字符驱动

设备三种类型

1、字符设备

2、块设备

3、网络设备

字符设备：在IO传输过程中以字符为单位，传输速率较慢

内核代码：demo_chr_dev.c

#include <linux/module.h>//define THIS_MODULE
#include <linux/kernel.h>
#include <linux/fs.h>  //define file_operations
#include <linux/cdev.h>//define cdev 

static struct cdev chr_dev;
static dev_t ndev;//char device main id and sub id

static int chr_open(struct inode * nd, struct file *filp)
{
    int major = MAJOR(nd->i_rdev);
    int minor = MINOR(nd->i_rdev);
    printk("char_open, major=%d,minor=%d
", major, minor);
    return 0;
};

static ssize_t chr_read(struct file *f, char __user *u, size_t sz, loff_t *off)
{
    printk("In the char_read() function!
");
    return 0;
}

struct file_operations chr_ops =
{
    .owner = THIS_MODULE,
    .open = chr_open,
    .read = chr_read,
};


static int demo_init(void)
{
    int ret;
    cdev_init(&chr_dev, &chr_ops);
    ret = alloc_chrdev_region(&ndev, 0, 1, "chr_dev");
    if(ret < 0)
        return ret;
    printk("demo_init(): major=%d,minor=%d
", MAJOR(ndev), MINOR(ndev));
    ret = cdev_add(&chr_dev, ndev, 1);
    if(ret < 0)
        return ret;
    return 0;
}

static void demo_exit(void)
{
    printk("Removing char_dev module...
");
    cdev_del(&chr_dev);
    unregister_chrdev_region(ndev, 1);
}

module_init(demo_init);
module_exit(demo_exit);
 

MODULE_LICENSE("GPL");
MODULE_AUTHOR("Sain");
MODULE_DESCRIPTION("a char device driver as an example");

//Makefile
ifeq ($(KERNELRELEASE),)
KERNELDIR ?= /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)
modules:
        $(MAKE) -C $(KERNELDIR) M=$(PWD) modules
modules_install:
        $(MAKE) -C $(KERNELDIR) M=$(PWD) modules_install
clean:
        rm -rf *.o *~ core .depend .*.cmd *.ko *.mod.c .tmp_versions Module* modules*
.PHONY: modules modules_install clean
else
obj-m := demo_chr_dev.o
endif

整个代码相较于hello world 主要增加了下面两个响应函数

.open = chr_open,
.read = chr_read,

动态加载内核模块

sudo insmod demo_chr_dev.ko

dmesg能看到打出来相应日志了：

[ 4833.212757] demo_init(): major=245,minor=0

驱动测试程序

//main.c


#include <stdio.h>
#include <fcntl.h>   //define O_RDONLY  
#include <unistd.h>  //use posix function open read
#define CHR_DEV "/dev/chr_dev"
int main(void){

    int ret;
    char buff[32]={0};
    int fd = open(CHR_DEV, O_RDONLY | O_NDELAY);
    if(fd < 0){
        printf("open file %s failed
", CHR_DEV);
        return -1;
    }

    read(fd,buff,32);
    printf("read:[%s]", buff);
    close(fd);
    return 0;

}

gcc main.c -o main( 注意：不要gcc -c main.c -o main，加上-c完全时另外一个意思了)

比较坑的是，直接执行./main 函数会提示打开失败

还要根据设备号信息用mknod 命令在系统的/dev/目录创建新的设备节点

ret = alloc_chrdev_region(&ndev, 0, 1, "chr_dev");

根据前面的log创建设备节点sudo mknod chr_dev c 245 0

ninjame@ubuntu1604:/dev$ ls -l chr_dev
crw-r--r-- 1 root root 245, 2 9月   1 00:13 chr_dev

重新执行./main

 可以看到应用程序成功调用到了设备驱动程序实现的函数

[ 4833.212757] demo_init(): major=245,minor=0
[ 5069.412260] char_open, major=245,minor=0
[ 5069.412269] In the char_read() function!

ps：

1、实际中发现创建节点的名字可以随意 ，只要主设备号、次设备号对上就可以

sudo mknod chr_devmode c 245 0  也是可以的

2、测试代码open 函数在打开失败时是完全不会调到内核函数的

chr_open 函数不存在出错分支，可见内核的调用和我们普通调用函数还是有很大区别的

static int chr_open(struct inode * nd, struct file *filp)
{
    int major = MAJOR(nd->i_rdev);
    int minor = MINOR(nd->i_rdev);
    printk("char_open, major=%d,minor=%d
", major, minor);
    return 0;
};