基于linux-2.6.35的class_create,device_create解析 (可自动生成设备文件)

转：基于linux-2.6.35的class_create(),device_create解析
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。提醒一点，udev是应用层的，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device_create创建对应的设备。大致用法如下：
struct class *myclass ；

class_create(THIS_MODULE, “my_device_driver”);

device_create(myclass, NULL, MKDEV(major_num, minor_num), 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。
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2.6.35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;

struct class_attribute *class_attrs;

struct device_attribute *dev_attrs;

struct kobject *dev_kobj;

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

char *(*devnode)(struct device *dev, mode_t *mode);

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);

const struct kobj_ns_type_operations *ns_type;

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

const struct dev_pm_ops *pm;

struct class_private *p;

};

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 lock_class_key *key)

{

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, key);

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, void *drvdata, const char *fmt, ...)

{

va_list vargs;

struct device *dev;

va_start(vargs, fmt);

dev = device_create_vargs(class, parent, devt, drvdata, 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：

/*

* Copyright (C) 2005 Farsight

*

* This program is free software; you can redistribute it and/or modify

* it under the terms of the GNU General Public License as published by

* the Free Software Foundation; either version 2 of the License, or

* (at your option) any later version.

* This program is distributed in the hope that it will be useful,

* but WITHOUT ANY WARRANTY; without even the implied warranty of

* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the

* GNU General Public License for more details.

* You should have received a copy of the GNU General Public License

* along with this program; if not, write to the Free Software

* Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA

*

*/

#include <linux/module.h>

#include <linux/kernel.h>

#include <linux/init.h>

#include <linux/fs.h>

#include <linux/cdev.h>

#include <asm/uaccess.h>

#include <linux/device.h>

MODULE_LICENSE ("GPL");

int hello_major = 250;

int hello_minor = 0;

int number_of_devices = 1;

struct cdev cdev;

dev_t dev = 0;

struct file_operations hello_fops = {

.owner = THIS_MODULE,

};

struct class *my_class;

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);

/* creating your own class */

my_class =class_create(THIS_MODULE, "farsight_class");

if(IS_ERR(my_class)) {

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

return ;

}

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

device_create(my_class,NULL, devno, NULL,"hello");

}

static int __init hello_2_init (void)

{

int result;

dev = MKDEV (hello_major, hello_minor);

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

if (result<0) {

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

return result;

}

char_reg_setup_cdev ();

printk (KERN_INFO "char device registered\n");

return 0;

}

static void __exit hello_2_exit (void)

{

dev_t devno = MKDEV (hello_major, hello_minor);

cdev_del (&cdev);

unregister_chrdev_region (devno, number_of_devices);

device_destroy(my_class, devno);

class_destroy(my_class);

}

module_init (hello_2_init);

module_exit (hello_2_exit);
当加载模块的时候，会在/dev/hello这个设备文件