Handling Interrupts in Linux Drivers

When developing Linux device drivers, one of the pivotal aspects to manage is handling hardware interrupts. Interrupts allow devices to signal the CPU that they need attention, enabling an efficient way for CPUs to react to hardware events without constant polling. In this article, we will delve into the intricacies of handling interrupts, covering essential concepts like IRQs (interrupt request lines) and bottom halves.

Understanding IRQs

Before we dive into interrupt handling, we need to understand the role of IRQs in the Linux kernel. An IRQ is a signal sent to the CPU by hardware devices indicating that they require processing. Each IRQ corresponds to a specific hardware line and is unique to each device in the system. The Linux kernel uses these IRQ lines to manage and prioritize hardware interrupts seamlessly.

Types of IRQs

Edge-triggered: These interrupts are generated by a change in signal level (high to low or vice versa). An edge-triggered interrupt is triggered when the signal transitions.
Level-triggered: These interrupts continue to be signaled as long as the interrupt line remains active. The CPU must clear the interrupt before it can be acknowledged again.

On Linux systems, the interrupt management is handled via the PIC (Programmable Interrupt Controller) or APIC (Advanced Programmable Interrupt Controller), depending on the architecture.

Requesting IRQs

When you write a device driver, one of the first things you will do is request an IRQ using the request_irq() function. This function not only associates an IRQ number with a handler function but also provides the kernel information about the context of the interrupt.

Here’s a brief example of how to request an IRQ:

#include <linux/interrupt.h>

#define MY_IRQ 10

irqreturn_t my_irq_handler(int irq, void *dev_id) {
    // Handling the interrupt
    return IRQ_HANDLED;
}

int my_driver_init(void) {
    if (request_irq(MY_IRQ, my_irq_handler, IRQF_SHARED, "my_device", my_device_id)) {
        printk(KERN_ERR "Failed to register IRQ\n");
        return -EIO;
    }
    return 0;
}

In the request_irq() function, the parameters are:

IRQ number: The IRQ number to be used.
Interrupt handler: A pointer to the function that will handle the interrupt.
Flags: Various flags that can modify the behavior of the interrupt (e.g., IRQF_SHARED for sharing IRQ with other devices).
Dev ID: A pointer used to identify the device that the handler is associated with.

If the request_irq() call is successful, the provided handler will be called whenever the device signals an interrupt.

Writing the Interrupt Handler

The interrupt handler you define should be efficient. It’s crucial to keep any work done in the handler minimal because time spent here can affect overall system performance.

Here’s an example of a simple interrupt handler:

irqreturn_t my_irq_handler(int irq, void *dev_id) {
    // Acknowledge the interrupt
    // Perform quick processing (like setting flags, reading a data register, etc.)
    
    // If there’s more work to do, schedule a bottom half
    schedule_work(&my_work);
    
    return IRQ_HANDLED;
}

The schedule_work() function queues work that can be processed later in a different context, letting the interrupt handler return quickly.

Bottom Halves

Since interrupts can interrupt any kernel process, it’s generally advisable to minimize processing time in the IRQ handler itself. For this reason, Linux provides a mechanism called "bottom halves," which allows you to defer work that does not need to be executed in the interrupt context to a later time.

There are two primary bottom half mechanisms in the Linux kernel:

Tasklets: These are lightweight and allow you to schedule functions to run later. They are not executed in a thread and are run in the same context as the interrupt handler, meaning they do not block IRQs.
Workqueues: Workqueues allow the deferrable work to execute in a kernel thread context. This means you can block and sleep in a workqueue handler without causing issues that might affect the responsiveness of the system.

Example of Using Workqueues

#include <linux/workqueue.h>

struct work_struct my_work;

void my_work_handler(struct work_struct *work) {
    // Perform longer operations here
}

int my_driver_init(void) {
    // Request the IRQ as shown previously

    INIT_WORK(&my_work, my_work_handler);
    return 0;
}

irqreturn_t my_irq_handler(int irq, void *dev_id) {
    // Acknowledge the interrupt and block until it’s processed
    schedule_work(&my_work);
    
    return IRQ_HANDLED;
}

Acknowledging Interrupts

In certain scenarios, explicitly acknowledging or clearing the interrupt may be necessary depending on the hardware. If the device uses level-triggered interrupts, the device must be informed to stop sending them. This typically involves writing specific registers associated with the hardware.

Here’s an example of clearing an interrupt:

void clear_device_interrupt(void) {
    // Write to the device register to acknowledge or clear the interrupt
    write_register(device_base_address, INTERRUPT_CLEAR_REGISTER);
}

Error Handling

Error handling is an essential aspect of interrupt management. You will need to ensure that if a request for an IRQ fails, proper cleanup and rollback occur. Make sure to release the IRQ in your driver exit function using free_irq().

Here’s a basic cleanup example:

void my_driver_exit(void) {
    // Free the IRQ which was previously allocated
    free_irq(MY_IRQ, my_device_id);
}

Conclusion

Handling hardware interrupts is a foundational skill in Linux device driver development. By understanding how to request IRQs, write efficient handlers, and utilize bottom halves appropriately, you can ensure your drivers interact with devices seamlessly. Always remember to keep your interrupt handlers short and offload longer processing tasks to later stages in the kernel execution flow. This practice not only results in more responsive systems but also maintains system stability. Happy coding!

Networking & Infrastructure - Linux Driver Development