One of the main problems with interrupt handling is how to perform longish tasks within a handler. Often a substantial amount of work must be done in response to a device interrupt, but interrupt handlers need to finish up quickly and not keep interrupts blocked for long. These two needs (work and speed) conflict with each other, leaving the driver writer in a bit of a bind.

Linux (along with many other systems) resolves this problem by splitting the interrupt handler into two halves. The so-called top half is the routine that actually responds to the interrupt—the one you register with request_irq. The bottom half is a routine that is scheduled by the top half to be executed later, at a safer time. The use of the term bottom half in the 2.4 kernel can be a bit confusing, in that it can mean either the second half of an interrupt handler or one of the mechanisms used to implement this second half, or both. When we refer to a bottom half we are speaking generally about a bottom half; the old Linux bottom-half implementation is referred to explicitly with the acronym BH.

But what is a bottom half useful for?

The big difference between the top-half handler and the bottom half is that all interrupts are enabled during execution of the bottom half—that’s why it runs at a safer time. In the typical scenario, the top half saves device data to a device-specific buffer, schedules its bottom half, and exits: this is very fast. The bottom half then ...