How to add poll function to the kernel module code?

You can find some good examples in kernel itself. Take a look at next files:

• drivers/rtc/dev.c, drivers/rtc/interface.c
• kernel/printk/printk.c
• drivers/char/random.c

To add poll() function to your code follow next steps.

1. Include needed headers:

    #include <linux/wait.h>
    #include <linux/poll.h>
   

2. Declare waitqueue variable:

    static DECLARE_WAIT_QUEUE_HEAD(fortune_wait);
   

3. Add fortune_poll() function and add it (as .poll callback) to your file operations structure:

    static unsigned int fortune_poll(struct file *file, poll_table *wait)
    {
        poll_wait(file, &fortune_wait, wait);
        if (new-data-is-ready)
            return POLLIN | POLLRDNORM;
        return 0;
    }
   
    static const struct file_operations proc_test_fops = {
        ....
        .poll = fortune_poll,
    };
   

   Note that you should return POLLIN | POLLRDNORM if you have some new data to read, and 0 in case there is no new data to read (poll() call timed-out). See man 2 poll for details.

4. Notify your waitqueue once you have new data:

    wake_up_interruptible(&fortune_wait);
   

That’s the basic stuff about implementing poll() operation. Depending on your task, you may be needed to use some waitqueue API in your .read function (like wait_event_interruptible()).

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See also related question: Implementing poll in a Linux kernel module.