq= line from
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your PCI configuration::
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echo -n pirq=; echo `scanpci | grep T_L | cut -c56-` | sed 's/ /,/g'
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note that this script won't work if you have skipped a few slots or if your
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board does not do default daisy-chaining. (or the IO-APIC has the PIRQ pins
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connected in some strange way). E.g. if in the above case you have your SCSI
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card (IRQ11) in Slot3, and have Slot1 empty::
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append="pirq=0,9,11"
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[value '0' is a generic 'placeholder', reserved for empty (or non-IRQ emitting)
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slots.]
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Generally, it's always possible to find out the correct pirq= settings, just
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permute all IRQ numbers properly ... it will take some time though. An
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'incorrect' pirq line will cause the booting process to hang, or a device
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won't function properly (e.g. if it's inserted as a module).
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If you have 2 PCI buses, then you can use up to 8 pirq values, although such
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boards tend to have a good configuration.
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Be prepared that it might happen that you need some strange pirq line::
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append="pirq=0,0,0,0,0,0,9,11"
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Use smart trial-and-error techniques to find out the correct pirq line ...
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Good luck and mail to linux-smp@vger.kernel.org or
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linux-kernel@vger.kernel.org if you have any problems that are not covered
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by this document.
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