Linux Device Drivers

Installing an Interrupt Handler

Interrupt lines are a precious and often limited resource, particularly when there are only 15 or 16 of them. The kernel keeps a registry of interrupt lines, similar to the registry of I/O ports. A module is allowed to request an interrupt channel (or IRQ, for Interrupt ReQuest) and release it when it’s done. The following functions, declared in <linux/sched.h>, implement the interface:

int request_irq(unsigned int irq,
                void (*handler)(int, void *, struct pt_regs *),
                unsigned long flags, 
                const char *device,
                void *dev_id);
void free_irq(unsigned int irq, void *dev_id);

Note that version 1.2 featured different prototypes. See Section 9.8 later in this chapter for portability issues.

The value returned to the requesting function is 0 to indicate success or a negative error code, as usual. It’s not uncommon for the function to return -EBUSY to signal that another driver is already using the requested interrupt line. The arguments to the functions are as follows:

unsigned int irq

This is the interrupt number. Sometimes the mapping from the Linux number to the hardware number isn’t one-to-one. Look, for example, at arch/ alpha/kernel/irq.c to see the Alpha mapping. The argument to the kernel functions is the Linux number rather than the hardware number.

void (*handler)(int, void *, struct pt_regs *)

The pointer to the handling function being installed.

unsigned long flags

As you might expect, a bitmask of options related to interrupt management. ...