在驅動模塊初始化函數中實現設備節點的自動創建

我們在剛開始寫Linux設備驅動程序的時候，很多時候都是利用mknod命令手動創建設備節點，實際上Linux內核為我們提供了一組函數，可以用來在模塊加載的時候自動在/dev目錄下創建相應設備節點，并在卸載模塊時刪除該節點，當然前提條件是用戶空間移植了udev。

內核中定義了STruct class結構體，顧名思義，一個struct class結構體類型變量對應一個類，內核同時提供了class_create(…)函數，可以用它來創建一個類，這個類存放于sysfs下面，一旦創建好了這個類，再調用device_create(…)函數來在/dev目錄下創建相應的設備節點。這樣，加載模塊的時候，用戶空間中的udev會自動響應device_create(…)函數，去/sysfs下尋找對應的類從而創建設備節點。

注意，在2.6較早的內核版本中，device_create(…)函數名稱不同，是class_device_create(…)，所以在新的內核中編譯以前的模塊程序有時會報錯，就是因為函數名稱不同，而且里面的參數設置也有一些變化。

struct class和device_create(…) 以及device_create(…)都定義在/include/linux/device.h中，使用的時候一定要包含這個頭文件，否則編譯器會報錯。

在2.6.26.6內核版本中，struct class定義在頭文件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中實現：

/**

* 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);

}

第一個參數指定類的所有者是哪個模塊，第二個參數指定類名。

在class.c中，還定義了class_destroy(…)函數，用于在模塊卸載時刪除類。

device_create(…)函數在/drivers/base/core.c中實現：

/**

* 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;

}

第一個參數指定所要創建的設備所從屬的類，第二個參數是這個設備的父設備，如果沒有就指定為NULL，第三個參數是設備號，第四個參數是設備名稱，第五個參數是從設備號。

下面以一個簡單字符設備驅動來展示如何使用這幾個函數

#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);

這樣，模塊加載后，就能在/dev目錄下找到hello0這個設備節點了。