diff --git a/compatibility_matrices/compatibility_matrix.6.xml b/compatibility_matrices/compatibility_matrix.6.xml
index 9c42cb0..e19d2dd 100644
--- a/compatibility_matrices/compatibility_matrix.6.xml
+++ b/compatibility_matrices/compatibility_matrix.6.xml
@@ -238,7 +238,10 @@
             <instance>default</instance>
         </interface>
     </hal>
-    <hal format="hidl" optional="false">
+    <!-- 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, tested in vts_treble_vintf_vendor_test -->
         <version>2.0</version>