diff --git a/graphics/mapper/4.0/IMapper.hal b/graphics/mapper/4.0/IMapper.hal
index 03dfef1..d024c6e 100644
--- a/graphics/mapper/4.0/IMapper.hal
+++ b/graphics/mapper/4.0/IMapper.hal
@@ -206,6 +206,9 @@
      * outside of @p accessRegion is undefined, except that it must not cause
      * process termination.
      *
+     * An accessRegion of all-zeros means the entire buffer. That is, it is
+     * equivalent to '(0,0)-(buffer width, buffer height)'.
+     *
      * This function can lock both single-planar and multi-planar formats. The caller
      * should use get() to get information about the buffer they are locking.
      * get() can be used to get information about the planes, offsets, stride,