linux 2.6 驱动笔记（二）

字符设备驱动

linux 2.6的字符驱动由cdev结构体描述，具体参考globalmem例子，代码主要分以下几部分：

1. 定义一个字符类型设备驱动的结构体

struct globalmem_dev{
    struct cdev cdev; /*linux 2.6 内核表示一个字符设备结构体 */
    unsigned char mem[200];
};

其中cdev在cdev.h文件中定义如下：

struct cdev {
    struct kobject kobj;
    struct module *owner;
    struct file_operations *ops; //文件操作结构体
    struct list_head list;
    dev_t dev; //设备号 unsigned long
    unsigned int count;
};

2. 在驱动模块的加载、卸载函数中注册设备，同时绑定文件操作结构体

static const struct file_operations globalmem_fops = {
    .owner     = THIS_MODULE,
    .llseek    = globalmem_llseek,
    .read   = globalmem_read,
    .write  = globalmem_write,
    .open   = globalmem_open,
    .release= globalmem_release};

//设备驱动加载
int globalmem_init(void){
    int result;
    dev_t devno = MKDEV(200, 0);

    result = register_chrdev_region(devno, 1, "globalmem");//注册字符设备编号
    globalmem_devp= kmalloc(sizeof(struct globalmem_dev), GFP_KERNEL);
    memset(globalmem_devp, 0, sizeof(struct globalmem_dev));

    cdev_init(&globalmem_devp->cdev, &globalmem_fops);
    globalmem_devp->cdev.owner = THIS_MODULE;

    int err = cdev_add(&globalmem_devp->cdev, devno, 1);

    return result;
}

3. 实现文件操作的具体接口，以文件写为例：

static ssize_t globalmem_read(struct file *flip, char __user *buf, size_t size, loff_t *ppos){
       //....
    
    struct globalmem_dev *dev = flip->private_data;
    copy_to_user(buf, (void*)(dev->mem+p), count); //内核空间拷贝至用户空间
    *ppos += count;

测试用应用代码如下：

#include<sys/types.h>
#include<sys/stat.h>
#include<stdio.h>
#include<fcntl.h>

void main(){
    int fd,num;

    fd = open("/dev/globalmem", O_RDWR);

    if(fd != -1){
        read(fd, &num, sizeof(int));
        printf("the globalmem is %d
", num);

        scanf("%d", &num);
        printf("the input is %d
", num);
        write(fd, &num, sizeof(int));

        read(fd, &num, sizeof(int));
        printf("the new globalmem is %d/n", num);
    
        close(fd);
        
    }else{
        printf("device err!");
    }
    return;
}