USB: make usb class a const structure

Now that the driver core allows for struct class to be in read-only
memory, remove the usb_class structure and create the usbmisc_class
const class structure declared at build time which places it into
read-only memory, instead of having it to be dynamically allocated
at load time.

Additionally, now we register usb class at startup and unregister it
when shutting down, so we don't have to count uses of the class.
Therefore we don't need the 'usb_class' structure anymore. Due to this
fact, remove all static functions related to class initialization and
deinitialization. We can't use them in 'usb.c' since they are static
and we don't really need them anymore.

Since we have to register the class in usb_init function in 'usb.c'
and use it in 'file.c' as well, declare the usbmisc_class structure
as 'export' in the 'usb.h' file.

Debatable moment: the class registration and unregistration functions
could be extracted to the 'file.c'. I think we don't want to do this
since it would be one-line functions. They would make the code paths
more confusing and add calling overhead.

Suggested-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Ivan Orlov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/usb/core/file.c

···
 #define MAX_USB_MINORS	256
 static const struct file_operations *usb_minors[MAX_USB_MINORS];
 static DECLARE_RWSEM(minor_rwsem);
-static DEFINE_MUTEX(init_usb_class_mutex);
 
 static int usb_open(struct inode *inode, struct file *file)
 {
···
 	.llseek =	noop_llseek,
 };
 
-static struct usb_class {
-	struct kref kref;
-	struct class *class;
-} *usb_class;
-
 static char *usb_devnode(const struct device *dev, umode_t *mode)
 {
 	struct usb_class_driver *drv;
···
 	return drv->devnode(dev, mode);
 }
 
-static int init_usb_class(void)
-{
-	int result = 0;
-
-	if (usb_class != NULL) {
-		kref_get(&usb_class->kref);
-		goto exit;
-	}
-
-	usb_class = kmalloc(sizeof(*usb_class), GFP_KERNEL);
-	if (!usb_class) {
-		result = -ENOMEM;
-		goto exit;
-	}
-
-	kref_init(&usb_class->kref);
-	usb_class->class = class_create("usbmisc");
-	if (IS_ERR(usb_class->class)) {
-		result = PTR_ERR(usb_class->class);
-		printk(KERN_ERR "class_create failed for usb devices\n");
-		kfree(usb_class);
-		usb_class = NULL;
-		goto exit;
-	}
-	usb_class->class->devnode = usb_devnode;
-
-exit:
-	return result;
-}
-
-static void release_usb_class(struct kref *kref)
-{
-	/* Ok, we cheat as we know we only have one usb_class */
-	class_destroy(usb_class->class);
-	kfree(usb_class);
-	usb_class = NULL;
-}
-
-static void destroy_usb_class(void)
-{
-	mutex_lock(&init_usb_class_mutex);
-	kref_put(&usb_class->kref, release_usb_class);
-	mutex_unlock(&init_usb_class_mutex);
-}
+const struct class usbmisc_class = {
+	.name		= "usbmisc",
+	.devnode	= usb_devnode,
+};
 
 int usb_major_init(void)
 {
···
 int usb_register_dev(struct usb_interface *intf,
 		     struct usb_class_driver *class_driver)
 {
-	int retval;
+	int retval = 0;
 	int minor_base = class_driver->minor_base;
 	int minor;
 	char name[20];
···
 		return -EINVAL;
 	if (intf->minor >= 0)
 		return -EADDRINUSE;
-
-	mutex_lock(&init_usb_class_mutex);
-	retval = init_usb_class();
-	mutex_unlock(&init_usb_class_mutex);
-
-	if (retval)
-		return retval;
 
 	dev_dbg(&intf->dev, "looking for a minor, starting at %d\n", minor_base);
 
···
 
 	/* create a usb class device for this usb interface */
 	snprintf(name, sizeof(name), class_driver->name, minor - minor_base);
-	intf->usb_dev = device_create(usb_class->class, &intf->dev,
+	intf->usb_dev = device_create(&usbmisc_class, &intf->dev,
 				      MKDEV(USB_MAJOR, minor), class_driver,
 				      "%s", kbasename(name));
 	if (IS_ERR(intf->usb_dev)) {
···
 		return;
 
 	dev_dbg(&intf->dev, "removing %d minor\n", intf->minor);
-	device_destroy(usb_class->class, MKDEV(USB_MAJOR, intf->minor));
+	device_destroy(&usbmisc_class, MKDEV(USB_MAJOR, intf->minor));
 
 	down_write(&minor_rwsem);
 	usb_minors[intf->minor] = NULL;
···
 
 	intf->usb_dev = NULL;
 	intf->minor = -1;
-	destroy_usb_class();
 }
 EXPORT_SYMBOL_GPL(usb_deregister_dev);

drivers/usb/core/usb.c

···
 	retval = usb_major_init();
 	if (retval)
 		goto major_init_failed;
+	retval = class_register(&usbmisc_class);
+	if (retval)
+		goto class_register_failed;
 	retval = usb_register(&usbfs_driver);
 	if (retval)
 		goto driver_register_failed;
···
 usb_devio_init_failed:
 	usb_deregister(&usbfs_driver);
 driver_register_failed:
+	class_unregister(&usbmisc_class);
+class_register_failed:
 	usb_major_cleanup();
 major_init_failed:
 	bus_unregister_notifier(&usb_bus_type, &usb_bus_nb);
···
 	usb_deregister(&usbfs_driver);
 	usb_devio_cleanup();
 	usb_hub_cleanup();
+	class_unregister(&usbmisc_class);
 	bus_unregister_notifier(&usb_bus_type, &usb_bus_nb);
 	bus_unregister(&usb_bus_type);
 	usb_acpi_unregister();

drivers/usb/core/usb.h

···
 
 #endif
 
+extern const struct class usbmisc_class;
 extern const struct bus_type usb_bus_type;
 extern struct mutex usb_port_peer_mutex;
 extern struct device_type usb_device_type;