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Rename the ia64 documentation files to ReST, add an index for them and adjust in order to produce a nice html output via the Sphinx build system. There are two upper case file names. Rename them to lower case, as we're working to avoid upper case file names at Documentation. At its new index.rst, let's add a :orphan: while this is not linked to the main index.rst file, in order to avoid build warnings. Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
81 lines
2.6 KiB
ReStructuredText
81 lines
2.6 KiB
ReStructuredText
==============================
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IRQ affinity on IA64 platforms
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==============================
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07.01.2002, Erich Focht <efocht@ess.nec.de>
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By writing to /proc/irq/IRQ#/smp_affinity the interrupt routing can be
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controlled. The behavior on IA64 platforms is slightly different from
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that described in Documentation/IRQ-affinity.txt for i386 systems.
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Because of the usage of SAPIC mode and physical destination mode the
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IRQ target is one particular CPU and cannot be a mask of several
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CPUs. Only the first non-zero bit is taken into account.
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Usage examples
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==============
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The target CPU has to be specified as a hexadecimal CPU mask. The
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first non-zero bit is the selected CPU. This format has been kept for
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compatibility reasons with i386.
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Set the delivery mode of interrupt 41 to fixed and route the
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interrupts to CPU #3 (logical CPU number) (2^3=0x08)::
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echo "8" >/proc/irq/41/smp_affinity
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Set the default route for IRQ number 41 to CPU 6 in lowest priority
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delivery mode (redirectable)::
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echo "r 40" >/proc/irq/41/smp_affinity
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The output of the command::
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cat /proc/irq/IRQ#/smp_affinity
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gives the target CPU mask for the specified interrupt vector. If the CPU
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mask is preceded by the character "r", the interrupt is redirectable
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(i.e. lowest priority mode routing is used), otherwise its route is
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fixed.
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Initialization and default behavior
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===================================
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If the platform features IRQ redirection (info provided by SAL) all
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IO-SAPIC interrupts are initialized with CPU#0 as their default target
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and the routing is the so called "lowest priority mode" (actually
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fixed SAPIC mode with hint). The XTP chipset registers are used as hints
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for the IRQ routing. Currently in Linux XTP registers can have three
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values:
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- minimal for an idle task,
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- normal if any other task runs,
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- maximal if the CPU is going to be switched off.
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The IRQ is routed to the CPU with lowest XTP register value, the
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search begins at the default CPU. Therefore most of the interrupts
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will be handled by CPU #0.
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If the platform doesn't feature interrupt redirection IOSAPIC fixed
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routing is used. The target CPUs are distributed in a round robin
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manner. IRQs will be routed only to the selected target CPUs. Check
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with::
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cat /proc/interrupts
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Comments
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========
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On large (multi-node) systems it is recommended to route the IRQs to
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the node to which the corresponding device is connected.
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For systems like the NEC AzusA we get IRQ node-affinity for free. This
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is because usually the chipsets on each node redirect the interrupts
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only to their own CPUs (as they cannot see the XTP registers on the
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other nodes).
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