[Linux driver] Automatically create/delete device nodes

After we load the device driver into the kernel, the corresponding device node file will not be generated in the /dev directory. The application program reads and writes the device, and the essence is to read and write the corresponding device node file under /dev.

Therefore, we need to automatically create device nodes when loading into the driver. This needs to be realized by relying on mdev, which is a simplified version of udev, which can detect the status of hardware devices in the system, so as to create or delete device files.

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Table of contents

1. Class creation/destruction

1. Create a class

2. Destruction class 

2. Creation/removal of device nodes

1. Create a device node

2. Remove the device node

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1. Class creation/destruction

1. Create a class

A device can have its own type. For example, touch, mouse, and keyboard all belong to input devices. This is the most typical class. You can see other similar classes in /sys/class.

The class structure is used in the kernel to represent a class, and the prototype used to create a class is as follows (essentially a macro)

/*
 * @description: 创建一个类
 * @param - owner
 * @param - name
 */
#define class_create(owner, name)		\
({						\
	static struct lock_class_key __key;	\
	__class_create(owner, name, &__key);	\
})

extern struct class * __must_check __class_create(struct module *owner,
						  const char *name,
						  struct lock_class_key *key);
                            

owner: generally THIS_MODULE

name: The name of the class. After the call is successful, the corresponding class directory will be generated in the /sys/class directory

Return value: Returns a pointer to a class structure

static struct class* _class;

_class = class_create(THIS_MODULE, CHRDEVBASE_NAME);
if (IS_ERR(_class))     // 判断指针是否有效，IS_ERR 在 #include <linux/err.h> 中定义   
{
    return -1;
}

2. Destruction class 

Destroying a class will remove the class directory from /sys/class, the prototype is as follows. The parameter cls represents the class to be destroyed

void class_destroy(struct class *cls);

2. Creation/removal of device nodes

1. Create a device node

Creating a device node will generate a device directory under the corresponding class directory, and will also generate the corresponding device node file under the /dev directory.

The device structure is used in the kernel to represent the device node, and the prototype used to create the device is as follows:

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

class: the class to which the device node belongs

parent: Indicates the parent device, generally NULL, representing no parent device

devt: device number

drvdata: the data required by the device, usually NULL

fmt: device name, if fmt=xxx is set, the device file /dev/xxx will be generated

Return value: the device node pointer of the creation number

2. Remove the device node

When uninstalling the driver, you need to delete the created device, and the corresponding device file under /dev/xxx will also be deleted. The function prototype is as follows:

void device_destroy(struct class *class, dev_t devt);

class: the class to which the device node belongs

devt: device number

Reference article:

Linux device driver model - Zhihu