在驱动模块初始化函数中实现设备节点的自动创建

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

#include

#include

#include

#include

#include

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 (result0) {

printk (KERN_WARNING hello: can't get major number %dn, 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 drivern);

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 upn);

}

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。