linux驱动开发（三） 字符设备驱动框架

#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/cdev.h>
#include <linux/fs.h>



int demo_major = 250;
int demo_minor = 0;
int demo_count = 1;

struct cdev cdev;

int  demo_open(struct inode *inodep, struct file * filep) // 打开设备
{
    printk("%s,%d\n", __func__, __LINE__);
    return 0;
}

int demo_release(struct inode * inodep, struct file * filep)  // 关闭设备
{
    printk("%s,%d\n", __func__, __LINE__);
    return 0;
}

struct file_operations  fops =
{
    .owner = THIS_MODULE,
    .open = demo_open,
    .release = demo_release,
};

static int __init demo_init(void)
{
    int ret = 0;
    dev_t  devno;
    
    printk("%s,%d\n", __func__, __LINE__);

    //使用下列宏则可以通过主设备号和次设备号生成 dev_t
    devno = MKDEV(demo_major, demo_minor);

    printk("devno:%d\n", devno);
    printk("demo_major:%d\n", demo_major);

    /**在调用 cdev_add()函数向系统注册字符设备之前，
      *应首先调用 register_chrdev_region()或alloc_chrdev_region()函数向系统申请设备号
      **/
    if (demo_major)
    {
        ret = register_chrdev_region(devno, 1, "demo");
    }
    else 
    {
        ret = alloc_chrdev_region(&devno, 0, 1, "demo");
    }

    if(ret)
    {
        printk("Failed to register_chrdev_region.\n");
        return ret;
    }

    //cdev_init()函数用于初始化 cdev 的成员，并建立 cdev 和 file_operations 之间的连接
    cdev_init(&cdev, &fops);
    cdev.owner = THIS_MODULE;

    //系统添加一个 cdev，完成字符设备的注册。
    ret = cdev_add(&cdev, devno, demo_count);
    if(ret)
    {
        printk(KERN_NOTICE " Failed to cdev_add [Error] %d adding demo%d", ret, demo_count);
        unregister_chrdev_region(devno, demo_count);
        return ret;
    }

    return 0;
}

static void __exit demo_exit(void)
{
    printk("%s,%d\n", __func__, __LINE__);
    //删除一个 cdev，完成字符设备的注销。
    cdev_del(&cdev);
    //在调用cdev_del()函数从系统注销字符设备之后，unregister_chrdev_region()应该被调用以释放原先申请的设备号
    unregister_chrdev_region( MKDEV(demo_major, demo_minor), demo_count );
}

module_init(demo_init);
module_exit(demo_exit);

MODULE_AUTHOR(" libra13179 ");
MODULE_LICENSE("GPL v2");

VERS = $(shell uname -r)

# Kernel modules
obj-m += demo.o

# Specify flags for the module compilation.
#EXTRA_CFLAGS=-g -O0

build: kernel_modules

kernel_modules:
    make -C /lib/modules/$(KVERS)/build M=$(CURDIR) modules

clean:
    make -C /lib/modules/$(KVERS)/build M=$(CURDIR) clean