char

Character Device Drivers

Overview

Character devices are those who send and receive single characters at a time. Some examples include serial ports, parallel ports, sounds cards.

I2C device files are character device files with major device number 89 and a minor device number corresponding to the number assigned (by the operating system). In order to converse with I2C character devices, you go through #include <linux/i2c-dev.h> You can use the i2c-tools i2cdetect -l to determine the adapter number. You also will need the I2C address in your program for communicating (e.g. ioctl(file, I2C_SLAVE, addr)). SMBus commands or plain I2C is then used to communicate with device. You can find more information and example under I2C devices.

Code changes

The following may eventually be integrated into the character driver example code. A newer initialization for character drivers is as follows:

dev_t dev_num;

static int __init my_init(void)
{
    int i;
    dev_t curr_dev;

    /* Request the kernel for N_MINOR devices */
    alloc_chrdev_region(&dev_num, 0, N_MINORS, "my_driver");

    /* Create a class : appears at /sys/class */
    my_class = class_create(THIS_MODULE, "my_driver_class");

    /* Initialize and create each of the device(cdev) */
    for (i = 0; i < N_MINORS; i++) {

        /* Associate the cdev with a set of file_operations */
        cdev_init(&my_cdev[i], &fops);

        /* Build up the current device number. To be used further */
        curr_dev = MKDEV(MAJOR(dev_num), MINOR(dev_num) + i);

        /* Create a device node for this device. Look, the class is
         * being used here. The same class is associated with N_MINOR
         * devices. Once the function returns, device nodes will be
         * created as /dev/my_dev0, /dev/my_dev1,... You can also view
         * the devices under /sys/class/my_driver_class.
         */
        device_create(my_class, NULL, curr_dev, NULL, "my_dev%d", i);

        /* Now make the device live for the users to access */
        cdev_add(&my_cdev[i], curr_dev, 1); 
    }

    return 0;
}