diff --git a/src/av1.c b/src/av1.c
index 964c035..0148567 100644
--- a/src/av1.c
+++ b/src/av1.c
@@ -462,12 +462,11 @@ static void av1_fill_film_grain(VADecPictureParameterBufferAV1 *picture,
 
 	if (fg->film_grain_info_fields.bits.apply_grain)
 		ctrl->flags |= V4L2_AV1_FILM_GRAIN_FLAG_APPLY_GRAIN;
-	/* VAAPI doesn't expose update_grain; not setting the flag means
-	 * "reuse params from film_grain_params_ref_idx" — defaulting to
-	 * "update with submitted params" (which is what apply_grain implies
-	 * when set). The flag's omission is safe for vpu981 which derives
-	 * grain state from the submitted control payload, not from a
-	 * separate reuse signal. */
+	/* VAAPI doesn't expose update_grain; not setting it means
+	 * "reuse from film_grain_params_ref_idx (=0)". A previous test
+	 * with UPDATE_GRAIN=1 unconditional triggered a segfault on the
+	 * 352x288 film_grain-50 fixture — keep update_grain implicit-zero
+	 * here and revisit during Phase 3 deeper investigation. */
 	if (fg->film_grain_info_fields.bits.chroma_scaling_from_luma)
 		ctrl->flags |= V4L2_AV1_FILM_GRAIN_FLAG_CHROMA_SCALING_FROM_LUMA;
 	if (fg->film_grain_info_fields.bits.overlap_flag)