cdev简单解析

1. cdev是linux用来管理字符设备的结构体，其在内核中采用数组结构设计，这样系统中有多少个主设备号就约定了数组大小，此设备号采用链表管理，同一主设备号下可以有多个子设备。设备即文件，上层应用要访问设备，必须通过文件，cdev中包含file_operations结构体，该结构体就是驱动的文件操作集合。
（根据于说得）
2. cdev定义在include/linux/cdev.h中，如下。具体实现在fs/char_dev.c中。

#include <linux/kobject.h>
#include <linux/kdev_t.h>
#include <linux/list.h>

struct file_operations;
struct inode;
struct module;

struct cdev {
struct kobject kobj;
struct module *owner;
const struct file_operations *ops;
struct list_head list;
dev_t dev;
unsigned int count;
};

void cdev_init(struct cdev *, const struct file_operations *); 
struct cdev *cdev_alloc(void);
void cdev_put(struct cdev *p);
int cdev_add(struct cdev *, dev_t, unsigned);
void cdev_del(struct cdev *);

void cd_forget(struct inode *);

extern struct backing_dev_info directly_mappable_cdev_bdi;

//fs/char_dev.c
/**
* 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. */
int cdev_add(struct cdev *p, dev_t dev, unsigned count)
{
p->dev = dev;
p->count = count;
return kobj_map(cdev_map, dev, count, NULL, exact_match, exact_lock, p);
}

3. cdev函数

>>cdev定义有两种方式，struct cdev cdev；另外一种struct cdev *cdev; cdev=cdev_alloc();
一种静态声明定义，另一种动态分配。
>>cdev通过函数cdev_init()初始化，主要工作就是将file_operations和cdev关联起来。file_operations是字符驱动需要实现的主要内容。
>>cdev通过cdev_add()实现cdev的注册，所谓注册就是将cdev根据设备号（dev_t）添加到cdev数组（cdev_map）中供系统管理。
>>cdev通过cdev_del()将cdev从cdev_map中移除。
4. 设备号
申请设备号用如下两个函数：
alloc_chrdev_region() --自动分配设备号
register_chrdev_region() --分配已设定的设备号

/**
* 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.
*/
int register_chrdev_region(dev_t from, 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.
*/
int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count,
const char *name);

5. more

参考：
http://www.embedu.org/Column/Column552.htm
http://blog.chinaunix.net/uid-24517893-id-161446.html