diff --git a/src/picture.c b/src/picture.c
index a0c82a2..bab34f3 100644
--- a/src/picture.c
+++ b/src/picture.c
@@ -349,6 +349,24 @@ VAStatus RequestEndPicture(VADriverContextP context, VAContextID context_id)
 	if (rc != VA_STATUS_SUCCESS)
 		return rc;
 
+	/*
+	 * DEBUG INSTRUMENTATION (0011): write a sentinel pattern into
+	 * the CAPTURE buffer's first 32 bytes BEFORE QBUF. If after
+	 * DQBUF the sentinel survives (per surface.c hex dump), the
+	 * kernel never wrote to this buffer. If the sentinel is gone
+	 * (replaced by zeros), the kernel did write but our CPU read
+	 * sees stale-cached data — cache-coherency issue.
+	 */
+	{
+		unsigned char *p = (unsigned char *)
+			surface_object->destination_map[0];
+		if (p != NULL) {
+			unsigned int i;
+			for (i = 0; i < 32; i++)
+				p[i] = 0xab;
+		}
+	}
+
 	rc = v4l2_queue_buffer(driver_data->video_fd, -1, capture_type, NULL,
 			       surface_object->destination_index, 0,
 			       surface_object->destination_buffers_count);