Linux Device Drivers Development

The solution – bottom halves

This idea consists of splitting the handler into two parts:

• The first part, called the top half or hard-IRQ, which is the registered function using request_irq() that will eventually mask/hide interrupts (on the current CPU, except the one being serviced since it is already disabled by the kernel before running the handler) depending on the needs, performs quick and fast operations (essentially time-sensitive tasks, read/write hardware registers, and fast processing of this data), schedules the second and next part, and then acknowledges the line. All interrupts that are disabled must have been re-enabled just before exiting the bottom half.
• The second part, called the bottom half, will process time-consuming ...