diff --git a/sys/arm/arm/nexus.c b/sys/arm/arm/nexus.c
--- a/sys/arm/arm/nexus.c
+++ b/sys/arm/arm/nexus.c
@@ -87,6 +87,8 @@
     rman_res_t, rman_res_t, rman_res_t, u_int);
 static	int nexus_activate_resource(device_t, device_t, int, int,
     struct resource *);
+static	int nexus_adjust_resource(device_t, device_t, int, struct resource *,
+    rman_res_t, rman_res_t);
 static bus_space_tag_t nexus_get_bus_tag(device_t, device_t);
 static bus_dma_tag_t nexus_get_dma_tag(device_t dev, device_t child);
 #ifdef SMP
@@ -126,6 +128,7 @@
 	DEVMETHOD(bus_add_child,	nexus_add_child),
 	DEVMETHOD(bus_alloc_resource,	nexus_alloc_resource),
 	DEVMETHOD(bus_activate_resource,	nexus_activate_resource),
+	DEVMETHOD(bus_adjust_resource,	nexus_adjust_resource),
 	DEVMETHOD(bus_config_intr,	nexus_config_intr),
 	DEVMETHOD(bus_deactivate_resource,	nexus_deactivate_resource),
 	DEVMETHOD(bus_release_resource,	nexus_release_resource),
@@ -263,6 +266,27 @@
 	return (rv);
 }
 
+static int
+nexus_adjust_resource(device_t bus __unused, device_t child __unused, int type,
+    struct resource *r, rman_res_t start, rman_res_t end)
+{
+	struct rman *rm;
+
+	switch (type) {
+	case SYS_RES_IRQ:
+		rm = &irq_rman;
+		break;
+	case SYS_RES_MEMORY:
+		rm = &mem_rman;
+		break;
+	default:
+		return (EINVAL);
+	}
+	if (rman_is_region_manager(r, rm) == 0)
+		return (EINVAL);
+	return (rman_adjust_resource(r, start, end));
+}
+
 static int
 nexus_release_resource(device_t bus, device_t child, int type, int rid,
     struct resource *res)
diff --git a/sys/arm64/arm64/nexus.c b/sys/arm64/arm64/nexus.c
--- a/sys/arm64/arm64/nexus.c
+++ b/sys/arm64/arm64/nexus.c
@@ -104,6 +104,8 @@
     rman_res_t, rman_res_t, rman_res_t, u_int);
 static	int nexus_activate_resource(device_t, device_t, int, int,
     struct resource *);
+static	int nexus_adjust_resource(device_t, device_t, int, struct resource *,
+    rman_res_t, rman_res_t);
 static	int nexus_map_resource(device_t, device_t, int, struct resource *,
     struct resource_map_request *, struct resource_map *);
 static int nexus_config_intr(device_t dev, int irq, enum intr_trigger trig,
@@ -135,6 +137,7 @@
 	DEVMETHOD(bus_add_child,	nexus_add_child),
 	DEVMETHOD(bus_alloc_resource,	nexus_alloc_resource),
 	DEVMETHOD(bus_activate_resource,	nexus_activate_resource),
+	DEVMETHOD(bus_adjust_resource,	nexus_adjust_resource),
 	DEVMETHOD(bus_map_resource,	nexus_map_resource),
 	DEVMETHOD(bus_config_intr,	nexus_config_intr),
 	DEVMETHOD(bus_get_resource_list, nexus_get_reslist),
@@ -273,6 +276,27 @@
 	return (rv);
 }
 
+static int
+nexus_adjust_resource(device_t bus __unused, device_t child __unused, int type,
+    struct resource *r, rman_res_t start, rman_res_t end)
+{
+	struct rman *rm;
+
+	switch (type) {
+	case SYS_RES_IRQ:
+		rm = &irq_rman;
+		break;
+	case SYS_RES_MEMORY:
+		rm = &mem_rman;
+		break;
+	default:
+		return (EINVAL);
+	}
+	if (rman_is_region_manager(r, rm) == 0)
+		return (EINVAL);
+	return (rman_adjust_resource(r, start, end));
+}
+
 static int
 nexus_release_resource(device_t bus, device_t child, int type, int rid,
     struct resource *res)
diff --git a/sys/dev/ofw/ofwbus.c b/sys/dev/ofw/ofwbus.c
--- a/sys/dev/ofw/ofwbus.c
+++ b/sys/dev/ofw/ofwbus.c
@@ -63,22 +63,14 @@
  * hang from the Open Firmware root node and adds them as devices to this bus
  * (except some special nodes which are excluded) so that drivers can be
  * attached to them.
- *
  */
 
-struct ofwbus_softc {
-	struct simplebus_softc simplebus_sc;
-	struct rman	sc_intr_rman;
-	struct rman	sc_mem_rman;
-};
-
 #ifndef __aarch64__
 static device_identify_t ofwbus_identify;
 #endif
 static device_probe_t ofwbus_probe;
 static device_attach_t ofwbus_attach;
 static bus_alloc_resource_t ofwbus_alloc_resource;
-static bus_adjust_resource_t ofwbus_adjust_resource;
 static bus_release_resource_t ofwbus_release_resource;
 
 static device_method_t ofwbus_methods[] = {
@@ -91,14 +83,14 @@
 
 	/* Bus interface */
 	DEVMETHOD(bus_alloc_resource,	ofwbus_alloc_resource),
-	DEVMETHOD(bus_adjust_resource,	ofwbus_adjust_resource),
+	DEVMETHOD(bus_adjust_resource,	bus_generic_adjust_resource),
 	DEVMETHOD(bus_release_resource,	ofwbus_release_resource),
 
 	DEVMETHOD_END
 };
 
 DEFINE_CLASS_1(ofwbus, ofwbus_driver, ofwbus_methods,
-    sizeof(struct ofwbus_softc), simplebus_driver);
+    sizeof(struct simplebus_softc), simplebus_driver);
 EARLY_DRIVER_MODULE(ofwbus, nexus, ofwbus_driver, 0, 0,
     BUS_PASS_BUS + BUS_PASS_ORDER_MIDDLE);
 MODULE_VERSION(ofwbus, 1);
@@ -133,12 +125,9 @@
 static int
 ofwbus_attach(device_t dev)
 {
-	struct ofwbus_softc *sc;
 	phandle_t node;
 	struct ofw_bus_devinfo obd;
 
-	sc = device_get_softc(dev);
-
 	node = OF_peer(0);
 
 	/*
@@ -152,15 +141,6 @@
 	 * ofw_bus_devinfo from it. Pass node to simplebus_init directly.
 	 */
 	simplebus_init(dev, node);
-	sc->sc_intr_rman.rm_type = RMAN_ARRAY;
-	sc->sc_intr_rman.rm_descr = "Interrupts";
-	sc->sc_mem_rman.rm_type = RMAN_ARRAY;
-	sc->sc_mem_rman.rm_descr = "Device Memory";
-	if (rman_init(&sc->sc_intr_rman) != 0 ||
-	    rman_init(&sc->sc_mem_rman) != 0 ||
-	    rman_manage_region(&sc->sc_intr_rman, 0, ~0) != 0 ||
-	    rman_manage_region(&sc->sc_mem_rman, 0, BUS_SPACE_MAXADDR) != 0)
-		panic("%s: failed to set up rmans.", __func__);
 
 	/*
 	 * Allow devices to identify.
@@ -182,15 +162,12 @@
 ofwbus_alloc_resource(device_t bus, device_t child, int type, int *rid,
     rman_res_t start, rman_res_t end, rman_res_t count, u_int flags)
 {
-	struct ofwbus_softc *sc;
-	struct rman *rm;
 	struct resource *rv;
 	struct resource_list_entry *rle;
-	int isdefault, passthrough;
+	bool isdefault, passthrough;
 
 	isdefault = RMAN_IS_DEFAULT_RANGE(start, end);
 	passthrough = (device_get_parent(child) != bus);
-	sc = device_get_softc(bus);
 	rle = NULL;
 	if (!passthrough && isdefault) {
 		rle = resource_list_find(BUS_GET_RESOURCE_LIST(bus, child),
@@ -206,28 +183,11 @@
 		end = ummax(rle->end, start + count - 1);
 	}
 
-	switch (type) {
-	case SYS_RES_IRQ:
-		rm = &sc->sc_intr_rman;
-		break;
-	case SYS_RES_MEMORY:
-		rm = &sc->sc_mem_rman;
-		break;
-	default:
-		return (NULL);
-	}
-
-	rv = rman_reserve_resource(rm, start, end, count, flags & ~RF_ACTIVE,
-	    child);
+	/* Let nexus handle the allocation. */
+	rv = bus_generic_alloc_resource(bus, child, type, rid, start, end,
+	    count, flags);
 	if (rv == NULL)
 		return (NULL);
-	rman_set_rid(rv, *rid);
-
-	if ((flags & RF_ACTIVE) != 0 && bus_activate_resource(child, type,
-	    *rid, rv) != 0) {
-		rman_release_resource(rv);
-		return (NULL);
-	}
 
 	if (!passthrough && rle != NULL) {
 		rle->res = rv;
@@ -239,42 +199,12 @@
 	return (rv);
 }
 
-static int
-ofwbus_adjust_resource(device_t bus, device_t child __unused, int type,
-    struct resource *r, rman_res_t start, rman_res_t end)
-{
-	struct ofwbus_softc *sc;
-	struct rman *rm;
-	device_t ofwbus;
-
-	ofwbus = bus;
-	while (strcmp(device_get_name(device_get_parent(ofwbus)), "root") != 0)
-		ofwbus = device_get_parent(ofwbus);
-	sc = device_get_softc(ofwbus);
-	switch (type) {
-	case SYS_RES_IRQ:
-		rm = &sc->sc_intr_rman;
-		break;
-	case SYS_RES_MEMORY:
-		rm = &sc->sc_mem_rman;
-		break;
-	default:
-		return (EINVAL);
-	}
-	if (rm == NULL)
-		return (ENXIO);
-	if (rman_is_region_manager(r, rm) == 0)
-		return (EINVAL);
-	return (rman_adjust_resource(r, start, end));
-}
-
 static int
 ofwbus_release_resource(device_t bus, device_t child, int type,
     int rid, struct resource *r)
 {
 	struct resource_list_entry *rle;
-	int passthrough;
-	int error;
+	bool passthrough;
 
 	passthrough = (device_get_parent(child) != bus);
 	if (!passthrough) {
@@ -285,10 +215,6 @@
 			rle->res = NULL;
 	}
 
-	if ((rman_get_flags(r) & RF_ACTIVE) != 0) {
-		error = bus_deactivate_resource(child, type, rid, r);
-		if (error)
-			return (error);
-	}
-	return (rman_release_resource(r));
+	/* Let nexus handle the release. */
+	return (bus_generic_release_resource(bus, child, type, rid, r));
 }
diff --git a/sys/powerpc/powerpc/nexus.c b/sys/powerpc/powerpc/nexus.c
--- a/sys/powerpc/powerpc/nexus.c
+++ b/sys/powerpc/powerpc/nexus.c
@@ -63,12 +63,18 @@
  * mapping. All direct subdevices of nexus are attached by DEVICE_IDENTIFY().
  */
 
+static struct rman intr_rman;
+static struct rman mem_rman;
+
 static device_probe_t nexus_probe;
 static device_attach_t nexus_attach;
 static bus_setup_intr_t nexus_setup_intr;
 static bus_teardown_intr_t nexus_teardown_intr;
+static bus_alloc_resource_t nexus_alloc_resource;
 static bus_activate_resource_t nexus_activate_resource;
 static bus_deactivate_resource_t nexus_deactivate_resource;
+static bus_adjust_resource_t nexus_adjust_resource;
+static bus_release_resource_t nexus_release_resource;
 static  int nexus_map_resource(device_t bus, device_t child, int type,
 			       struct resource *r,
 			       struct resource_map_request *argsp,
@@ -92,8 +98,11 @@
 
 	/* Bus interface */
 	DEVMETHOD(bus_add_child,	bus_generic_add_child),
+	DEVMETHOD(bus_alloc_resource,	nexus_alloc_resource),
 	DEVMETHOD(bus_activate_resource,	nexus_activate_resource),
 	DEVMETHOD(bus_deactivate_resource,	nexus_deactivate_resource),
+	DEVMETHOD(bus_adjust_resource,	nexus_adjust_resource),
+	DEVMETHOD(bus_release_resource,	nexus_release_resource),
 	DEVMETHOD(bus_map_resource,	nexus_map_resource),
 	DEVMETHOD(bus_unmap_resource,   nexus_unmap_resource),
 	DEVMETHOD(bus_setup_intr,	nexus_setup_intr),
@@ -128,6 +137,15 @@
 nexus_attach(device_t dev)
 {
 
+	intr_rman.rm_type = RMAN_ARRAY;
+	intr_rman.rm_descr = "Interrupts";
+	mem_rman.rm_type = RMAN_ARRAY;
+	mem_rman.rm_descr = "I/O memory addresses";
+	if (rman_init(&intr_rman) != 0 || rman_init(&mem_rman) != 0 ||
+	    rman_manage_region(&intr_rman, 0, ~0) != 0 ||
+	    rman_manage_region(&mem_rman, 0, BUS_SPACE_MAXADDR) != 0)
+		panic("%s: failed to set up rmans.", __func__);
+
 	bus_generic_probe(dev);
 	bus_generic_attach(dev);
 
@@ -233,6 +251,44 @@
 	return (intr);
 }
 
+/*
+ * Allocate a resource on behalf of child.  NB: child is usually going to be a
+ * child of one of our descendants, not a direct child of nexus0.
+ */
+static struct resource *
+nexus_alloc_resource(device_t bus, device_t child, int type, int *rid,
+    rman_res_t start, rman_res_t end, rman_res_t count, u_int flags)
+{
+	struct rman *rm;
+	struct resource *rv;
+
+	switch (type) {
+	case SYS_RES_IRQ:
+		rm = &intr_rman;
+		break;
+	case SYS_RES_MEMORY:
+		rm = &mem_rman;
+		break;
+	default:
+		return (NULL);
+	}
+
+	rv = rman_reserve_resource(rm, start, end, count, flags & ~RF_ACTIVE,
+	    child);
+	if (rv == NULL)
+		return (NULL);
+	rman_set_rid(rv, *rid);
+
+	if ((flags & RF_ACTIVE) != 0) {
+		if (bus_activate_resource(child, type, *rid, rv) != 0) {
+			rman_release_resource(rv);
+			return (NULL);
+		}
+	}
+
+	return (rv);
+}
+
 static int
 nexus_activate_resource(device_t bus __unused, device_t child __unused,
     int type, int rid __unused, struct resource *r)
@@ -275,6 +331,42 @@
 	return (rman_deactivate_resource(r));
 }
 
+static int
+nexus_adjust_resource(device_t bus, device_t child __unused, int type,
+    struct resource *r, rman_res_t start, rman_res_t end)
+{
+	struct rman *rm;
+
+	switch (type) {
+	case SYS_RES_IRQ:
+		rm = &intr_rman;
+		break;
+	case SYS_RES_MEMORY:
+		rm = &mem_rman;
+		break;
+	default:
+		return (EINVAL);
+	}
+	if (rm == NULL)
+		return (ENXIO);
+	if (rman_is_region_manager(r, rm) == 0)
+		return (EINVAL);
+	return (rman_adjust_resource(r, start, end));
+}
+
+static int
+nexus_release_resource(device_t bus, device_t child, int type,
+    int rid, struct resource *r)
+{
+	int error;
+
+	if ((rman_get_flags(r) & RF_ACTIVE) != 0) {
+		error = bus_deactivate_resource(child, type, rid, r);
+		if (error)
+			return (error);
+	}
+	return (rman_release_resource(r));
+}
 
 static int
 nexus_map_resource(device_t bus, device_t child, int type, struct resource *r,
diff --git a/sys/riscv/riscv/nexus.c b/sys/riscv/riscv/nexus.c
--- a/sys/riscv/riscv/nexus.c
+++ b/sys/riscv/riscv/nexus.c
@@ -85,6 +85,8 @@
     rman_res_t, rman_res_t, rman_res_t, u_int);
 static	int nexus_activate_resource(device_t, device_t, int, int,
     struct resource *);
+static	int nexus_adjust_resource(device_t, device_t, int, struct resource *,
+    rman_res_t, rman_res_t);
 static	int nexus_map_resource(device_t, device_t, int, struct resource *,
     struct resource_map_request *, struct resource_map *);
 static int nexus_config_intr(device_t dev, int irq, enum intr_trigger trig,
@@ -118,6 +120,7 @@
 	DEVMETHOD(bus_add_child,	nexus_add_child),
 	DEVMETHOD(bus_alloc_resource,	nexus_alloc_resource),
 	DEVMETHOD(bus_activate_resource,	nexus_activate_resource),
+	DEVMETHOD(bus_adjust_resource,	nexus_adjust_resource),
 	DEVMETHOD(bus_map_resource,	nexus_map_resource),
 	DEVMETHOD(bus_config_intr,	nexus_config_intr),
 	DEVMETHOD(bus_get_resource_list, nexus_get_reslist),
@@ -265,6 +268,27 @@
 	return (rv);
 }
 
+static int
+nexus_adjust_resource(device_t bus __unused, device_t child __unused, int type,
+    struct resource *r, rman_res_t start, rman_res_t end)
+{
+	struct rman *rm;
+
+	switch (type) {
+	case SYS_RES_IRQ:
+		rm = &irq_rman;
+		break;
+	case SYS_RES_MEMORY:
+		rm = &mem_rman;
+		break;
+	default:
+		return (EINVAL);
+	}
+	if (rman_is_region_manager(r, rm) == 0)
+		return (EINVAL);
+	return (rman_adjust_resource(r, start, end));
+}
+
 static int
 nexus_release_resource(device_t bus, device_t child, int type, int rid,
     struct resource *res)