Index: sys/sys/bus_dma.h
===================================================================
--- sys/sys/bus_dma.h
+++ sys/sys/bus_dma.h
@@ -282,13 +282,17 @@
 void bus_dmamem_free(bus_dma_tag_t dmat, void *vaddr, bus_dmamap_t map);
 
 /*
- * Perform a synchronization operation on the given map.
+ * Perform a synchronization operation on the given map. If the map
+ * is NULL we have a fully IO-coherent system. Place a memory barrier
+ * then to ensure all data accesses are visible before going any further.
  */
 void _bus_dmamap_sync(bus_dma_tag_t, bus_dmamap_t, bus_dmasync_op_t);
 #define bus_dmamap_sync(dmat, dmamap, op) 			\
 	do {											\
 		if ((dmamap) != NULL)						\
 			_bus_dmamap_sync(dmat, dmamap, op);		\
+		else										\
+			mb();									\
 	} while (0)
 
 /*