class_create()函数解析

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 PART
a5d1
ICULAR 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);