fdt: Add a function to remove unused strings from a device tree

Property names are stored in a string table. When a node property is
removed, the string table is not updated since other nodes may have a
property with the same name.

Thus it is possible for the string table to build up a number of unused
strings. Add a function to remove these. This works by building a new device
tree from the old one, adding strings one by one as needed.

Signed-off-by: Simon Glass <sjg@chromium.org>

2 files changed, 49 insertions, 0 deletions

diff --git a/include/libfdt.h b/include/libfdt.h
index 421d64fd8b..d0017d8865 100644
--- a/include/libfdt.h
+++ b/include/libfdt.h
@@ -1646,6 +1646,23 @@ int fdt_del_node(void *fdt, int nodeoffset);
 
 const char *fdt_strerror(int errval);
 
+/**
+ * fdt_remove_unused_strings() - Remove any unused strings from an FDT
+ *
+ * This creates a new device tree in @new with unused strings removed. The
+ * called can then use fdt_pack() to minimise the space consumed.
+ *
+ * @old:	Old device tree blog
+ * @new:	Place to put new device tree blob, which must be as large as
+ *		@old
+ * @return
+ *	0, on success
+ *	-FDT_ERR_BADOFFSET, corrupt device tree
+ *	-FDT_ERR_NOSPACE, out of space, which should not happen unless there
+ *		is something very wrong with the device tree input
+ */
+int fdt_remove_unused_strings(const void *old, void *new);
+
 struct fdt_region {
 	int offset;
 	int size;
diff --git a/lib/libfdt/fdt_rw.c b/lib/libfdt/fdt_rw.c
index bec8b8ad89..1a358a8ca0 100644
--- a/lib/libfdt/fdt_rw.c
+++ b/lib/libfdt/fdt_rw.c
@@ -449,3 +449,35 @@ int fdt_pack(void *fdt)
 
 	return 0;
 }
+
+int fdt_remove_unused_strings(const void *old, void *new)
+{
+	const struct fdt_property *old_prop;
+	struct fdt_property *new_prop;
+	int size = fdt_totalsize(old);
+	int next_offset, offset;
+	const char *str;
+	int ret;
+	int tag = FDT_PROP;
+
+	/* Make a copy and remove the strings */
+	memcpy(new, old, size);
+	fdt_set_size_dt_strings(new, 0);
+
+	/* Add every property name back into the new string table */
+	for (offset = 0; tag != FDT_END; offset = next_offset) {
+		tag = fdt_next_tag(old, offset, &next_offset);
+		if (tag != FDT_PROP)
+			continue;
+		old_prop = fdt_get_property_by_offset(old, offset, NULL);
+		new_prop = (struct fdt_property *)(unsigned long)
+			fdt_get_property_by_offset(new, offset, NULL);
+		str = fdt_string(old, fdt32_to_cpu(old_prop->nameoff));
+		ret = _fdt_find_add_string(new, str);
+		if (ret < 0)
+			return ret;
+		new_prop->nameoff = cpu_to_fdt32(ret);
+	}
+
+	return 0;
+}