register_ Chrdev and CDEV_ init cdev_ Add usage differences

register_chrdev
 shortcoming ：
 A kernel can only have 255 Character driver 
2.6 After the kernel, it is recommended to register_chrdev an .
 Previously, I wanted to take the main equipment number as the subscript , stay chrdevs Find the previously registered file_operations, Now find the structure with the main equipment number and the secondary equipment number .
 Begin to 1.register_chrdev_region Registration area 
       /alloc_chrdev_region  region Area means a certain （ Lord , Time ） To a certain （ Lord , Time +n) The device number corresponds to this driver . It used to be （ Lord ,0） To （ Lord ,255） All corresponding to this structure all at once , After expansion, you can narrow the range from the primary equipment number to a secondary equipment number 
       
     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;
}

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;
}
---------------------- Use --------------------------
/* 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");// If the master device number is specified, use this function devid For where to start （(major, 0) Corresponding ops,(major, 1-255) Other mismatches 
	} else {
    
		rc = alloc_chrdev_region(&devid, 0, MAX_PINS, "scx200_gpio");// The second parameter is the base address of the secondary device number ,MAX_PINS Number of 
		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// If you let the system automatically create an equipment node , You need to create classes and device
	return 0; /* succeed */