在驱动模块初始化函数中实现设备节点的自动创建

作者：杨硕，华清远见嵌入式学院讲师。

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。