Index: head/sys/dev/bhnd/bhndb/bhndb.c
===================================================================
--- head/sys/dev/bhnd/bhndb/bhndb.c
+++ head/sys/dev/bhnd/bhndb/bhndb.c
@@ -1144,11 +1144,16 @@
 {
 	struct bhndb_softc		*sc;
 	struct rman			*rm;
+	rman_res_t			 mstart, mend;
 	int				 error;
 	
 	sc = device_get_softc(dev);
 	error = 0;
 
+	/* Verify basic constraints */
+	if (end <= start)
+		return (EINVAL);
+
 	/* Fetch resource manager */
 	rm = bhndb_get_rman(sc, child, type);
 	if (rm == NULL)
@@ -1157,16 +1162,29 @@
 	if (!rman_is_region_manager(r, rm))
 		return (ENXIO);
 
-	/* If active, adjustment is limited by the assigned window. */
 	BHNDB_LOCK(sc);
 
-	// TODO: Currently unsupported
-	error = ENODEV;
+	/* If not active, allow any range permitted by the resource manager */
+	if (!(rman_get_flags(r) & RF_ACTIVE))
+		goto done;
 
-	BHNDB_UNLOCK(sc);
+	/* Otherwise, the range is limited to the existing register window
+	 * mapping */
+	error = bhndb_find_resource_limits(sc->bus_res, r, &mstart, &mend);
+	if (error)
+		goto done;
+
+	if (start < mstart || end > mend) {
+		error = EINVAL;
+		goto done;
+	}
+
+	/* Fall through */
+done:
 	if (!error)
 		error = rman_adjust_resource(r, start, end);
 
+	BHNDB_UNLOCK(sc);
 	return (error);
 }
 
@@ -1536,7 +1554,8 @@
 	if (bhndb_get_addrspace(sc, child) == BHNDB_ADDRSPACE_BRIDGED) {
 		bhndb_priority_t r_prio;
 
-		region = bhndb_find_resource_region(sc->bus_res, r_start, r_size);
+		region = bhndb_find_resource_region(sc->bus_res, r_start,
+		    r_size);
 		if (region != NULL)
 			r_prio = region->priority;
 		else
Index: head/sys/dev/bhnd/bhndb/bhndb_private.h
===================================================================
--- head/sys/dev/bhnd/bhndb/bhndb_private.h
+++ head/sys/dev/bhnd/bhndb/bhndb_private.h
@@ -71,6 +71,11 @@
 				     bhndb_priority_t priority,
 				     const struct bhndb_regwin *static_regwin);
 
+int				 bhndb_find_resource_limits(
+				     struct bhndb_resources *br,
+				     struct resource *r, rman_res_t *start,
+				     rman_res_t *end);
+
 struct bhndb_region		*bhndb_find_resource_region(
 				     struct bhndb_resources *br,
 				     bhnd_addr_t addr, bhnd_size_t size);
@@ -133,7 +138,7 @@
  * Dynamic register window allocation reference.
  */
 struct bhndb_dw_rentry {
-	struct resource			*dw_res;		/**< child resource */
+	struct resource			*dw_res;	/**< child resource */
 	LIST_ENTRY(bhndb_dw_rentry)	 dw_link;
 };
 
Index: head/sys/dev/bhnd/bhndb/bhndb_subr.c
===================================================================
--- head/sys/dev/bhnd/bhndb/bhndb_subr.c
+++ head/sys/dev/bhnd/bhndb/bhndb_subr.c
@@ -563,8 +563,52 @@
 	return (0);
 }
 
+
+/**
+ * Find the maximum start and end limits of the register window mapping
+ * resource @p r.
+ * 
+ * If the memory range is not mapped by an existing dynamic or static register
+ * window, ENOENT will be returned.
+ * 
+ * @param br The resource state to search.
+ * @param r The resource to search for in @p br.
+ * @param addr The requested starting address.
+ * @param size The requested size.
+ * 
+ * @retval bhndb_region A region that fully contains the requested range.
+ * @retval NULL If no mapping region can be found.
+ */
+int
+bhndb_find_resource_limits(struct bhndb_resources *br, struct resource *r,
+    rman_res_t *start, rman_res_t *end)
+{
+	struct bhndb_dw_alloc	*dynamic;
+	struct bhndb_region	*sregion;
+
+	/* Check for an enclosing dynamic register window */
+	if ((dynamic = bhndb_dw_find_resource(br, r))) {
+		*start = dynamic->target;
+		*end = dynamic->target + dynamic->win->win_size - 1;
+		return (0);
+	}
+
+	/* Check for a static region */
+	sregion = bhndb_find_resource_region(br, rman_get_start(r),
+	    rman_get_size(r));
+	if (sregion != NULL && sregion->static_regwin != NULL) {
+		*start = sregion->addr;
+		*end = sregion->addr + sregion->size - 1;
+
+		return (0);
+	}
+
+	/* Not found */
+	return (ENOENT);
+}
+
 /**
- * Find a bus region that maps @p size bytes at @p addr.
+ * Find the bus region that maps @p size bytes at @p addr.
  * 
  * @param br The resource state to search.
  * @param addr The requested starting address.