Handling Asynchronous Hardware Interrupts via Signals

In this lesson, you will learn how to transition your embedded application from inefficient "busy-wait" polling to an event-driven model using asynchronous hardware signals.

The Problem with Polling

In previous lessons, we discussed performing direct I/O and using ioctl to monitor hardware states. If you rely on a while(1) loop to repeatedly check a sensor or a GPIO status, your CPU remains pinned at 100% utilization. This is not only wasteful of battery power but also prevents the processor from entering low-power sleep states.

The Signal-Driven Alternative

Instead of asking the hardware, "Are you ready yet?" thousands of times per second, we tell the Linux kernel to notify our process only when a specific hardware event occurs. This is achieved by mapping hardware interrupts to POSIX Signals (like SIGIO).

When the hardware triggers an interrupt (e.g., a button press or a data-ready flag), the kernel sends a signal to your user-space application. Your application pauses its current task, jumps to a pre-defined signal handler function to process the event, and then returns to its previous state.

Implementation Concept

To use this, you must configure the file descriptor associated with your hardware peripheral to be "asynchronous." Here is the logical flow:

1. Register a handler: Use sigaction() to define a callback function that triggers when a signal is received.
2. Set the Owner: Use fcntl(fd, F_SETOWN, getpid()) to tell the kernel that your process is the recipient of signals generated by this specific hardware file descriptor.
3. Enable Asynchronous I/O: Use fcntl(fd, F_SETFL, FASYNC) to activate the asynchronous notification mode.

// Example: Signal handler prototype
void hardware_event_handler(int sig) {
    // Read the hardware here because the signal 
    // indicates the data is ready!
}

// In main():
struct sigaction sa;
sa.sa_handler = hardware_event_handler;
sigaction(SIGIO, &sa, NULL);

fcntl(fd, F_SETOWN, getpid());
int flags = fcntl(fd, F_GETFL);
fcntl(fd, F_SETFL, flags | FASYNC);

By offloading the "waiting" to the kernel, your application can sleep() or perform other calculations, waking up only when the hardware actually requires attention.

Key Takeaway: By using asynchronous signals, you replace CPU-intensive polling with a reactive architecture that maximizes system efficiency and power savings.