libfdt: Add phandle generation helper

The new fdt_generate_phandle() function can be used to generate a new,
unused phandle given a specific device tree blob. The implementation is
somewhat naive in that it simply walks the entire device tree to find
the highest phandle value and then returns a phandle value one higher
than that. A more clever implementation might try to find holes in the
current set of phandle values and fill them. But this implementation is
relatively simple and works reliably.

Also add a test that validates that phandles generated by this new API
are indeed unique.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Message-Id: <20190320151003.28941-1-thierry.reding@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

libfdt/fdt_ro.c

@@ -170,6 +170,37 @@ uint32_t fdt_get_max_phandle(const void *fdt)
 	return 0;
 }
 
+int fdt_generate_phandle(const void *fdt, uint32_t *phandle)
+{
+	uint32_t max = 0;
+	int offset = -1;
+
+	while (true) {
+		uint32_t value;
+
+		offset = fdt_next_node(fdt, offset, NULL);
+		if (offset < 0) {
+			if (offset == -FDT_ERR_NOTFOUND)
+				break;
+
+			return offset;
+		}
+
+		value = fdt_get_phandle(fdt, offset);
+
+		if (value > max)
+			max = value;
+	}
+
+	if (max == FDT_MAX_PHANDLE)
+		return -FDT_ERR_NOPHANDLES;
+
+	if (phandle)
+		*phandle = max + 1;
+
+	return 0;
+}
+
 static const struct fdt_reserve_entry *fdt_mem_rsv(const void *fdt, int n)
 {
 	int offset = n * sizeof(struct fdt_reserve_entry);

libfdt/libfdt.h

@@ -140,6 +140,10 @@
 
 #define FDT_ERR_MAX		17
 
+/* constants */
+#define FDT_MAX_PHANDLE 0xfffffffe
+	/* Valid values for phandles range from 1 to 2^32-2. */
+
 /**********************************************************************/
 /* Low-level functions (you probably don't need these)                */
 /**********************************************************************/
@@ -376,6 +380,21 @@ const char *fdt_string(const void *fdt, int stroffset);
  */
 uint32_t fdt_get_max_phandle(const void *fdt);
 
+/**
+ * fdt_generate_phandle - return a new, unused phandle for a device tree blob
+ * @fdt: pointer to the device tree blob
+ * @phandle: return location for the new phandle
+ *
+ * Walks the device tree blob and looks for the highest phandle value. On
+ * success, the new, unused phandle value (one higher than the previously
+ * highest phandle value in the device tree blob) will be returned in the
+ * @phandle parameter.
+ *
+ * Returns:
+ *   0 on success or a negative error-code on failure
+ */
+int fdt_generate_phandle(const void *fdt, uint32_t *phandle);
+
 /**
  * fdt_num_mem_rsv - retrieve the number of memory reserve map entries
  * @fdt: pointer to the device tree blob

libfdt/libfdt_env.h

@@ -52,6 +52,7 @@
  *     EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <stdbool.h>
 #include <stddef.h>
 #include <stdint.h>
 #include <stdlib.h>

libfdt/version.lds

@@ -66,6 +66,7 @@ LIBFDT_1.2 {
 		fdt_resize;
 		fdt_overlay_apply;
 		fdt_get_string;
+		fdt_generate_phandle;
 	local:
 		*;
 };