Questions about register_chrdev_region() in linux device driver

Question

I'm learning about the registration of a kernel module using register_chrdev_region(dev_t from, unsigned count, const char * name);.

I notice that with or without this function, my kernel module worked as expected. The code I used for testing:

first = MKDEV(MAJOR_NUM, MINOR_NUM);
register_chrdev_region(first, count, DEVICE_NAME);//<---with and without

mycdev=cdev_alloc();
mycdev->ops= &fops;
mycdev->owner = THIS_MODULE;

if (cdev_add(mycdev,first, count) == 0)
{printk(KERN_ALERT "driver loaded\n");}

I commented out the line register_chrdev_region(first, count, DEVICE_NAME);, and the printk message still appeared. I tried to communicate with the driver with or without this from user space, and both are successful.

So my question is, is this function register_chrdev_region() only used to make my driver a good kernel citizen, just like telling the others that "I'm using up the major number, please don't use"?

I tried to have a look in the kernel source char_dev.c to understand the function, but I find it too difficult to understand, anyone that's familiar with this?

Thanks!

Answer 1

That will work because it's not actually necessary to allocate your device numbers up front. In fact, it's considered preferable by many kernel developers to use the dynamic (on-the-fly, as-needed) allocation function alloc_chrdev_region.

Whether you do it statically up front or dynamically as needed, it is something you should do to avoid conflict with other device drivers which may have played by the rules and been allocated the numbers you're trying to use. Even if your driver works perfectly well without it, that won't necessarily be true on every machine or at any time in the future.

The rules are there for a reason and, especially with low-level stuff, you are well advised to follow them.

See here for more details on the set-up process.

Answer 2

If the major number for your devices clash with any other device already in use, then the driver won't have the allocation done.

If you have already tested which major number is free and used it, it might generally not throw up an error and you will face no problem as u load the driver.

But if you run on various systems and if the major number is already captured and used by some other system., Then your driver loading can fail.

Its always better to use dynamic allocation !!