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Problem with writing from kernel to user space - linux device driver

Posted by a6lkd 
I'm trying to write a simple Raspberry Pi GPIO driver, with four switches connected to four of the GPIO pins, that reads each switch state. The problem is, I'm not sure how to write from kernel to user space, I'm not getting anything when I insert my device kernel module and try to read the device file with "cat" command. The device_read function is as follows:

static ssize_t gpio_driver_read(struct file *filp, char *buf, size_t len, loff_t *f_pos)
    /* Size of valid data in gpio_driver - data to send in user space. */
    int data_size = 0;

    /* Counter for 'for' loop. */
    int i;

    /* Print to kernel space. */
    printk(KERN_INFO "Reading active Switch state...\n"winking smiley;

    for (i = 0; i < 4; i = i+1)
        printk(KERN_INFO "Loop number %d...\n", i);
        /* TODO: fill gpio_driver_buffer here. */
        if (i == 0 && mySwitches[0])
            sprintf(gpio_driver_buffer, "gpio_driver: gpio12 value: %d\n", GetGpioPinValue(GPIO_12));
        else if (i == 1 && mySwitches[1])
            sprintf(gpio_driver_buffer, "gpio_driver: gpio16 value: %d\n", GetGpioPinValue(GPIO_16));
        else if (i == 2 && mySwitches[2])
            sprintf(gpio_driver_buffer, "gpio_driver: gpio20 value: %d\n", GetGpioPinValue(GPIO_20));
        else if (i == 3 && mySwitches[3])
            sprintf(gpio_driver_buffer, "gpio_driver: gpio21 value: %d\n", GetGpioPinValue(GPIO_21));

        printk(KERN_INFO "%s\n", gpio_driver_buffer);

        /* Get size of valid data. */
        data_size = strlen(gpio_driver_buffer);

        printk(KERN_INFO "%d\n", data_size);

        /* Send data to user space. */
        if (copy_to_user(buf, gpio_driver_buffer, data_size) != 0)
            return -EFAULT;

    return 0;

"gpio_driver_buffer" is an array of some default size (I put it to 80).
"mySwitches" is an array of 4 elements, each one with value 0 or 1 (I'm passing that as an argument when inserting the kernel module, 1 meaning I want to watch the state of the switch and 0 meaning I'm not watching the switch).
"GetGpioPinValue" is a function that returns switch state.

The problem is, when I try to read the device file with cat command, I'm not getting anything. However, as you can see, I kind of debugged the program with "printk" commands and everything is written correctly in kernel space. Where could the problem be?

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