字符设备驱动范例

模拟一个虚拟寄存器设备驱动

1.编写驱动

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

int dev1_registers[5];
int dev2_registers[5];

struct cdev cdev;
dev_t devno;

/*文件打开函数*/
int mem_open(struct inode *inode, struct file *filp)
{

/*获取次设备号*/
int num = MINOR(inode->i_rdev);

if (num==0)
filp->private_data = dev1_registers;
else if(num == 1)
filp->private_data = dev2_registers;
else
return -ENODEV; //无效的次设备号

return 0;
}

/*文件释放函数*/
int mem_release(struct inode *inode, struct file *filp)
{
return 0;
}

/*读函数*/
static ssize_t mem_read(struct file *filp, char __user *buf, size_t size, loff_t *ppos)
{
unsigned long p = *ppos;
unsigned int count = size;
int ret = 0;
int *register_addr = filp->private_data; /*获取设备的寄存器基地址*/

/*判断读位置是否有效*/
if (p >= 5*sizeof(int))
return 0;
if (count > 5*sizeof(int) - p)
count = 5*sizeof(int) - p;

/*读数据到用户空间*/
if (copy_to_user(buf, register_addr+p, count))
{
ret = -EFAULT;
}
else
{
*ppos += count;
ret = count;
}

return ret;
}

/*写函数*/
static ssize_t mem_write(struct file *filp, const char __user *buf, size_t size, loff_t *ppos)
{
unsigned long p = *ppos;
unsigned int count = size;
int ret = 0;
int *register_addr = filp->private_data; /*获取设备的寄存器地址*/

/*分析和获取有效的写长度*/
if (p >= 5*sizeof(int))
return 0;
if (count > 5*sizeof(int) - p)
count = 5*sizeof(int) - p;

/*从用户空间写入数据*/
if (copy_from_user(register_addr + p, buf, count))
ret = -EFAULT;
else
{
*ppos += count;
ret = count;
}

return ret;
}

/* seek文件定位函数 */
static loff_t mem_llseek(struct file *filp, loff_t offset, int whence)
{
loff_t newpos;

switch(whence) {
case SEEK_SET:
newpos = offset;
break;

case SEEK_CUR:
newpos = filp->f_pos + offset;
break;

case SEEK_END:
newpos = 5*sizeof(int)-1 + offset;
break;

default:
return -EINVAL;
}
if ((newpos<0) || (newpos>5*sizeof(int)))
return -EINVAL;

filp->f_pos = newpos;
return newpos;

}

/*文件操作结构体*/
static const struct file_operations mem_fops =
{
.llseek = mem_llseek,
.read = mem_read,
.write = mem_write,
.open = mem_open,
.release = mem_release,
};

/*设备驱动模块加载函数*/
static int memdev_init(void)
{
/*初始化cdev结构*/
cdev_init(&cdev, &mem_fops);

/* 注册字符设备 */
alloc_chrdev_region(&devno, 0, 2, "memdev");
cdev_add(&cdev, devno, 2);
}

/*模块卸载函数*/
static void memdev_exit(void)
{
cdev_del(&cdev); /*注销设备*/
unregister_chrdev_region(devno, 2); /*释放设备号*/
}

MODULE_LICENSE("GPL");

module_init(memdev_init);
module_exit(memdev_exit);

2.创建设备文件，首先查看开发板上文件系统中的设备编号

cat /proc/devices 找到memdev 的设备编号，我这里是252

mknod dev/memdev0 c 252 0

3.编写write.c文件

vim write.c
#include<stdio.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<fcntl.h>
int main()
{
int fd =0;//
int src = 2013;
fd=open("/dev/memdev0",O_RDWR);//以可读可写的方式打开文件，打开之
后会返回一个fd,
write(fd,&src,sizeof(int));向文件中写入数据，首先写入fd,然后写入
数据，数据的大小。
close(fd);
}

4.编写读取数据read.c文件

vim read_mem.c
#include<stdio.h>
#include<sys/types.h>
#include<sys/stat.h>
#include<fcntl.h>
int main()
{
int fd =0;
int dst=0;
fd = open("/dev/memdev0",O_RDWR);
read (fd,&dst,sizeof(int));
printf("dst is %d\n",dst);
return 0;
}

5.编写makefile

这里就省略，很简单。 

make

arm-linux-gcc -static write.c -o write

arm-linux-gcc -static read.c -o read

6.将编译好的模块,以及write read 拷贝到nfs文件系统中

安装内核模块 insmod memdev.ko

./write

./read