diff --git a/Documentation/devicetree/bindings/clock/rockchip,rk3399-cru.txt b/Documentation/devicetree/bindings/clock/rockchip,rk3399-cru.txt
index 3888dd33fcbd89cd9fdda39afbf90bdced614e39..3bc56fae90ac98b4cf626e7d433210b952db815a 100644
--- a/Documentation/devicetree/bindings/clock/rockchip,rk3399-cru.txt
+++ b/Documentation/devicetree/bindings/clock/rockchip,rk3399-cru.txt
@@ -13,6 +13,12 @@ Required Properties:
 - #clock-cells: should be 1.
 - #reset-cells: should be 1.
 
+Optional Properties:
+
+- rockchip,grf: phandle to the syscon managing the "general register files".
+  It is used for GRF muxes, if missing any muxes present in the GRF will not
+  be available.
+
 Each clock is assigned an identifier and client nodes can use this identifier
 to specify the clock which they consume. All available clocks are defined as
 preprocessor macros in the dt-bindings/clock/rk3399-cru.h headers and can be