diff --git a/sys/dev/fdt/fdt_common.h b/sys/dev/fdt/fdt_common.h
--- a/sys/dev/fdt/fdt_common.h
+++ b/sys/dev/fdt/fdt_common.h
@@ -79,8 +79,8 @@
 typedef void (*fdt_mem_region_cb)(const struct mem_region *, void *);
 
 int fdt_addrsize_cells(phandle_t, int *, int *);
-u_long fdt_data_get(void *, int);
-int fdt_data_to_res(pcell_t *, int, int, u_long *, u_long *);
+u_long fdt_data_get(const void *, int);
+int fdt_data_to_res(const pcell_t *, int, int, u_long *, u_long *);
 phandle_t fdt_find_compatible(phandle_t, const char *, int);
 phandle_t fdt_depth_search_compatible(phandle_t, const char *, int);
 int fdt_foreach_mem_region(fdt_mem_region_cb, void *);
diff --git a/sys/dev/fdt/fdt_common.c b/sys/dev/fdt/fdt_common.c
--- a/sys/dev/fdt/fdt_common.c
+++ b/sys/dev/fdt/fdt_common.c
@@ -303,13 +303,13 @@
 }
 
 u_long
-fdt_data_get(void *data, int cells)
+fdt_data_get(const void *data, int cells)
 {
 
 	if (cells == 1)
-		return (fdt32_to_cpu(*((uint32_t *)data)));
+		return (fdt32_to_cpu(*((const uint32_t *)data)));
 
-	return (fdt64_to_cpu(*((uint64_t *)data)));
+	return (fdt64_to_cpu(*((const uint64_t *)data)));
 }
 
 int
@@ -336,22 +336,22 @@
 }
 
 int
-fdt_data_to_res(pcell_t *data, int addr_cells, int size_cells, u_long *start,
-    u_long *count)
+fdt_data_to_res(const pcell_t *data, int addr_cells, int size_cells,
+    u_long *start, u_long *count)
 {
 
 	/* Address portion. */
 	if (addr_cells > 2)
 		return (ERANGE);
 
-	*start = fdt_data_get((void *)data, addr_cells);
+	*start = fdt_data_get((const void *)data, addr_cells);
 	data += addr_cells;
 
 	/* Size portion. */
 	if (size_cells > 2)
 		return (ERANGE);
 
-	*count = fdt_data_get((void *)data, size_cells);
+	*count = fdt_data_get((const void *)data, size_cells);
 	return (0);
 }