libfdt: Add fdt_setprop_empty()

Device trees can contain empty (zero length) properties, which are often
used as boolean flags.  These can already be created using fdt_setprop()
passing a length of zero and a pointer which is ignored.  It is safe to
pass NULL, but that may not be obvious from the interface.  To make it
clearer, add an fdt_setprop_empty() helper macro.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

libfdt/libfdt.h

@@ -1527,6 +1527,36 @@ static inline int fdt_setprop_cell(void *fdt, int nodeoffset, const char *name,
 #define fdt_setprop_string(fdt, nodeoffset, name, str) \
 	fdt_setprop((fdt), (nodeoffset), (name), (str), strlen(str)+1)
 
+
+/**
+ * fdt_setprop_empty - set a property to an empty value
+ * @fdt: pointer to the device tree blob
+ * @nodeoffset: offset of the node whose property to change
+ * @name: name of the property to change
+ *
+ * fdt_setprop_empty() sets the value of the named property in the
+ * given node to an empty (zero length) value, or creates a new empty
+ * property if it does not already exist.
+ *
+ * This function may insert or delete data from the blob, and will
+ * therefore change the offsets of some existing nodes.
+ *
+ * returns:
+ *	0, on success
+ *	-FDT_ERR_NOSPACE, there is insufficient free space in the blob to
+ *		contain the new property value
+ *	-FDT_ERR_BADOFFSET, nodeoffset did not point to FDT_BEGIN_NODE tag
+ *	-FDT_ERR_BADLAYOUT,
+ *	-FDT_ERR_BADMAGIC,
+ *	-FDT_ERR_BADVERSION,
+ *	-FDT_ERR_BADSTATE,
+ *	-FDT_ERR_BADSTRUCTURE,
+ *	-FDT_ERR_BADLAYOUT,
+ *	-FDT_ERR_TRUNCATED, standard meanings
+ */
+#define fdt_setprop_empty(fdt, nodeoffset, name) \
+	fdt_setprop((fdt), (nodeoffset), (name), NULL, 0)
+
 /**
  * fdt_appendprop - append to or create a property
  * @fdt: pointer to the device tree blob

tests/setprop.c

@@ -66,7 +66,7 @@ int main(int argc, char *argv[])
 			     TEST_STRING_1);
 
 	verbose_printf("Old string value was \"%s\"\n", strp);
-	err = fdt_setprop(fdt, 0, "prop-str", NULL, 0);
+	err = fdt_setprop_empty(fdt, 0, "prop-str");
 	if (err)
 		FAIL("Failed to empty \"prop-str\": %s",
 		     fdt_strerror(err));