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Just note in the docs: 1) A PCI device as an example for shared interrupts 2) A sparse tree can be used for interrupts too 3) i8259s which have 8 pins [ tglx: Fix up subject prefix ] Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Link: https://lore.kernel.org/all/20250319092951.37667-55-jirislaby@kernel.org
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===============
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What is an IRQ?
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===============
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An IRQ is an interrupt request from a device. Currently, they can come
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in over a pin, or over a packet. Several devices may be connected to
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the same pin thus sharing an IRQ. Such as on legacy PCI bus: All devices
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typically share 4 lanes/pins. Note that each device can request an
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interrupt on each of the lanes.
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An IRQ number is a kernel identifier used to talk about a hardware
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interrupt source. Typically, this is an index into the global irq_desc
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array or sparse_irqs tree. But except for what linux/interrupt.h
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implements, the details are architecture specific.
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An IRQ number is an enumeration of the possible interrupt sources on a
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machine. Typically, what is enumerated is the number of input pins on
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all of the interrupt controllers in the system. In the case of ISA,
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what is enumerated are the 8 input pins on each of the two i8259
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interrupt controllers.
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Architectures can assign additional meaning to the IRQ numbers, and
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are encouraged to in the case where there is any manual configuration
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of the hardware involved. The ISA IRQs are a classic example of
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assigning this kind of additional meaning.
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