1IRQ affinity on IA64 platforms
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3                           07.01.2002, Erich Focht <efocht@ess.nec.de>
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6By writing to /proc/irq/IRQ#/smp_affinity the interrupt routing can be
7controlled. The behavior on IA64 platforms is slightly different from
8that described in Documentation/IRQ-affinity.txt for i386 systems.
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10Because of the usage of SAPIC mode and physical destination mode the
11IRQ target is one particular CPU and cannot be a mask of several
12CPUs. Only the first non-zero bit is taken into account.
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14
15Usage examples:
16
17The target CPU has to be specified as a hexadecimal CPU mask. The
18first non-zero bit is the selected CPU. This format has been kept for
19compatibility reasons with i386.
20
21Set the delivery mode of interrupt 41 to fixed and route the
22interrupts to CPU #3 (logical CPU number) (2^3=0x08):
23     echo "8" >/proc/irq/41/smp_affinity
24
25Set the default route for IRQ number 41 to CPU 6 in lowest priority
26delivery mode (redirectable):
27     echo "r 40" >/proc/irq/41/smp_affinity
28
29The output of the command
30     cat /proc/irq/IRQ#/smp_affinity
31gives the target CPU mask for the specified interrupt vector. If the CPU
32mask is preceded by the character "r", the interrupt is redirectable
33(i.e. lowest priority mode routing is used), otherwise its route is
34fixed.
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38Initialization and default behavior:
39
40If the platform features IRQ redirection (info provided by SAL) all
41IO-SAPIC interrupts are initialized with CPU#0 as their default target
42and the routing is the so called "lowest priority mode" (actually
43fixed SAPIC mode with hint). The XTP chipset registers are used as hints
44for the IRQ routing. Currently in Linux XTP registers can have three
45values:
46	- minimal for an idle task,
47	- normal if any other task runs,
48	- maximal if the CPU is going to be switched off.
49The IRQ is routed to the CPU with lowest XTP register value, the
50search begins at the default CPU. Therefore most of the interrupts
51will be handled by CPU #0.
52
53If the platform doesn't feature interrupt redirection IOSAPIC fixed
54routing is used. The target CPUs are distributed in a round robin
55manner. IRQs will be routed only to the selected target CPUs. Check
56with
57        cat /proc/interrupts
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60
61Comments:
62
63On large (multi-node) systems it is recommended to route the IRQs to
64the node to which the corresponding device is connected.
65For systems like the NEC AzusA we get IRQ node-affinity for free. This
66is because usually the chipsets on each node redirect the interrupts
67only to their own CPUs (as they cannot see the XTP registers on the
68other nodes).
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