usb: interface authorization: Documentation part

This part adds the documentation for the interface authorization.
Signed-off-by: Stefan Koch <stefan.koch10@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-bus-usb

+What:		/sys/bus/usb/devices/INTERFACE/authorized
+Date:		August 2015
+Description:
+		This allows to authorize (1) or deauthorize (0)
+		individual interfaces instead a whole device
+		in contrast to the device authorization.
+		If a deauthorized interface will be authorized
+		so the driver probing must be triggered manually
+		by writing INTERFACE to /sys/bus/usb/drivers_probe
+		This allows to avoid side-effects with drivers
+		that need multiple interfaces.
+		A deauthorized interface cannot be probed or claimed.
+
+What:		/sys/bus/usb/devices/usbX/interface_authorized_default
+Date:		August 2015
+Description:
+		This is used as value that determines if interfaces
+		would be authorized by default.
+		The value can be 1 or 0. It's by default 1.
+
 What:		/sys/bus/usb/device/.../authorized
 Date:		July 2008
 KernelVersion:	2.6.26

Documentation/usb/authorization.txt

@@ -90,3 +90,34 @@ etc, but you get the idea. Anybody with access to a device gadget kit
 can fake descriptors and device info. Don't trust that. You are
 welcome.
 
+
+Interface authorization
+-----------------------
+There is a similar approach to allow or deny specific USB interfaces.
+That allows to block only a subset of an USB device.
+
+Authorize an interface:
+$ echo 1 > /sys/bus/usb/devices/INTERFACE/authorized
+
+Deauthorize an interface:
+$ echo 0 > /sys/bus/usb/devices/INTERFACE/authorized
+
+The default value for new interfaces
+on a particular USB bus can be changed, too.
+
+Allow interfaces per default:
+$ echo 1 > /sys/bus/usb/devices/usbX/interface_authorized_default
+
+Deny interfaces per default:
+$ echo 0 > /sys/bus/usb/devices/usbX/interface_authorized_default
+
+Per default the interface_authorized_default bit is 1.
+So all interfaces would authorized per default.
+
+Note:
+If a deauthorized interface will be authorized so the driver probing must
+be triggered manually by writing INTERFACE to /sys/bus/usb/drivers_probe
+
+For drivers that need multiple interfaces all needed interfaces should be
+authroized first. After that the drivers should be probed.
+This avoids side effects.