Detailed explanation of the asynchronous notification mechanism in embedded Linux driver

Preface

In embedded Linux development,Asynchronous notifications(Asynchronous Notification) is an efficient device access mechanism, especially suitable for needsReal-time responseHardware events.

Compared with traditional blocking I/O and polling methods, asynchronous notifications can actively notify applications when events occur, thereby significantly reducing CPU usage and improving system response speed.

1. The core concept of asynchronous notification

1. What is asynchronous notification

Asynchronous notification is a kind of Linux kernel providedInterrupt simulation mechanism at the software level. When a specific event occurs in the device (such as readable data and pressing a key), the driver will send a signal to the user space (such asSIGIO), notify the application to process the event immediately.

This mechanism is similar to hardware interrupts, but occurs between user space and kernel space.

2. Key components of asynchronous notifications

• Signal: The Linux kernel communicates with user processes through signals.SIGIOIt is a commonly used signal in asynchronous notifications.
• fasync_struct: The kernel-maintained asynchronous notification queue structure is used to store process information for registering asynchronous notifications.
• fasyncMethod: The interface in the driver used to manage the asynchronous notification queue.
• kill_fasyncFunction: The core function that drives the sending signal to the user process.

2. The implementation principle of asynchronous notification

In the user program, the following three steps need to be completed to enable asynchronous notifications, that is, define an asynchronous queue and add it in the device structurestruct fasync_struct *async_queue;。

accomplishfasyncMethod, byfasync_helperManage asynchronous queues.

Trigger signal, called when an event occurskill_fasyncsendSIGIOSignal.

// 1. Set signal processing functionsignal(SIGIO, handler); // handler is a custom signal processing function
// 2. Bind process and file descriptorfcntl(fd, F_SETOWN, getpid()); // Set the current process ID as the owner of the file descriptor
// 3. Enable asynchronous modefcntl(fd, F_SETFL, fcntl(fd, F_GETFL) | FASYNC); // set upFASYNCLogo

3. Code sample analysis

Here is some code for a key-driven asynchronous notification implementation that is being learned:

1. Equipment structure definition

struct imx6uirq_dev {
    ...
    struct fasync_struct *async_queue; // Asynchronous notification queue};

2. Fasync method implementation

• fasync_helper: Standard functions provided by the kernel to register or remove processes into asynchronous queues.
• on=1: Register asynchronous notifications.
• on=0: Remove asynchronous notifications.

static int imx6uirq_fasync(int fd, struct file *filp, int on) {
    struct imx6uirq_dev *dev = (struct imx6uirq_dev *)filp->private_data;
    return fasync_helper(fd, filp, on, &dev->async_queue);
}

3. Trigger signal

• kill_fasync: Send to processes in the asynchronous queueSIGIOSignals to notify the application that data is readable.

void timer_function(unsigned long arg) {
    ...
    if (atomic_read(&dev->releasekey)) { /* key release event */
        if (dev->async_queue)
            kill_fasync(&dev->async_queue, SIGIO, POLL_IN); // Send SIGIO signal    }
}

4. Free up resources

static int imx6uirq_release(struct inode *inode, struct file *filp) {
    return imx6uirq_fasync(-1, filp, 0); // Clean up the asynchronous queue}

Summarize

Features of asynchronous notifications:

• Low-latency sounding, that is, key detection, sensor data update, and real-time monitoring system.
• Ability to avoid polling or blocking waiting, reducing latency

Comparison of traditional methods:

• Blocking I/O: The application goes to sleep and waits for the device to be ready.
• Non-blocking I/O: Applications poll regularly, consuming a large amount of CPU resources.
• Asynchronous notification: Proactive notification when the device is ready without polling or blocking.

The above is personal experience. I hope you can give you a reference and I hope you can support me more.