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- ===============
- 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. Such as on legacy PCI bus: All devices
- typically share 4 lanes/pins. Note that each device can request an
- interrupt on each of the lanes.
- 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 or sparse_irqs tree. 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 controllers in the system. In the case of ISA,
- what is enumerated are the 8 input pins on each of 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.
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