Solving the -EINVAL Error When Requesting GPIO Interrupts from I2C IO Expanders in Linux

Описание: Discover the solution to handling GPIO interrupts on Linux using I2C IO expanders like the MCP23008 on Raspberry Pi. Learn how to avoid the `-EINVAL` errors and successfully implement interrupt requests in your kernel module.
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Handling GPIO Interrupt Requests with I2C IO Expanders on Linux

When developing kernel modules that interact with hardware components, ensuring that interrupts are triggered correctly from GPIOs (General Purpose Input/Output) can often become a tricky challenge. One common issue that arises is the failure to request GPIO interrupts from an I2C IO expander, such as the MCP23008, leading to the dreaded -EINVAL error.

In this post, we'll explore the problem in depth and walk through a structured solution to overcome this issue, enabling you to handle interrupts effectively in your Linux kernel module.

The Problem: Understanding the -EINVAL Error

You might encounter an -EINVAL error (which translates to "Invalid Argument") when your code tries to request interrupts on GPIOs that are part of an I2C IO expander. This situation arises particularly in systems like the Raspberry Pi 3B+ when interfacing with modules like pinctrl-mcp23s08.

Key Symptoms:

The call to request_irq() fails, returning -22.

Your module works fine with regular GPIOs but fails with the expander-based GPIOs.

The interrupts for the I2C IO expander appear in /proc/interrupts, confirming that they are set up correctly.

The Root Cause

The primary reason for encountering the -EINVAL error is an incompatibility regarding threaded interrupt handlers. The parent interrupt controller is configured to use threaded handlers, which means that the child interrupt handlers also need to be threaded to function properly.

This incompatibility is checked in the __setup_irq() function where the conditions for nesting thread handlers are verified:

[[See Video to Reveal this Text or Code Snippet]]

If the new->thread_fn is NULL, then the call to request_irq() will lead to the -EINVAL return value.

The Solution: Requesting Threaded IRQs

To resolve the issue, you can change your outgoing IRQ request from request_irq() to request_threaded_irq(). This way, you indicate that the handler is to be executed in a threaded context.

Here’s How to Implement It:

In your initialization code, modify the IRQ request as follows:

[[See Video to Reveal this Text or Code Snippet]]

Explanation:

Replacing request_irq with request_threaded_irq: This allows your driver to use a threaded interrupt handler.

Setting the first argument to NULL: Since there’s no need for a hard IRQ handler because it's a purely threaded context.

Alternative Solution: Using request_any_context_irq

If you prefer flexibility, consider using request_any_context_irq(), which can automatically select the appropriate IRQ handler type based on the context requirements:

[[See Video to Reveal this Text or Code Snippet]]

This function intelligently decides whether to use a threaded or hard IRQ handler based on the interrupt request context.

Conclusion

Interrupt handling in Linux can be daunting, especially when dealing with GPIOs on I2C IO expanders. By understanding the interactions between your driver, the interrupt controller, and using the appropriate IRQ request methods, you can successfully manage GPIO interrupts in your kernel module.

With these solutions in hand, you should now be able to sidestep the -EINVAL error and enhance your device's functionality effectively. Happy coding!