diff --git a/Documentation/fb/udlfb.txt b/Documentation/fb/udlfb.txt
index 7fdde2a..473ceed 100644
--- a/Documentation/fb/udlfb.txt
+++ b/Documentation/fb/udlfb.txt
@@ -105,6 +105,11 @@
 		the first framebuffer it finds, which isn't usually what the
 		user wants in the case of USB displays.
 
+shadow		Allocate a 2nd framebuffer to shadow what's currently across
+		the USB bus in device memory. If any pixels are unchanged,
+		do not transmit. Spends host memory to save USB transfers.
+		Enabled by default. Only disable on very low memory systems.
+
 Sysfs Attributes
 ================