Index: sys/dev/pci/pci.c
===================================================================
--- sys/dev/pci/pci.c
+++ sys/dev/pci/pci.c
@@ -4744,7 +4744,7 @@
 
 static struct resource *
 pci_reserve_map(device_t dev, device_t child, int type, int *rid,
-    rman_res_t start, rman_res_t end, rman_res_t count, u_int num,
+    rman_res_t *start, rman_res_t *end, rman_res_t *count, u_int num,
     u_int flags)
 {
 	struct pci_devinfo *dinfo = device_get_ivars(child);
@@ -4811,30 +4811,49 @@
 	 * situation where we might allocate the excess to
 	 * another driver, which won't work.
 	 */
-	count = ((pci_addr_t)1 << mapsize) * num;
+	*count = ((pci_addr_t)1 << mapsize) * num;
 	if (RF_ALIGNMENT(flags) < mapsize)
 		flags = (flags & ~RF_ALIGNMENT_MASK) | RF_ALIGNMENT_LOG2(mapsize);
 	if (PCI_BAR_MEM(map) && (map & PCIM_BAR_MEM_PREFETCH))
 		flags |= RF_PREFETCHABLE;
 
+	if ((*start == 0UL) && (*end == ~0UL)) {
+
+		/* Mask the information bits */
+		if (PCI_BAR_MEM(map))
+			map &= PCIM_BAR_MEM_BASE;
+		else
+			map &= PCIM_BAR_IO_BASE;
+
+		if (PCI_BAR_MEM(testval))
+			testval &= PCIM_BAR_MEM_BASE;
+		else
+			testval &= PCIM_BAR_IO_BASE;
+
+		*start = map;
+		if (*count == 0)
+			*count = (~testval) + 1;
+		*end = *start + *count - 1;
+	}
+
 	/*
 	 * Allocate enough resource, and then write back the
 	 * appropriate BAR for that resource.
 	 */
-	resource_list_add(rl, type, *rid, start, end, count);
-	res = resource_list_reserve(rl, dev, child, type, rid, start, end,
-	    count, flags & ~RF_ACTIVE);
+	resource_list_add(rl, type, *rid, *start, *end, *count);
+	res = resource_list_reserve(rl, dev, child, type, rid, *start, *end,
+	    *count, flags & ~RF_ACTIVE);
 	if (res == NULL) {
 		resource_list_delete(rl, type, *rid);
 		device_printf(child,
 		    "%#lx bytes of rid %#x res %d failed (%#lx, %#lx).\n",
-		    count, *rid, type, start, end);
+		    *count, *rid, type, *start, *end);
 		goto out;
 	}
 	if (bootverbose)
 		device_printf(child,
 		    "Lazy allocation of %#lx bytes rid %#x type %d at %#lx\n",
-		    count, *rid, type, rman_get_start(res));
+		    *count, *rid, type, rman_get_start(res));
 	map = rman_get_start(res);
 	pci_write_bar(child, pm, map);
 out:
@@ -4907,8 +4926,8 @@
 		/* Reserve resources for this BAR if needed. */
 		rle = resource_list_find(rl, type, *rid);
 		if (rle == NULL) {
-			res = pci_reserve_map(dev, child, type, rid, start, end,
-			    count, num, flags);
+			res = pci_reserve_map(dev, child, type, rid, &start,
+			    &end, &count, num, flags);
 			if (res == NULL)
 				return (NULL);
 		}