What is the purpose of class and class device?

Question

I followed some tutorials that explained how to write Linux kernel modules and I am a bit confused. Even after reading the official "documentation", I have poor understanding of the concepts.

After creating a character device (register_chrdev), I see it is common to use a combination of the following functions:

class_create

class_device_create

device_create

I was not able to understand, what is a class, device and, class device and driver?

Which one of these actually responsible to create an entry under /proc/?

Carol

Forget about /proc, nowadays devices are advertised through sysfs. – Proteolysis

@Proteolysis Can you suggest any good reading about the correct and modern way of writing device drivers? – Carol

Answer 1

Rather than going into what's a class, or what's a device (I'm no expert in Linux kernel), I will address the question as follows.

After creating the character device, you want to be able to access it from the user space. To do this, you need to add a device node under /dev. You can do this in two ways.

Use mknod to manually add a device node (old)

mknod /dev/<name> c <major> <minor>

OR

Use udev

This is where the class_create and device_create or class_device_create (old) come in.

To notify udev from your kernel module, you first create a virtual device class using

struct class * class_create(owner, name)

Now, the name will appear in /sys/class/<name>.

Then, create a device and register it with sysfs.

struct device *device_create(struct class *class, struct device *parent, dev_t devt, void *drvdata, const char *fmt, ...)

Now, device name will appear in /sys/devices/virtual/<class name>/<device name> and /dev/<device name>

It's not clear what you are asking about the /proc entry.

After your module is loaded, it will appear in /proc/modules (do a cat /proc/modules to see it). And, after you allocate the device numbers, say with

int register_chrdev_region(dev_t first, unsigned int count, char *name)

, the name will appear in /proc/devices (do a cat /proc/devices to see it).

And, please check the kernel sources for these functions as well, as they provide a good description of what they do in their comments.

The good old LDD3 does not provide these mechanisms, but it's a very good source.

Araliaceous

is there any logic in class names? for example this one: /sys/class/leds/omega2p:amber:system what is :amber:system mean?! thanks – Jerid

@Jerid I think you can give any descriptive name for the class. Here, amber could be the led color, and system could be related to the led's designated functionality. I just did a search and found this. If you have the sources, you can see how it is implemented. – Araliaceous

@Araliaceous any points on why class.h file is empty in kernel headers? – Hennahane

@Hennahane Can you please provide a link to the particular file you are talking about (e.g. from Elixir Bootlin)? – Araliaceous

sure @dhanushka...this is the link elixir.bootlin.com/linux/latest/source/include/linux/device/…. So in my system the class.h is empty(no contents) i dont know why and how class_create gets linked without that header. Thanks. – Hennahane

@Hennahane Then the class_create macro should be in a different header, probably device.h? You can perform a search and verify it's there in a header. – Araliaceous

thanks much @Araliaceous its there in device.h. – Hennahane

Answer 2

From Essential Linux Device Drivers:

class_create() -> It add the device entry in sysfs.

device_create() -> It generates a uevent, which is then captured by udevd, leading to the creation of the node in /dev.

/proc/ entry is created when you initialize the device.

alloc_chardev_region(...) or register_chrdev_region(...);

Cognition

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