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    Subject[PATCH 25/25] irq: Document what an IRQ is.
    Date
    From: Eric W. Biederman <ebiederman@lnxi.com>

    Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
    ---
    Documentation/IRQ.txt | 22 ++++++++++++++++++++++
    1 files changed, 22 insertions(+), 0 deletions(-)

    diff --git a/Documentation/IRQ.txt b/Documentation/IRQ.txt
    new file mode 100644
    index 0000000..237235d
    --- /dev/null
    +++ b/Documentation/IRQ.txt
    @@ -0,0 +1,22 @@
    +What is an IRQ?
    +
    +An IRQ is an interrupt request from a device.
    +Currently they can come in over a pin, or over a packet.
    +Several devices may be connected to the same pin thus
    +sharing an IRQ.
    +
    +An IRQ number is a kernel identifier used to talk about a hardware
    +interrupt source. Typically this is an index into the global irq_desc
    +array, but except for what linux/interrupt.h implements the details
    +are architecture specific.
    +
    +An IRQ number is an enumeration of the possible interrupt sources on a
    +machine. Typically what is enumerated is the number of input pins on
    +all of the interrupt controller in the system. In the case of ISA
    +what is enumerated are the 16 input pins on the two i8259 interrupt
    +controllers.
    +
    +Architectures can assign additional meaning to the IRQ numbers, and
    +are encouraged to in the case where there is any manual configuration
    +of the hardware involved. The ISA IRQs are a classic example of
    +assigning this kind of additional meaning.
    --
    1.4.0.gc07e
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