Handlers registered through request_irq() are executed by the interrupt-handling path of the kernel. This code path is asynchronous, and runs by suspending scheduler preemption and hardware interrupts on the local processor, and so is referred to as a hard IRQ context. Thus, it is imperative to program the driver's interrupt handler routines to be short (do as little work as possible) and atomic (non blocking), to ensure responsiveness of the system. However, not all hardware interrupt handlers can be short and atomic: there are a magnitude of convoluted devices generating interrupt events, whose responses involve complex variable-time operations.
Conventionally, drivers are programmed to handle such complications ...
