diff --git a/sys/dev/pci/pci_pci.c b/sys/dev/pci/pci_pci.c
--- a/sys/dev/pci/pci_pci.c
+++ b/sys/dev/pci/pci_pci.c
@@ -452,6 +452,20 @@
 	if (!pcib_is_window_open(w))
 		return;
 
+	/*
+	 * Assume that a window where both the base and limit read as
+	 * zero is not really open, or at least not assigned a valid
+	 * range by the firmware.  If devices behind this bridge have
+	 * firmware-assigned resources in this range then the window
+	 * will be allocated on-demand.
+	 */
+	if (w->base == 0 && w->limit == (pci_addr_t)1 << w->step) {
+		w->base = max_address;
+		w->limit = 0;
+		pcib_write_windows(sc, w->mask);
+		return;
+	}
+
 	if (w->base > max_address || w->limit > max_address) {
 		device_printf(sc->dev,
 		    "initial %s window has too many bits, ignoring\n", w->name);