字符设备驱动

编写Linux字符设备驱动程序的流程基本步骤如下：
　　1、查看原理图、数据手册，了解设备的操作方法；
　　2、在内核中找到相近的驱动程序，以它作为模板进行开发，有时候需要从零开始；
　　3、注册驱动程序，给用户空间提供访问的接口，如在/dev/目录下生成设备文件；
　　4、设计所要实现的操作，如open（），close（），read（），write（）、ioctl()等函数；
　　5、如果有需要实现中断服务；
　　6、编译该驱动程序到内核中，编写应用程序测试驱动程序；

    在Linux内核中使用cdev结构体来描述字符设备，通过其成员dev_t来定义设备号（分为主、次设备号）以确定字符设备的唯一性。通过其成员file_operations来定义字符设备驱动提供给VFS的接口函数，如常见的open()、read()、write()等。

在Linux字符设备驱动中，模块加载函数通过register_chrdev_region( ) 或alloc_chrdev_region( )来静态或者动态获取设备号，通过cdev_init( )建立cdev与file_operations之间的连接，通过cdev_add( )向系统添加一个cdev以完成注册。模块卸载函数通过cdev_del( )来注销cdev，通过unregister_chrdev_region( )来释放设备号。

用户空间访问该设备的程序通过Linux系统调用，如open( )、read( )、write( )，来“调用”file_operations来定义字符设备驱动提供给VFS的接口函数。

详尽实现代码如下：

/*
 * Copyright (C) Continential- weizhen.diao@conti.engineering.com
 *
 *The purpose of the following program is to create char devive drivern for realizing
 * the read,write,control and orientation funtion by using Linux system call 
 */

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

#define GLOBALMEM_SIZE    0x1000
#define MEM_CLEAR 0x1
#define CONTI_globalmem_major 230

static int CONTI_globalmem_major = CONTI_globalmem_major;

/*module_param is used to define the parameters for the specified module, three
 * parameters are name, data type and permission respectively*/
module_param(CONTI_globalmem_major, int, S_IRUGO);

/*For the purpose of packaging, cdev and global memory for the specified module is 
 *integreted into the CONTI_globalmem_dev object*/
struct CONTI_globalmem_dev {
    struct cdev cdev;
    unsigned char mem[GLOBALMEM_SIZE];
};

struct CONTI_globalmem_dev *CONTI_globalmem_devp;

static int CONTI_globalmem_open(struct inode *inode, struct file *filp)
{
    filp->private_data = CONTI_globalmem_devp;
    return 0;
}

static int CONTI_globalmem_release(struct inode *inode, struct file *filp)
{
    return 0;
}

static long CONTI_globalmem_ioctl(struct file *filp, unsigned int cmd,
                unsigned long arg)
{
    struct CONTI_globalmem_dev *dev = filp->private_data;

    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;
}

static ssize_t CONTI_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 CONTI_globalmem_dev *dev = filp->private_data;

    if (p >= GLOBALMEM_SIZE)
        return 0;
    if (count > GLOBALMEM_SIZE - p)
        count = GLOBALMEM_SIZE - p;

    if (copy_to_user(buf, dev->mem + p, count)) {
        ret = -EFAULT;
    } else {
        *ppos += count;
        ret = count;

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

    return ret;
}

static ssize_t CONTI_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 CONTI_globalmem_dev *dev = filp->private_data;

    if (p >= GLOBALMEM_SIZE)
        return 0;
    if (count > GLOBALMEM_SIZE - p)
        count = GLOBALMEM_SIZE - p;

    if (copy_from_user(dev->mem + p, buf, count))
        ret = -EFAULT;
    else {
        *ppos += count;
        ret = count;

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

    return ret;
}

static loff_t CONTI_globalmem_llseek(struct file *filp, loff_t offset, int orig)
{
    loff_t ret = 0;
    switch (orig) {
    case 0:
        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:
        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;
}

static const struct file_operations CONTI_globalmem_fops = {
    .owner = THIS_MODULE,
    .llseek = CONTI_globalmem_llseek,
    .read = CONTI_globalmem_read,
    .write = CONTI_globalmem_write,
    .unlocked_ioctl = CONTI_globalmem_ioctl,
    .open = CONTI_globalmem_open,
    .release = CONTI_globalmem_release,
};

static void globalmem_setup_cdev(struct CONTI_globalmem_dev *dev, int index)
{
    int err, devno = MKDEV(CONTI_globalmem_major, index);

    cdev_init(&dev->cdev, &CONTI_globalmem_fops);
    dev->cdev.owner = THIS_MODULE;
    err = cdev_add(&dev->cdev, devno, 1);
    if (err)
        printk(KERN_NOTICE "Error %d adding globalmem%d", err, index);
}

static int __init globalmem_init(void)
{
    int ret;
    dev_t devno = MKDEV(CONTI_globalmem_major, 0);

    if (CONTI_globalmem_major)
        ret = register_chrdev_region(devno, 1, "globalmem");
    else {
        ret = alloc_chrdev_region(&devno, 0, 1, "globalmem");
        CONTI_globalmem_major = MAJOR(devno);
    }
    if (ret < 0)
        return ret;

    CONTI_globalmem_devp = kzalloc(sizeof(struct CONTI_globalmem_dev), GFP_KERNEL);
    if (!CONTI_globalmem_devp) {
        ret = -ENOMEM;
        goto fail_malloc;
    }

    globalmem_setup_cdev(CONTI_globalmem_devp, 0);
    return 0;

 fail_malloc:
    unregister_chrdev_region(devno, 1);
    return ret;
}
module_init(globalmem_init);

static void __exit globalmem_exit(void)
{
    cdev_del(&CONTI_globalmem_devp->cdev);
    kfree(CONTI_globalmem_devp);
    unregister_chrdev_region(MKDEV(CONTI_globalmem_major, 0), 1);
}
module_exit(globalmem_exit);

MODULE_AUTHOR("Hemingway <weizhen.diao@conti.engineering.com>");
MODULE_LICENSE("GPL v2");