device_create 转自华清

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