一、字符设备驱动内部实现原理

用户层调用open函数时，内核层的sys_open()会根据用户层传递的文件路径参数找到该文件的文件信息结构体struct inode{}，这个文件信息结构体存放的是该文件的相关信息，里面有一个成员是字符设备驱动结构体struct cdev{}，这个字符设备驱动结构体struct cdev{}里面有一个操作方法结构体指针struct  file_openations，基于这个操作方法结构体指针struct  file_openations就可以找到操作方法mycdev_open()然后回调用户层的open函数。

二、字符设备驱动分步注册流程

1.为字符设备驱动对象申请空间

①struct cdev cdev;//直接分配一个变量空间

②struct cdev *cdev=cdev_alloc();

   struct cdev *cdev_alloc(void);//手动申请字符设备驱动对象空间 

2.字符设备驱动对象的初始化

①void cdev_init(struct cdev *cdev, const struct file_operations *fops);

                cdev:字符设备驱动对象指针

                fops:操作方法结构体指针

②申请设备号 int register_chrdev_region(dev_t from, unsigned count, const char *name)

                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

         int alloc_chrdev_region(dev_t *dev, unsigned baseminor, unsigned count, const char *name)

                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

3.将字符设备驱动对象注册进内核 int cdev_add(struct cdev *cdev, dev_t dev, unsigned count);

                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

三.字符设备驱动分步注销流程

1.字符设备驱动对象的注销void cdev_del(struct cdev *);

                 scdev *字符设备驱动对象指针

2.释放申请的设备号 void unregister_chrdev_region(dev_t from, unsigned count)

                from:要释放的设备号 count:设备的数量

3.释放对象空间void kfree(void *p) 

                p :对象空间首地址

四、代码实例

#include <linux/init.h>
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/slab.h>
#include <linux/device.h>
#include <linux/uaccess.h>struct cdev *cdev;
unsigned int minor = 0;
unsigned int major = 500;
dev_t devno;
char kbuf[128] = {0};
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;
}
ssize_t mycdev_read(struct file *file, char *ubuf, size_t size, loff_t *lof)
{printk("%s:%s:%d\n", __FILE__, __func__, __LINE__);unsigned long ret;// 向用户空间读取拷贝if (size > sizeof(kbuf)) // 用户空间期待读取的大小内核满足不了，那就给内核支持的最大大小size = sizeof(kbuf);ret = copy_to_user(ubuf, kbuf, size);if (ret) // 拷贝失败{printk("copy_to_user filed\n");return ret;}return 0;
}
ssize_t mycdev_write(struct file *file, const char *ubuf, size_t size, loff_t *lof)
{unsigned long ret;// 从用户空间读取数据if (size > sizeof(kbuf)) // 用户空间期待读取的大小内核满足不了，那就给内核支持的最大大小size = sizeof(kbuf);ret = copy_from_user(kbuf, ubuf, size);if (ret) // 拷贝失败{printk("copy_to_user filed\n");return ret;}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,.read = mycdev_read,.write = mycdev_write,.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){ret = -EFAULT;goto OUT1;}printk("字符设备驱动对象实例化成功\n");// 2.部分初始化字符设备驱动对象cdev_init(cdev, &fops);// 3.申请设备号if (major == 0){ret = alloc_chrdev_region(&devno, minor, 3, "mycdev");if (ret){printk("动态申请设备号失败\n");goto OUT2;}major = MAJOR(devno);minor = MINOR(devno);}else{devno = MKDEV(major, 0);ret = register_chrdev_region(devno, 3, "mycdev");if (ret){printk("静态指定设备号失败\n");goto OUT2;}}printk("设备号申请成功\n");// 4.将字符设备驱动对象注册进内核ret = cdev_add(cdev, devno, 3);if (ret){printk("字符设备驱动对象注册内核失败\n");goto OUT3;}printk("注册进内核成功\n");// 5.自动创建设备节点cls = class_create(THIS_MODULE, "mycdev");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for (i = 0; i < 3; i++){dev = device_create(cls, NULL, MKDEV(major, i), NULL, "mycdev%d", i);if (IS_ERR(dev)){printk("向上提交设备信息失败\n");ret = PTR_ERR(dev);goto OUT5;}}printk("自动创建设备节点成功\n");return 0;
OUT5:for (--i; i >= 0; i--){device_destroy(cls, MKDEV(major, i));}// 销毁目录class_destroy(cls);
OUT4:cdev_del(cdev);
OUT3:unregister_chrdev_region(devno, 3);
OUT2:kfree(cdev);
OUT1:return ret;
}
static void __exit mycdev_exit(void)
{// 销毁设备节点信息int i;for (i = 0; i < 3; i++){device_destroy(cls, MKDEV(major, i));}class_destroy(cls);// 注销字符设备驱动对象cdev_del(cdev);// 释放设备号unregister_chrdev_region(devno, 3);// 释放对象空间kfree(cdev);
}
module_init(mycdev_init);
module_exit(mycdev_exit);
MODULE_LICENSE("GPL");

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