Processing Hardware Interrupts in Kernel Space

Learners will register interrupt service routines to handle asynchronous hardware events and execute specific code paths when a device requires attention.

In this lesson, you will learn how to bridge the gap between physical hardware signals and kernel-space execution by registering an Interrupt Service Routine (ISR) to respond immediately to device events.

The Mechanism of Hardware Interrupts

In user space, we often rely on polling or epoll to check if a device needs attention. However, these methods are inefficient for latency-critical hardware. When a peripheral needs to communicate with the CPU, it asserts an electrical signal on an Interrupt Request (IRQ) line. The CPU then pauses its current execution context, saves its state, and jumps to a specific address defined in the Interrupt Vector Table.

In kernel space, our objective is to write an ISR—a function that executes in the context of this interrupt—to handle the event rapidly and then return control to the system.

Registering an ISR

To handle an interrupt, we must request the kernel to associate a specific IRQ line with our function. In a kernel module, we use request_irq(). This function links an IRQ number to an ISR and defines the trigger type (e.g., rising edge or level-sensitive).

#include <linux/interrupt.h>

static irqreturn_t my_interrupt_handler(int irq, void *dev_id) {
    // 1. Acknowledge the hardware interrupt
    // 2. Perform time-critical data processing
    return IRQ_HANDLED;
}

// Inside module_init
int result = request_irq(my_irq_number, my_interrupt_handler, 
                         IRQF_TRIGGER_RISING, "my_device_name", NULL);

Constraints in Kernel Space

When writing an ISR, you must follow strict rules because you are operating in an "atomic" context. You cannot perform operations that might sleep, such as calling mutex_lock() or waiting for I/O, because the scheduler cannot safely transition the system while an interrupt is being serviced. If you have significant processing to perform, the ISR should only perform the bare minimum (like clearing a status register) and then schedule a "bottom half" (such as a tasklet or workqueue) to perform the heavier data processing later.

Key Takeaway: Hardware interrupts provide a low-latency mechanism for the CPU to react to hardware events, but ISRs must remain lean and non-blocking to avoid stalling the entire operating system.