FreeBSD Architecture Handbook
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# 10.12 xxx_intr

The interrupt handler is called when an interrupt is received which may be from this particular device. The ISA bus does not support interrupt sharing (except in some special cases) so in practice if the interrupt handler is called then the interrupt almost for sure came from its device. Still, the interrupt handler must poll the device registers and make sure that the interrupt was generated by its device. If not it should just return.

The old convention for the ISA drivers was getting the device unit number as an argument. This is obsolete, and the new drivers receive whatever argument was specified for them in the attach routine when calling `bus_setup_intr()`. By the new convention it should be the pointer to the structure softc. So the interrupt handler commonly starts as:

              static void
              xxx_intr(struct xxx_softc *sc)
              {

It runs at the interrupt priority level specified by the interrupt type parameter of `bus_setup_intr()`. That means that all the other interrupts of the same type as well as all the software interrupts are disabled.

To avoid races it is commonly written as a loop:

              while(xxx_interrupt_pending(sc)) {
                  xxx_process_interrupt(sc);
                  xxx_acknowledge_interrupt(sc);
              }

The interrupt handler has to acknowledge interrupt to the device only but not to the interrupt controller, the system takes care of the latter.

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