Linux字符设备驱动

Linux字符设备驱动

1. 字符设备驱动的组成

在Linux系统中，字符设备驱动由如下几部分组成：设备相关结构体、字符设备驱动模块的加载与卸载函数、字符设备驱动中file_operations的成员函数。

1）设备相关结构体

在Linux驱动程序中，工程师一般习惯为相关设备建立一个结构体，其中包含cdev、私有数据、信号量等信息，如下：

<pre name="code" class="cpp">/*驱动结构体*/
struct xxx_dev_t{
struct cdev cdev;
......
}xxx_dev;

2) 字符设备驱动模块的加载与卸载函数

加载函数的功能为：实现cdev结构体的注册和设备号的申请、设备的初始化等；卸载函数功能：相应结构体的注销和设备号的注销、资源的释放等；

加载

/***********************************************************************
*   Function Name:  globalmem_init
*   Paramter:       type name [IN]: void
*   Function Descrition:
*                   Module init
*   Return: 0
*   Error:
*   Author: Vinvian 2014/11/11
***********************************************************************/
static int globalmem_init(void)
{
printk(KERN_INFO " Globlamem Driver for init!\n");
/*init cdev*/
...
/*register dev no*/
...
return 0;
}

卸载

/***********************************************************************
*   Function Name:  globalmem_exit
*   Paramter:       type name [IN]: void
*   Function Descrition:
*                   Module exit
*   Return: void
*   Error:
*   Author: Vinvian 2014/11/11
***********************************************************************/
static void globalmem_exit(void)
{
printk(KERN_INFO " Globlamem Driver for exit!\n");

/*release dev no*/
...
/*del cdev*/
...

}

3）file_operations结构体

该结构体是字符设备驱动的主体内容，是Linux驱动与应用程序及与底层处理的桥梁。

4）字符设备驱动程序的结构

字符设备驱动程序的结构，如下图所示。

2. globalmem虚拟设备驱动实例

1) 头文件及宏定义

#include <linux/module.h>
#include <linux/types.h>
#include <linux/fs.h>
#include <linux/errno.h>
#include <linux/mm.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/cdev.h>
#include <asm/io.h>
#include <asm/system.h>
#include <asm/uaccess.h>

/*struct mem*/
#define GLOBALMEM_SIZE	0x1000	/*全局内存最大4K字节*/

#define MEM_CLEAR 0x1  /*清0全局内存*/
#define GLOBALMEM_MAJOR 254    /*预设的globalmem的主设备号*/

2）结构体定义

static globalmem_major = GLOBALMEM_MAJOR;

/*globalmem struct*/
struct globalmem_dev{
struct cdev cdev;   /*cdev struct*/
unsigned char mem[GLOBALMEM_SIZE];  /*全局内存，私有数据*/
};

/*dev struct piont*/
struct globalmem_dev *globalmem_devp = NULL;

3）函数主体

/*文件打开函数*/
int globalmem_open(struct inode *inode, struct file *filp)
{
/*将设备结构体指针赋值给文件私有数据指针*/
filp->private_data = globalmem_devp;
return 0;
}
/*文件释放函数*/
int globalmem_release(struct inode *inode, struct file *filp)
{
return 0;
}
/* ioctl设备控制函数 */
static int globalmem_ioctl(struct inode *inodep, struct file *filp, unsigned
int cmd, unsigned long arg)
{
struct globalmem_dev *dev = filp->private_data;/*获得设备结构体指针*/

switch (cmd)
{
case MEM_CLEAR:
memset(dev->mem, 0, GLOBALMEM_SIZE);
printk(KERN_INFO "globalmem is set to zero\n");
break;

default:
return  - EINVAL;
}
return 0;
}

/*读函数*/
static ssize_t globalmem_read(struct file *filp, char __user *buf, size_t size,
loff_t *ppos)
{
unsigned long p =  *ppos;
unsigned int count = size;
int ret = 0;
struct globalmem_dev *dev = filp->private_data; /*获得设备结构体指针*/

/*分析和获取有效的写长度*/
if (p >= GLOBALMEM_SIZE)
return count ?  - ENXIO: 0;
if (count > GLOBALMEM_SIZE - p)
count = GLOBALMEM_SIZE - p;

/*内核空间->用户空间*/
if (copy_to_user(buf, (void*)(dev->mem + p), count))
{
ret =  - EFAULT;
}
else
{
*ppos += count;
ret = count;

printk(KERN_INFO "read %d bytes(s) from %ld\n", count, p);
}

return ret;
}
/*写函数*/
static ssize_t globalmem_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;
struct globalmem_dev *dev = filp->private_data; /*获得设备结构体指针*/

/*分析和获取有效的写长度*/
if (p >= GLOBALMEM_SIZE)
return count ?  - ENXIO: 0;
if (count > GLOBALMEM_SIZE - p)
count = GLOBALMEM_SIZE - p;

/*用户空间->内核空间*/
if (copy_from_user(dev->mem + p, buf, count) )
ret =  - EFAULT;
else
{
*ppos += count;
ret = count;

printk(KERN_INFO "written %d bytes(s) from %d\n", count, p);
}

return ret;
}
/* seek文件定位函数 */
static loff_t globalmem_llseek(struct file *filp, loff_t offset, int orig)
{
loff_t ret = 0;
switch (orig)
{
case 0:   /*相对文件开始位置偏移*/
if (offset < 0)
{
ret =  - EINVAL;
break;
}
if ((unsigned int)offset > GLOBALMEM_SIZE)
{
ret =  - EINVAL;
break;
}
filp->f_pos = (unsigned int)offset;
ret = filp->f_pos;
break;
case 1:   /*相对文件当前位置偏移*/
if ((filp->f_pos + offset) > GLOBALMEM_SIZE)
{
ret =  - EINVAL;
break;
}
if ((filp->f_pos + offset) < 0)
{
ret =  - EINVAL;
break;
}
filp->f_pos += offset;
ret = filp->f_pos;
break;
default:
ret =  - EINVAL;
break;
}

return ret;
}

/*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,
.ioctl    = globalmem_ioctl,
.open     = globalmem_open,
.release  = globalmem_release,
};

/***********************************************************************
*   Function Name:  globalmem_setup_cdev
*   Paramter:
*           struct globalmem_dev *dev [INOUT];
*           int index   [IN];
*   Function Descrition:
*                   cdev init and register.
*   Return: void
*   Author: Vinvian 2014/11/11
***********************************************************************/
static void globalmem_setup_cdev(struct globalmem_dev *dev, int index)
{
int err = 0;
int devno = MKDEV(globalmem_major, index);

cdev_init(&dev->cdev, &globalmem_fops);       //dev init
dev->cdev.owner = THIS_MODULE;
dev->cdev.ops = &globalmem_fops;

err = cdev_add(&dev->cdev, devno, 1);
if (err)
printk(KERN_NOTICE "Error %d adding LED%d", err, index);
}
/***********************************************************************
*   Function Name:  globalmem_init
*   Paramter:       type name [IN]: void
*   Function Descrition:
*                   Module init
*   Return: 0
*   Error:
*   Author: Vinvian 2014/11/11
***********************************************************************/
static int globalmem_init(void)
{
printk(KERN_INFO " Globlamem Driver for init!\n");

int result;

dev_t devno = MKDEV(globalmem_major, 0); //Create det_t with major and minor devNo

/*Regsiter devNo*/
if (globalmem_major)    //has major no
result = register_chrdev_region(devno, 1, "globalmem");
else                    //no major no
{
result = alloc_chrdev_region(&devno, 0, 1, "globalmem");
globalmem_major = MAJOR(devno);
}
if (result < 0)
return result;

/*malloc dev struct mem*/
globalmem_devp = kmalloc(sizeof(struct globalmem_dev), GFP_KERNEL);
if (!globalmem_devp)    /*申请失败*/
{
result =  - ENOMEM;

goto fail_malloc;
}
memset(globalmem_devp, 0, sizeof(struct globalmem_dev));

/*init cdev*/
globalmem_setup_cdev(globalmem_devp, 0);
return 0;

fail_malloc: unregister_chrdev_region(devno, 1);

return result;

}
/***********************************************************************
*   Function Name:  globalmem_exit
*   Paramter:       type name [IN]: void
*   Function Descrition:
*                   Module exit
*   Return: void
*   Error:
*   Author: Vinvian 2014/11/11
***********************************************************************/
void globalmem_exit(void)
{
/*release cdev*/
cdev_del(&globalmem_devp->cdev);

/*release struct dev mem*/
kfree(globalmem_devp);

/*release devNo*/
unregister_chrdev_region(MKDEV(globalmem_major, 0), 1);
}

4）

module_init(globalmem_init);
module_exit(globalmem_exit);

module_param(globalmem_major, int, S_IRUGO);

MODULE_LICENSE("Dual BSD/GPL");
MODULE_AUTHOR("Vinvian");
MODULE_DESCRIPTION("A globalmem driver Module");
MODULE_ALIAS("A globalmem driver");

3. 扩展学习

1）cdev_init();
2) cdev_add();
3) 宏MKDEV();

4. 参考文献

1）Linux设备驱动开发详解 宋宝华著