diff --git a/compatibility_matrices/compatibility_matrix.current.xml b/compatibility_matrices/compatibility_matrix.current.xml
index bf27199..ce57660 100644
--- a/compatibility_matrices/compatibility_matrix.current.xml
+++ b/compatibility_matrices/compatibility_matrix.current.xml
@@ -277,9 +277,13 @@
             <instance>default</instance>
         </interface>
     </hal>
+    <!-- Either the AIDL or the HIDL allocator HAL must exist on the device.
+         If the HIDL composer HAL exists, it must be at least version 2.0.
+         See DeviceManifestTest.GrallocHal -->
     <hal format="hidl" optional="true">
         <name>android.hardware.graphics.allocator</name>
-        <!-- New, non-Go devices should use 4.0 or the AIDL hal. -->
+        <!-- New, non-Go devices should use 4.0 or the AIDL hal.
+             See DeviceManifestTest.GrallocVersionCompatibility. -->
         <version>2.0</version>
         <version>3.0</version>
         <version>4.0</version>