字符设备注册/注销

字符设备注册步骤：

1.分配cdev结构体

2.分配设备号，register_chrdev_region()/alloc_chrdev_region();

3.添加设备到系统，cdev_add();

字符设备注销步骤：

1. 从系统中删除设备，cdev_del();

2. 释放设备号，unregister_chrdev_region();

除了以上按步骤注册/注销字符设备外，系统也提供了函数直接完成设备的注册与注销。

/**
 * __register_chrdev() - create and register a cdev occupying a range of minors
 * @major: major device number or 0 for dynamic allocation
 * @baseminor: first of the requested range of minor numbers
 * @count: the number of minor numbers required
 * @name: name of this range of devices
 * @fops: file operations associated with this devices
 *
 * If @major == 0 this functions will dynamically allocate a major and return
 * its number.
 *
 * If @major > 0 this function will attempt to reserve a device with the given
 * major number and will return zero on success.
 *
 * Returns a -ve errno on failure.
 *
 * The name of this device has nothing to do with the name of the device in
 * /dev. It only helps to keep track of the different owners of devices. If
 * your module name has only one type of devices it's ok to use e.g. the name
 * of the module here.
 */
int __register_chrdev(unsigned int major, unsigned int baseminor,
              unsigned int count, const char *name,
              const struct file_operations *fops)
{
    struct char_device_struct *cd;
    struct cdev *cdev;
    int err = -ENOMEM;

    cd = __register_chrdev_region(major, baseminor, count, name);
    if (IS_ERR(cd))
        return PTR_ERR(cd);
    
    cdev = cdev_alloc();
    if (!cdev)
        goto out2;

    cdev->owner = fops->owner;
    cdev->ops = fops;
    kobject_set_name(&cdev->kobj, "%s", name);
        
    err = cdev_add(cdev, MKDEV(cd->major, baseminor), count);
    if (err)
        goto out;

    cd->cdev = cdev;

    return major ? 0 : cd->major;
out:
    kobject_put(&cdev->kobj);
out2:
    kfree(__unregister_chrdev_region(cd->major, baseminor, count));
    return err;
}
static inline int register_chrdev(unsigned int major, const char *name,
                  const struct file_operations *fops)
{
    return __register_chrdev(major, 0, 256, name, fops);
}

/**
 * __unregister_chrdev - unregister and destroy a cdev
 * @major: major device number
 * @baseminor: first of the range of minor numbers
 * @count: the number of minor numbers this cdev is occupying
 * @name: name of this range of devices
 *
 * Unregister and destroy the cdev occupying the region described by
 * @major, @baseminor and @count.  This function undoes what
 * __register_chrdev() did.
 */
void __unregister_chrdev(unsigned int major, unsigned int baseminor,
             unsigned int count, const char *name)
{
    struct char_device_struct *cd;

    cd = __unregister_chrdev_region(major, baseminor, count);
    if (cd && cd->cdev)
        cdev_del(cd->cdev);
    kfree(cd);
}
static inline void unregister_chrdev(unsigned int major, const char *name)
{
    __unregister_chrdev(major, 0, 256, name);
}