How to get major:minor for a platform_driver

Question

i've wrote a platform_driver for Linux. Now, i want it to be a char device under /dev/myDrv.

Right now, the driver doesn't appear in /dev/myDrv although I successfully load it with 'insmod'.

I guess, i have to create a char device with 'mknod /dev/myDrv c MAJOR MINOR'. Correct?

Well, the problem is i don't know how to find major:minor for my driver.

After 'insmod myDrv' there are some files generated under:

/sys/bus/platform/devices/myDrv
/sys/devices/platform/myDrv
/sys/module/myDrv

Do they help me in any way.

Or how to find major:minor for a platform_driver?

Crazy Psychild Crazy Psychild · Accepted Answer · 2016-01-21T13:09:01

When developing a driver you have to do two basic steps before anything else: register for <major,minor> range and then link the device file operations to the device driver functions.

Following is the working code for the init and exit modules of a basic char driver:

static int __init char_init(void)
{
        int ret;
        struct device *dev_ret;

        printk(KERN_INFO "Char device registered");
        if ((ret = alloc_chrdev_region(&first, 0, 3, "Abhinav")) < 0)
                {
                        return ret;
                }
        if (IS_ERR(cl = class_create(THIS_MODULE, "char_dev")))
        {
                unregister_chrdev_region(first, 1);
                return PTR_ERR(cl);
        }
        if (IS_ERR(dev_ret = device_create(cl, NULL, first, NULL, "mynull")))
        {
                class_destroy(cl);
                unregister_chrdev_region(first, 1);
                return PTR_ERR(dev_ret);
        }

        cdev_init(&c_dev, &file_ops);
        if ((ret = cdev_add(&c_dev, first, 1)) < 0)
        {
                device_destroy(cl, first);
                class_destroy(cl);
                unregister_chrdev_region(first, 1);
                return ret;
        }
        printk(KERN_INFO "<Major,Minor>: <%d, %d>\n", MAJOR(first), MINOR(first));
        return 0;
}

static void __exit char_exit(void)
{
        cdev_del(&c_dev);
        device_destroy(cl, first);
        class_destroy(cl);
        unregister_chrdev_region(first, 3);
        printk(KERN_INFO "Goodbye: Char device unregistered");
}

The probe and disconnect are left up to you.

In this code, the call to alloc_chrdev_region is done to register for the major and minor numbers.

And the class_create and device_create API's are used for the creation of device file mynull.

After executing, /dev/mynull will be created.

How to get major:minor for a platform_driver

1 Answers