| =============== |
| 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. |
