Index: sys/kern/device_if.m
===================================================================
--- sys/kern/device_if.m
+++ sys/kern/device_if.m
@@ -48,6 +48,11 @@
 # Default implementations of some methods.
 #
 CODE {
+	static int null_delay_attach(device_t dev)
+	{
+	    return FALSE;
+	}
+
 	static int null_shutdown(device_t dev)
 	{
 	    return 0;
@@ -188,6 +193,27 @@
 };
 
 /**
+ * @brief See if a device's dependencies are present.
+ * If not, delay attach for the future but continue claiming the device.
+
+ * To include this method in a device driver, use a line like this
+ * in the driver's method list:
+ *
+ * @code
+ * 	KOBJMETHOD(device_delay_attach, foo_delay_attach)
+ * @endcode
+ *
+ * @param dev		the device to probe
+ *
+ * @retval TRUE		postpone attachment
+ * @retval FALSE	attach now
+ * @see DEVICE_ATTACH()
+ */
+METHOD int delay_attach {
+	device_t dev;
+} DEFAULT null_delay_attach;
+
+/**
  * @brief Attach a device to a device driver
  *
  * Normally only called via device_probe_and_attach(), this is called
Index: sys/kern/subr_bus.c
===================================================================
--- sys/kern/subr_bus.c
+++ sys/kern/subr_bus.c
@@ -2908,6 +2908,12 @@
 		return (ENXIO);
 	}
 
+	if (DEVICE_DELAY_ATTACH(dev)) {
+		if (bootverbose)
+			 device_printf(dev, "attach postponed\n");
+		return (EAGAIN);
+	}
+
 	device_sysctl_init(dev);
 	if (!device_is_quiet(dev))
 		device_print_child(dev->parent, dev);
@@ -3962,6 +3968,7 @@
 	driverlink_t dl;
 	devclass_t dc;
 	device_t child;
+	int attachments_pending, last_attachments_pending;
 
 	dc = dev->devclass;
 	TAILQ_FOREACH(dl, &dc->drivers, link) {
@@ -3974,6 +3981,21 @@
 		else if (child->state == DS_NOTPRESENT)
 			device_probe_and_attach(child);
 	}
+
+	/*
+	 * Keep trying to attach pending devices so long as there are any
+	 * and we are making forward progress on dependency resolution.
+	 */
+	do {
+		last_attachments_pending = attachments_pending;
+		attachments_pending = 0;
+		TAILQ_FOREACH(child, &dev->children, link) {
+			if (child->state == DS_ALIVE)
+				if (device_attach(child) == EAGAIN)
+					attachments_pending++;
+		}
+	} while (attachments_pending > 0 &&
+	    attachments_pending != last_attachments_pending);
 }
 
 /**
Index: sys/powerpc/powermac/smu.c
===================================================================
--- sys/powerpc/powermac/smu.c
+++ sys/powerpc/powermac/smu.c
@@ -154,6 +154,7 @@
 /* regular bus attachment functions */
 
 static int	smu_probe(device_t);
+static int	smu_delay_attach(device_t);
 static int	smu_attach(device_t);
 static const struct ofw_bus_devinfo *
     smu_get_devinfo(device_t bus, device_t dev);
@@ -186,6 +187,7 @@
 static device_method_t  smu_methods[] = {
 	/* Device interface */
 	DEVMETHOD(device_probe,		smu_probe),
+	DEVMETHOD(device_delay_attach,	smu_delay_attach),
 	DEVMETHOD(device_attach,	smu_attach),
 
 	/* Clock interface */
@@ -279,6 +281,14 @@
 }
 
 static int
+smu_delay_attach(device_t dev)
+{
+	
+	/* Delay attachment until our GPIO has attached */
+	return (smu_doorbell == NULL);
+}
+
+static int
 smu_attach(device_t dev)
 {
 	struct smu_softc *sc;