Framework for registering Linux kernel driver as class device / How to add new class to Linux kernel

Following simple driver code shows, how to add a new class with name “hello_as_class” can be added to linux kernel.

#include <linux/module.h> /* Needed by all modules */
#include <linux/kernel.h> /* Needed for KERN_INFO */
#include <linux/init.h> /* Needed for the macros */
#include <linux/device.h> // for struct class
static struct class hello_class = {
        .name = "hello_as_class",
        .owner = THIS_MODULE,
static int __init hello_init(void) {
        printk(KERN_INFO "Hello, world\n");
        return 0;
static void __exit hello_exit(void) {
        printk(KERN_INFO "Goodbye, world\n");
 $ vim Makefile 
obj-m += hello.o

        make -C /lib/modules/$(shell uname -r)/build M=$(PWD) modules

        make -C /lib/modules/$(shell uname -r)/build M=$(PWD) clean

Compile this driver as,

 $ make 

Now insert the driver as,

 $ sudo insmod ./hello.ko 

We can check whether this driver is inserted or not as,

$ lsmod | grep hello 
hello                  16384  0 

This driver will create a directory as, /sys/class/hello_as_class

 $ cd /sys/class/hello_as_class 

Now, if we see kernel source code, struct class has been declared in include/linux/device.h as,

 * struct class - device classes
 * @name:	Name of the class.
 * @owner:	The module owner.
 * @class_groups: Default attributes of this class.
 * @dev_groups:	Default attributes of the devices that belong to the class.
 * @dev_kobj:	The kobject that represents this class and links it into the hierarchy.
 * @dev_uevent:	Called when a device is added, removed from this class, or a
 *		few other things that generate uevents to add the environment
 *		variables.
 * @devnode:	Callback to provide the devtmpfs.
 * @class_release: Called to release this class.
 * @dev_release: Called to release the device.
 * @shutdown_pre: Called at shut-down time before driver shutdown.
 * @ns_type:	Callbacks so sysfs can detemine namespaces.
 * @namespace:	Namespace of the device belongs to this class.
 * @pm:		The default device power management operations of this class.
 * @p:		The private data of the driver core, no one other than the
 *		driver core can touch this.
 * A class is a higher-level view of a device that abstracts out low-level
 * implementation details. Drivers may see a SCSI disk or an ATA disk, but,
 * at the class level, they are all simply disks. Classes allow user space
 * to work with devices based on what they do, rather than how they are
 * connected or how they work.
struct class {
	const char		*name;
	struct module		*owner;

	const struct attribute_group	**class_groups;
	const struct attribute_group	**dev_groups;
	struct kobject			*dev_kobj;

	int (*dev_uevent)(struct device *dev, struct kobj_uevent_env *env);
	char *(*devnode)(struct device *dev, umode_t *mode);

	void (*class_release)(struct class *class);
	void (*dev_release)(struct device *dev);

	int (*shutdown_pre)(struct device *dev);

	const struct kobj_ns_type_operations *ns_type;
	const void *(*namespace)(struct device *dev);

	const struct dev_pm_ops *pm;

	struct subsys_private *p;

extern int __must_check __class_register(struct class *class,
					 struct lock_class_key *key);
extern void class_unregister(struct class *class);

/* This is a #define to keep the compiler from merging different
 * instances of the __key variable */
#define class_register(class)			\
({						\
	static struct lock_class_key __key;	\
	__class_register(class, &__key);	\

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