diff --git a/vpx/vp8dx.h b/vpx/vp8dx.h
index 5cc25cd6a..c5475c478 100644
--- a/vpx/vp8dx.h
+++ b/vpx/vp8dx.h
@@ -85,7 +85,14 @@ enum vp8_dec_control_id {
    */
   VP9_SET_BYTE_ALIGNMENT,
 
-  /** For testing. */
+  /** control function to invert the decoding order to from right to left. The
+   * function is used in a test to confirm the decoding independence of tile
+   * columns. The function may be used in application where this order
+   * of decoding is desired.
+   *
+   * TODO(yaowu): Rework the unit test that uses this control, and in a future
+   *              release, this test-only control shall be removed.
+   */
   VP9_INVERT_TILE_DECODE_ORDER,
 
   VP8_DECODER_CTRL_ID_MAX