usb: gadget: Fix binding to UDC via configfs interface

By default user could store only valid UDC name in configfs UDC
attr by doing:

echo $UDC_NAME > UDC

Commit (855ed04a "usb: gadget: udc-core: independent registration of
gadgets and gadget drivers") broke this behavior and allowed to store
any arbitrary string in UDC file and udc core was waiting for such
controller to appear.

echo "any arbitrary string here" > UDC

This commit fix this by adding a flag which prevents configfs
gadget from being added to list of pending drivers if UDC with
given name has not been found.
Signed-off-by: Krzysztof Opasiak <k.opasiak@samsung.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>

drivers/usb/gadget/configfs.c

@@ -1401,6 +1401,7 @@ static const struct usb_gadget_driver configfs_driver_template = {
 		.owner          = THIS_MODULE,
 		.name		= "configfs-gadget",
 	},
+	.match_existing_only = 1,
 };
 
 static struct config_group *gadgets_make(

drivers/usb/gadget/udc/udc-core.c

@@ -603,11 +603,15 @@ int usb_gadget_probe_driver(struct usb_gadget_driver *driver)
 		}
 	}
 
+	if (!driver->match_existing_only) {
 		list_add_tail(&driver->pending, &gadget_driver_pending_list);
 		pr_info("udc-core: couldn't find an available UDC - added [%s] to list of pending drivers\n",
 			driver->function);
+		ret = 0;
+	}
+
 	mutex_unlock(&udc_lock);
-	return 0;
+	return ret;
 found:
 	ret = udc_bind_to_driver(udc, driver);
 	mutex_unlock(&udc_lock);

include/linux/usb/gadget.h

@@ -1034,6 +1034,8 @@ static inline int usb_gadget_activate(struct usb_gadget *gadget)
  * @udc_name: A name of UDC this driver should be bound to. If udc_name is NULL,
  *	this driver will be bound to any available UDC.
  * @pending: UDC core private data used for deferred probe of this driver.
+ * @match_existing_only: If udc is not found, return an error and don't add this
+ *      gadget driver to list of pending driver
  *
  * Devices are disabled till a gadget driver successfully bind()s, which
  * means the driver will handle setup() requests needed to enumerate (and
@@ -1097,6 +1099,7 @@ struct usb_gadget_driver {
 
 	char			*udc_name;
 	struct list_head	pending;
+	unsigned                match_existing_only:1;
 };