register_chrdev和cdev_init区别

register_chrdev
缺点：
一个内核只能有255个字符驱动程序
2.6内核后建议把register_chrdev展开.
以前想以主设备号为下标，在chrdevs里找到之前注册的file_operations,而现在以主设备号和次设备号来找到该结构体。
展开为1.register_chrdev_region注册区域
       /alloc_chrdev_region  region区域的意思为某个（主，次）到某个（主，次+n)设备号都对应这个驱动。以前是（主，0）到（主，255）都对应这个结构体一下子都占住位置，展开后可以缩小主设备号次设备号到某个次设备号范围
       
     2.cdev_init
     3.cdev_add


/**
 * __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;
}
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cdev_init

/**
 * cdev_init() - initialize a cdev structure
 * @cdev: the structure to initialize
 * @fops: the file_operations for this device
 *
 * Initializes @cdev, remembering @fops, making it ready to add to the
 * system with cdev_add().
 */
void cdev_init(struct cdev *cdev, const struct file_operations *fops)
{
	memset(cdev, 0, sizeof *cdev);
	INIT_LIST_HEAD(&cdev->list);
	kobject_init(&cdev->kobj, &ktype_cdev_default);
	cdev->ops = fops;
}
----------------------使用--------------------------
/* nsc_gpio uses dev_dbg(), so needs this */
	scx200_gpio_ops.dev = &pdev->dev;

	if (major) {
		devid = MKDEV(major, 0);
		rc = register_chrdev_region(devid, MAX_PINS, "scx200_gpio");//如果指定了主设备号用这函数devid为从哪开始（(major, 0)对应ops，(major, 1-255)其他不对应
	} else {
		rc = alloc_chrdev_region(&devid, 0, MAX_PINS, "scx200_gpio");//第二个参数为次设备号基地址，MAX_PINS为个数
		major = MAJOR(devid);
	}
	if (rc < 0) {
		dev_err(&pdev->dev, "SCx200 chrdev_region err: %d\n", rc);
		goto undo_platform_device_add;
	}

	cdev_init(&scx200_gpio_cdev, &scx200_gpio_fileops);
	cdev_add(&scx200_gpio_cdev, devid, MAX_PINS);
class_create
device_create//如果让系统自动创建设备节点，需要创建类和device
	return 0; /* succeed */

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