基于linux-2.6.35的class_create(),device_create解析

作者：苗德行,讲师。

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