class_create(),device_create自动创建设备文件结点

从linux 内核2.6的某个版本之后，devfs不复存在，udev成为devfs的替代。相比devfs，udev有很多优势，在此就不罗嗦了，提醒一 点，udev是应用层的东东，不要试图在内核的配置选项里找到它;加入对udev的支持很简单，以作者所写的一个字符设备驱动为例，在驱动初始化的代码里调用class_create为该设备创建一个class，再为每个设备调用
class_device_create创建对应的设备。(不太明白什么是devfs,udev,对linux中的文件系统没有什么概念呢。)

大致用法如下：

struct class *myclass = class_create(THIS_MODULE, “my_device_driver”);

class_device_create(myclass, NULL, MKDEV(major_num, 0), NULL, “my_device”);

这样的module被加载时，udev daemon就会自动在/dev下创建my_device设备文件。

class_create()

-------------------------------------------------

linux-2.6.22/include/linux/device.h

struct class *class_create(struct module *owner, const char *name)

class_create - create a struct class structure

@owner: pointer to the module that is to "own" this struct class

@name: pointer to a string for the name of this class.

在/sys/class/下创建类目录

class_device_create()

-------------------------------------------------

linux-2.6.22/include/linux/device.h

struct class_device *class_device_create(struct class *cls,

struct class_device *parent,

dev_t devt,

struct device *device,

const char *fmt, ...)

class_device_create - creates a class device and registers it with sysfs

@cls: pointer to the struct class that this device should be registered to.

@parent: pointer to the parent struct class_device of this new device, if any.

@devt: the dev_t for the char device to be added.

@device: a pointer to a struct device that is assiociated with this class device.

@fmt: string for the class device's name

在驱动模块初始化函数中实现设备节点的自动创建

我们在刚开始写Linux设备驱动程序的时候， 很多时候都是利用mknod命令手动创建设备节点，实际上Linux内核为我们提供了一组函数，可以用来在模块加载的时候自动在/dev目录下创建相应设 备节点，并在卸载模块时删除该节点，当然前提条件是用户空间移植了udev。

内核中定义了struct class结构体，顾名思义，一个struct class结构体类型变量对应一个类， 内核同时提供了class_create(…)函数，可以用它来创建一个类，这个类存放于sysfs下面，一旦创建好了这个类，再调用 device_create(…)函数来在/dev目录下创建相应的设备节点。这样，加载模块的时候，用户空间中的udev会自动响应
device_create(…)函数，去/sysfs下寻找对应的类从而创建设备节点。

注意，在2.6较早的内核版本中，device_create(…)函数名称不同，是class_device_create(…)，所以在新的内核中编译以前的模块程序有时会报错，就是因为函数名称不同，而且里面的参数设置也有一些变化。

struct class和device_create(…) 以及device_create(…)都定义在/include/linux/device.h中，使用的时候一定要包含这个头文件，否则编译器会报错。

在2.6.26.6内核版本中，struct class定义在头文件include/linux/device.h中

/*

* device classes

*/

struct class {

const char *name;

struct module *owner;

nbsp;struct kset subsys;

struct list_head devices;

struct list_head interfaces;

struct kset class_dirs;

struct semaphore sem; /* locks children, devices, interfaces */

struct class_attribute *class_attrs;

struct device_attribute *dev_attrs;

int (*dev_uevent)(struct device *dev, struct kobj_uevent_env *env);

void (*class_release)(struct class *class);

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

int (*suspend)(struct device *dev, pm_message_t state);

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

};

class_create(…)在/drivers/base/class.c中实现：

/**

* class_create - create a struct class structure

* @owner: pointer to the module that is to "own" this struct class

* @name: pointer to a string for the name of this class.

*

* This is used to create a struct class pointer that can then be used

* in calls to device_create().

*

* Note, the pointer created here is to be destroyed when finished by

* making a call to class_destroy().

*/

struct class *class_create(struct module *owner, const char *name)

{

struct class *cls;

int retval;

cls = kzalloc(sizeof(*cls), GFP_KERNEL);

if (!cls) {

retval = -ENOMEM;

goto error;

}

cls->name = name;

cls->owner = owner;

cls->class_release = class_create_release;

retval = class_register(cls);

if (retval)

goto error;

return cls;

error:

kfree(cls);

return ERR_PTR(retval);

}

第一个参数指定类的所有者是哪个模块，第二个参数指定类名。

在class.c中，还定义了class_destroy(…)函数，用于在模块卸载时删除类。

device_create(…)函数在/drivers/base/core.c中实现：

/**

* device_create - creates a device and registers it with sysfs

* @class: pointer to the struct class that this device should be registered to

* @parent: pointer to the parent struct device of this new device, if any

* @devt: the dev_t for the char device to be added

* @fmt: string for the device's name

*

* This function can be used by char device classes. A struct device

* will be created in sysfs, registered to the specified class.

*

* A "dev" file will be created, showing the dev_t for the device, if

* the dev_t is not 0,0.

* If a pointer to a parent struct device is passed in, the newly created

* struct device will be a child of that device in sysfs.

* The pointer to the struct device will be returned from the call.

* Any further sysfs files that might be required can be created using this

* pointer.

*

* Note: the struct class passed to this function must have previously

* been created with a call to class_create().

*/

struct device *device_create(struct class *class, struct device *parent,

dev_t devt, const char *fmt, ...)

{

va_list vargs;

struct device *dev;

va_start(vargs, fmt);

dev = device_create_vargs(class, parent, devt, NULL, fmt, vargs);

va_end(vargs);

return dev;

}

第一个参数指定所要创建的设备所从属的类，第二个参数是这个设备的父设备，如果没有就指定为NULL，第三个参数是设备号，第四个参数是设备名称，第五个参数是从设备号。

下面以一个简单字符设备驱动来展示如何使用这几个函数

#include <linux/module.h>

#include <linux/kernel.h>

#include <linux/init.h>

#include <linux/fs.h>

#include <linux/cdev.h>

#include <linux/device.h>

MODULE_LICENSE ("GPL");

int hello_major = 555;

int hello_minor = 0;

int number_of_devices = 1;

struct cdev cdev;

dev_t dev = 0;

struct file_operations hello_fops = {

.owner = THIS_MODULE

};

static void char_reg_setup_cdev (void)

{

int error, devno = MKDEV (hello_major, hello_minor);

cdev_init (&cdev, &hello_fops);

cdev.owner = THIS_MODULE;

cdev.ops = &hello_fops;

error = cdev_add (&cdev, devno , 1);

if (error)

printk (KERN_NOTICE "Error %d adding char_reg_setup_cdev", error);

}

struct class *my_class;

static int __init hello_2_init (void)

{

int result;

dev = MKDEV (hello_major, hello_minor);

result = register_chrdev_region (dev, number_of_devices, "hello");

if (result<0) {

printk (KERN_WARNING "hello: can't get major number %d\n", hello_major);

return result;

}

char_reg_setup_cdev ();

/* create your own class under /sysfs */

my_class = class_create(THIS_MODULE, "my_class");

if(IS_ERR(my_class))

{

printk("Err: failed in creating class.\n");

return -1;

}

/* register your own device in sysfs, and this will cause udev to create corresponding device node */

device_create( my_class, NULL, MKDEV(hello_major, 0), "hello" "%d", 0 );

printk (KERN_INFO "Registered character driver\n");

return 0;

}

static void __exit hello_2_exit (void)

{

dev_t devno = MKDEV (hello_major, hello_minor);

cdev_del (&cdev);

device_destroy(my_class, MKDEV(adc_major, 0)); //delete device node under /dev

class_destroy(my_class); //delete class created by us

unregister_chrdev_region (devno, number_of_devices);

printk (KERN_INFO "char driver cleaned up\n");

}

module_init (hello_2_init);

module_exit (hello_2_exit);

这样，模块加载后，就能在/dev目录下找到hello0这个设备节点了。

例子2

drivers/i2c/i2c-dev.c

/*

* module load/unload record keeping

*/

static int __init i2c_dev_init(void)

{

int res;

printk(KERN_INFO "i2c /dev entries driver\n");

res = register_chrdev(I2C_MAJOR, "i2c", &i2cdev_fops);

if (res)

goto out;

i2c_dev_class = class_create(THIS_MODULE, "i2c-dev"); //创建一个名称为i2c-dev的class

if (IS_ERR(i2c_dev_class)) {

res = PTR_ERR(i2c_dev_class);

goto out_unreg_chrdev;

}

res = i2c_add_driver(&i2cdev_driver);

if (res)

goto out_unreg_class;

return 0;

out_unreg_class:

class_destroy(i2c_dev_class);

out_unreg_chrdev:

unregister_chrdev(I2C_MAJOR, "i2c");

out:

printk(KERN_ERR "%s: Driver Initialisation failed\n", __FILE__);

return res;

}

在 i2c_dev_init创建类i2c_dev_class

i2c_dev_class = class_create(THIS_MODULE, "i2c-dev");

static int i2cdev_attach_adapter(struct i2c_adapter *adap)

{

struct i2c_dev *i2c_dev;

int res;

i2c_dev = get_free_i2c_dev(adap);

if (IS_ERR(i2c_dev))

return PTR_ERR(i2c_dev);

/* register this i2c device with the driver core */

i2c_dev->dev = device_create(i2c_dev_class, &adap->dev,

MKDEV(I2C_MAJOR, adap->nr), NULL,

"i2c-%d", adap->nr);

if (IS_ERR(i2c_dev->dev)) {

res = PTR_ERR(i2c_dev->dev);

goto error;

}

res = device_create_file(i2c_dev->dev, &dev_attr_name);

if (res)

goto error_destroy;

pr_debug("i2c-dev: adapter [%s] registered as minor %d\n",

adap->name, adap->nr);

return 0;

error_destroy:

device_destroy(i2c_dev_class, MKDEV(I2C_MAJOR, adap->nr));

error:

return_i2c_dev(i2c_dev);

return res;

}

在i2cdev_attach_adapter调用device_create(i2c_dev_class, &adap->dev,

MKDEV(I2C_MAJOR, adap->nr), NULL,

"i2c-%d", adap->nr);

这样在dev目录就产生i2c-0 或i2c-1节点

class_device_create（）；

class_device_destroy（）；

在2.6.27中变为：

device_create（）

device_destroy（）

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