FreeBSD Device Drivers

Interrupt Handlers in FreeBSD

Now that you know how to register an interrupt handler, let’s discuss how interrupt handlers are implemented.

In FreeBSD, interrupt handlers are composed of a filter routine, an ithread routine, or both. A filter routine executes in primary interrupt context (that is, it does not have its own context). Thus, it cannot block or context switch, and it can use only spin mutexes for synchronization. Due to these constraints, filter routines are typically used only with devices that require a nonpreemptive interrupt handler.

A filter routine may either completely handle an interrupt or defer the computationally expensive work to its associated ithread routine, assuming it has one. Table 8-2 details the values that a filter ...