字符设备驱动内部实现原理解析及分步注册流程和代码实例

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 
#include 
#include 
#include 
#include 
#include 
#include 

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");