字符设备驱动步骤

编写字符设备驱动框架的步骤

Step 1: 申请设备号（主要是申请主设备号）

有两种方式:

⑴静态申请

通过下面这个函数实现：

int register_chrdev_region(dev_t from, unsigned count, const char *name);

/* register_chrdev_region() - register a range of device numbers

* @from: the first in the desired range of device numbers; must include

* the major number.

* @count: the number of consecutive device numbers required

* @name: the name of the device or driver.

*

* Return value is zero on success, a negative error code on failure.*/

这种方式主要用于，驱动开发者事先知道该驱动主设备号的情况。

⑵动态申请

通过下面这个函数实现：

int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count, const char *name) /* alloc_chrdev_region() - register a range of char device numbers

* @dev: output parameter for first assigned number

* @baseminor: first of the requested range of minor numbers

* @count: the number of minor numbers required

* @name: the name of the associated device or driver

*

* Allocates a range of char device numbers. The major number will be

* chosen dynamically, and returned (along with the first minor number)

* in @dev. Returns zero or a negative error code.*/

这种方式由系统动态分配一个设备号，返回的设备号保存在参数dev中。

Step 2 :注册字符设备

在linux 内核中用struct cdev表示一个字符设备。

字符设备的注册与注销分别通过下面的两个函数来实现：

int cdev_add(struct cdev *p, dev_t dev, unsigned count)；

/**

* cdev_add() - add a char device to the system

* @p: the cdev structure for the device

* @dev: the first device number for which this device is responsible

* @count: the number of consecutive minor numbers corresponding to this

* device

*

* cdev_add() adds the device represented by @p to the system, making it

* live immediately. A negative error code is returned on failure.

*/

void cdev_del(struct cdev *p)；

不过，在注册一个字符设备之前，要调用下面这个函数来初始化struct cdev结构体:

void cdev_init(struct cdev *cdev, const struct file_operations *fops)

/**

* cdev_init() - initialize a cdev structure

* @cdev: the structure to initialize

* @fops: the file_operations for this device

*

* Initializes @cdev, remembering @fops, making it ready to add to the

* system with cdev_add().

*/

另外，struct cdev结构体变量可以声明为一个指针，内核提供了一个函数来申请：struct cdev *cdev_alloc(void)；

step 3:创建设备节点

有两种方法：

一是通过mknod命令来创建。如：

mknod /dev/yourname c major minor

其中“yourname”可以是任意符合unix下路径名的名字，不一定要是你代码里定义的驱动或设备的名字；c 表示创建字符设备节点，major是你成功申请的主设备号，minor是次设备号，这个可以是任意的（在次设备号范围内）

另外一种方法是通过udev自动生成。这种方法需要在你的代码里创建一个设备类，然后在这个设备类的基础上，创建一个设备；另外应用程序需要跑一个udevd的后台程序。struct class*class_create(owner, name);

struct device *device_create(struct class *class, struct device *parent,

dev_t devt, void *drvdata, const char *fmt, ...)