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; /* lockschildren, devices, interfaces */
struct class_attribute *class_attrs;
struct device_attribute *dev_attrs;
int (*dev_uevent)(structdevice *dev, struct kobj_uevent_env *env);
void (*class_release)(structclass *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 canthen be used
* in calls to device_create().
*
* Note, the pointer created here is to be destroyed whenfinished 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。
/**
*class_destroy - destroys a struct class structure
*@cs: pointer to the struct class that is to be destroyed
*
*Note, the pointer to be destroyed must have been created with a call
*to class_create().
*/
void class_destroy(struct class *cls)
{
if((cls == NULL) || (IS_ERR(cls)))
return;
class_unregister(cls);
}
device_create(…)函数在/drivers/base/core.c中实现：
/**
* device_create - creates a device and registersit with sysfs
* @class: pointer to the struct class that thisdevice should be registered to
* @parent: pointer to the parent struct device ofthis new device, if any
* @devt: the dev_t for the char device to beadded
* @fmt: string for the device's name
*
* This function can be used by char deviceclasses. A struct device
* will be created in sysfs, registered to thespecified class.
*
* A "dev" file will be created, showingthe dev_t for the device, if
* the dev_t is not 0,0.
* If a pointer to a parent struct device ispassed in, the newly created
* struct device will be a child of that device insysfs.
* The pointer to the struct device will bereturned from the call.
* Any further sysfs files that might be requiredcan be created using this
* pointer.
*
* Note: the struct class passed to this functionmust have previously
* been created with a call to class_create().
*/
struct device *device_create(struct class *class,struct device *parent,
dev_tdevt, 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。
Device_destroy函数
/**
*device_destroy - removes a device that was created with device_create()
*@class: pointer to the struct class that this device was registered with
*@devt: the dev_t of the device that was previously registered
*
*This call unregisters and cleans up a device that was created with a
*call to device_create().
*/
void device_destroy(struct class *class,dev_t devt)
{
structdevice *dev = NULL;
structdevice *dev_tmp;
down(&class->sem);
list_for_each_entry(dev_tmp,&class->devices, node) {
if(dev_tmp->devt == devt) {
dev= dev_tmp;
break;
}
}
up(&class->sem);
if(dev)
device_unregister(dev);
}
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);
returnresult;
}
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 devicein 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)); //deletedevice 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。