Table of contents
2.1 Registration process and API
3. Character device driver step-by-step registration\unregistration example
Internal implementation principle of character device driver 
https://blog.csdn.net/liu319293960_/article/details/131234447?spm=1001.2014.3001.5501
When a character device driver registers with the kernel, using the register_chrdev() function causes a waste of device resources.
1. Reasons
The register_chrdev() function will automatically apply for 256 device resources when registering. In actual development, so many resources may not be used, so it will cause a waste of resources.
2. Countermeasures
Adopt a step-by-step registration method to specify the number of equipment resources to apply for
字符设备驱动结构体：
struct cdev {
struct kobject kobj;
struct module *owner;//THIS_MODULE
const struct file_operations *ops;//操作方法结构体
struct list_head list;//构成链表
dev_t dev;//设备号
unsigned int count;//设备数量
}；2.1 Registration process and API
1. Apply for space for the character device driver structure
#include <linux/cdev.h>
struct cdev *cdev_alloc(void);//手动申请字符设备驱动对象空间
返回值：成功返回申请空间首地址，失败返回NULL
// 实例: struct cdev *cdev=cdev_alloc();
2. Initialization of some members of the character device driver structure
#include <linux/cdev.h>
void cdev_init(struct cdev *cdev, const struct file_operations *fops);
功能：实现字符设备驱动对象的部分初始化
参数：
cdev:字符设备驱动结构体指针
fops:操作方法结构体指针
返回值：无3. Apply for device number
int register_chrdev_region(dev_t from, unsigned count, const char *name);
功能：静态指定设备号
参数：
from:要申请的设备号
count:要申请的设备资源的数量
name:驱动名字
返回值：成功返回0，失败返回 错误码
int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count, const char *name);
功能：动态申请一定范围内的设备号
参数：
dev：申请的设备号填充在这个变量中
baseminor:次设备号的起始值
count:要申请的设备资源的数量
name:驱动名字
返回值：成功返回0，失败返回 错误码4. Register the character device driver structure into the kernel
int cdev_add(struct cdev *cdev, dev_t dev, unsigned count);
参数：
cdev:字符设备驱动对象指针
dev:申请的设备号的起始值
count：设备数量
返回值：成功返回0，失败返回错误码2.2 Logout process and API
1. Unregister the character device driver structure from the kernel
void cdev_del(struct cdev *);
功能：字符设备驱动结构体的注销
参数：字符设备驱动结构体指针
返回值：无2. Release the requested device number
void unregister_chrdev_region(dev_t from, unsigned count)
功能：释放申请的设备号
参数：
from:要释放的设备号
count:设备的数量
返回值:无3. Release character device driver structure space
void kfree(void *p)
参数：
p:字符设备驱动结构体空间首地址3. Character device driver step-by-step registration\unregistration example
#include <linux/init.h>
#include <linux/module.h>
#include<linux/fs.h>
#include<linux/io.h>
#include<linux/device.h>
#include<linux/cdev.h>
#include<linux/slab.h>
#include"led.h"
struct cdev *cdev;
unsigned int major=500;
unsigned int minor=0;
dev_t devno;
struct class *cls;
struct device * dev;
int mycdev_open(struct inode *inode, struct file *file)
{
printk("%s:%s:%d\n",__FILE__,__func__,__LINE__);
return 0;
}
long mycdev_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
{
return 0;
}
int mycdev_close(struct inode *inode, struct file *file)
{
printk("%s:%s:%d\n",__FILE__,__func__,__LINE__);
return 0;
}
//定义操作方法结构体变量并赋值
struct file_operations fops={
.open=mycdev_open,
.unlocked_ioctl=mycdev_ioctl,
.release=mycdev_close,
};
static int __init mycdev_init(void)
{
int ret,i;
//1.分配对象空间
cdev=cdev_alloc();
if(cdev==NULL)
{
printk("分配字符设备驱动对象失败\n");
ret=-EFAULT;
goto OUT1;
}
printk("分配对象空间成功\n");
//2.初始化对象
cdev_init(cdev,&fops);
//3.申请设备号
if(major>0)//静态指定设备号
{
ret= register_chrdev_region(MKDEV(major,minor),3,"myled");
if(ret)
{
printk("静态指定设备号失败\n");
goto OUT2;
}
}
else if(major==0)//动态申请设备号
{
ret=alloc_chrdev_region(&devno,minor,3,"myled");
if(ret)
{
printk("动态申请设备号失败\n");
goto OUT2;
}
//获取主设备号和次设备号
major=MAJOR(devno);
minor=MINOR(devno);
}
printk("申请设备号成功\n");
//4.注册字符设备驱动对象
ret=cdev_add(cdev,MKDEV(major,minor),3);
if(ret)
{
printk("注册字符设备驱动对象失败\n");
goto OUT3;
}
printk("注册字符设备驱动对象成功\n");
//向上提交目录
cls=class_create(THIS_MODULE,"myled");
if(IS_ERR(cls))
{
printk("向上提交目录失败\n");
ret=-PTR_ERR(cls);
goto OUT4;
}
printk("向上提交目录成功\n");
//向上提交设备节点信息
for(i=0;i<3;i++)
{
dev=device_create(cls,NULL,MKDEV(major,i),NULL,"myled%d",i);
if(IS_ERR(dev))
{
ret=-PTR_ERR(dev);
goto OUT5;
}
}
printk("向上提交设备节点成功\n");
//完成硬件寄存器地址的映射以及初始化
return 0;
OUT5:
//释放已经申请的设备节点信息
for(--i;i>=0;i--)
{
device_destroy(cls,MKDEV(major,i));
}
//释放目录空间
class_destroy(cls);
OUT4:
//注销字符设备驱动对象
cdev_del(cdev);
OUT3:
//释放设备号
unregister_chrdev_region(MKDEV(major,minor),3);
OUT2:
//释放对象空间
kfree(cdev);
OUT1:
return ret;
}
static void __exit mycdev_exit(void)
{
//销毁设备节点
int i;
for(i=0;i<3;i++)
{
device_destroy(cls,MKDEV(major,i));
}
//释放目录空间
class_destroy(cls);
//1.注销字符设备驱动对象
cdev_del(cdev);
//2.释放设备号
unregister_chrdev_region(MKDEV(major,minor),3);
//3.释放对象空间
kfree(cdev);
}
module_init(mycdev_init);
module_exit(mycdev_exit);
MODULE_LICENSE("GPL");