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标题: linux驱动程序学习3--字符设备驱动程序举例分析 [打印本页]

作者: 皇冠小子 时间: 2011-12-20 09:44
标题: linux驱动程序学习3--字符设备驱动程序举例分析

刚开始学习字符设备驱动程序，先在物理内存上分出一块内存，当做一块虚拟字符设备，然后对该设备进行读，写操作。

举例分析：

/*memdev.c */

#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>

#include "memdev.h"

static mem_major = MEMDEV_MAJOR;

module_param(mem_major, int, S_IRUGO);

struct mem_dev *mem_devp; /*设备结构体指针*/

struct cdev cdev;

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

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

    if (num >= MEMDEV_NR_DEVS)
            return -ENODEV;
    dev = &mem_devp[num];

    /*将设备描述结构指针赋值给文件私有数据指针*/
    filp->private_data = dev;

    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;
struct mem_dev *dev = filp->private_data; /*获得设备结构体指针*/

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

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

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

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;
struct mem_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 >= MEMDEV_SIZE)
    return 0;
if (count > MEMDEV_SIZE - p)
    count = MEMDEV_SIZE - p;

/*从用户空间写入数据*/
if (copy_from_user(dev->data + 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 mem_llseek(struct file *filp, loff_t offset, int whence)
{
loff_t newpos;

    switch(whence) {
      case 0: /* SEEK_SET */
        newpos = offset;
        break;

      case 1: /* SEEK_CUR */
        newpos = filp->f_pos + offset;
        break;

      case 2: /* SEEK_END */
        newpos = MEMDEV_SIZE -1 + offset;
        break;

      default: /* can't happen */
        return -EINVAL;
    }
    if ((newpos<0) || (newpos>MEMDEV_SIZE))
    return -EINVAL;

    filp->f_pos = newpos;
    return newpos;

}

/*文件操作结构体*/
static const struct file_operations mem_fops =
{
.owner = THIS_MODULE,
.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)
{
int result;
int i;

dev_t devno = MKDEV(mem_major, 0);

/* 静态申请设备号*/
if (mem_major)
    result = register_chrdev_region(devno, 2, "memdev");
else /* 动态分配设备号 */
{
    result = alloc_chrdev_region(&devno, 0, 2, "memdev");
    mem_major = MAJOR(devno);
}

if (result < 0)
    return result;

/*初始化cdev结构*/
cdev_init(&cdev, &mem_fops);
cdev.owner = THIS_MODULE;
cdev.ops = &mem_fops;

/* 注册字符设备 */
cdev_add(&cdev, MKDEV(mem_major, 0), MEMDEV_NR_DEVS);

/* 为设备描述结构分配内存*/
mem_devp = kmalloc(MEMDEV_NR_DEVS * sizeof(struct mem_dev), GFP_KERNEL);
if (!mem_devp)    /*申请失败*/
{
    result = - ENOMEM;
    goto fail_malloc;
}
memset(mem_devp, 0, sizeof(struct mem_dev));

/*为设备分配内存*/
for (i=0; i < MEMDEV_NR_DEVS; i++)
{
        mem_devp[i].size = MEMDEV_SIZE;
        mem_devp[i].data = kmalloc(MEMDEV_SIZE, GFP_KERNEL);
        memset(mem_devp[i].data, 0, MEMDEV_SIZE);
}

return 0;

fail_malloc:
unregister_chrdev_region(devno, 1);

return result;
}

/*模块卸载函数*/
static void memdev_exit(void)
{
cdev_del(&cdev); /*注销设备*/
kfree(mem_devp); /*释放设备结构体内存*/
unregister_chrdev_region(MKDEV(mem_major, 0), 2); /*释放设备号*/
}

MODULE_AUTHOR("David Xie");
MODULE_LICENSE("GPL");

module_init(memdev_init);
module_exit(memdev_exit);

/*memdev.h*/

#ifndef _MEMDEV_H_
#define _MEMDEV_H_

#ifndef MEMDEV_MAJOR
#define MEMDEV_MAJOR 254 /*预设的mem的主设备号*/
#endif

#ifndef MEMDEV_NR_DEVS
#define MEMDEV_NR_DEVS 2 /*设备数*/
#endif

#ifndef MEMDEV_SIZE
#define MEMDEV_SIZE 4096
#endif

/*mem设备描述结构体*/
struct mem_dev
{
char *data;
unsigned long size;
};

#endif /* _MEMDEV_H_ */

分析一个驱动程序或内核程序的方法是先找程序入口。

首先查找 module_init();因为module_init()这个宏所定义的函数是这个内核模块的入口，所以应从这里开始分析驱动程序。

分析驱动程序，应有层次：

1，分析初始化部分

2，分析设备操作，设备方法，设备函数

在分析设备操作的过程中，应按照文件打开，关闭，读，写，查找的顺序分析。

分析完成后，写一个测试程序，测试驱动是否成功。

/*app-mem.c*/

#include <stdio.h>

int main()
{
FILE *fp0 = NULL;
char Buf[4096];

/*初始化Buf*/
strcpy(Buf,"Mem is char dev!");
printf("BUF: %s\n",Buf);

/*打开设备文件*/
fp0 = fopen("/dev/memdev0","r+");
if (fp0 == NULL)
{
printf("Open Memdev0 Error!\n");
return -1;
}

/*写入设备*/
fwrite(Buf, sizeof(Buf), 1, fp0);

/*重新定位文件位置（思考没有该指令，会有何后果)*/
fseek(fp0,0,SEEK_SET);

/*清除Buf*/
strcpy(Buf,"Buf is NULL!");
printf("BUF: %s\n",Buf);

/*读出设备*/
fread(Buf, sizeof(Buf), 1, fp0);

/*检测结果*/
printf("BUF: %s\n",Buf);

return 0;

}

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