[XEN][RFC PATCH 03/13] libfdt: import fdt_overlay from Linux

[Vikram Garhwal <fnu.vikram@xxxxxxxxxx>]

Xen is missing fdt overlay functionalities. FDT overlay is required for changing
the device tree nodes during run-time.

fdt_overlay.c file is copied from Linux tree's following patch:
commit: 6e9c9686d826564f44c93cdd6f111b1c0a9dc224
scripts/dtc: Update to upstream version v1.6.0-31-gcbca977ea121

Signed-off-by: Vikram Garhwal <fnu.vikram@xxxxxxxxxx>
---
 xen/common/libfdt/fdt_overlay.c | 882 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 882 insertions(+)
 create mode 100644 xen/common/libfdt/fdt_overlay.c

diff --git a/xen/common/libfdt/fdt_overlay.c b/xen/common/libfdt/fdt_overlay.c
new file mode 100644
index 0000000..d217e79
--- /dev/null
+++ b/xen/common/libfdt/fdt_overlay.c
@@ -0,0 +1,882 @@
+// SPDX-License-Identifier: (GPL-2.0-or-later OR BSD-2-Clause)
+/*
+ * libfdt - Flat Device Tree manipulation
+ * Copyright (C) 2016 Free Electrons
+ * Copyright (C) 2016 NextThing Co.
+ */
+#include "libfdt_env.h"
+
+#include <fdt.h>
+#include <libfdt.h>
+
+#include "libfdt_internal.h"
+
+/**
+ * overlay_get_target_phandle - retrieves the target phandle of a fragment
+ * @fdto: pointer to the device tree overlay blob
+ * @fragment: node offset of the fragment in the overlay
+ *
+ * overlay_get_target_phandle() retrieves the target phandle of an
+ * overlay fragment when that fragment uses a phandle (target
+ * property) instead of a path (target-path property).
+ *
+ * returns:
+ *      the phandle pointed by the target property
+ *      0, if the phandle was not found
+ *     -1, if the phandle was malformed
+ */
+static uint32_t overlay_get_target_phandle(const void *fdto, int fragment)
+{
+       const fdt32_t *val;
+       int len;
+
+       val = fdt_getprop(fdto, fragment, "target", &len);
+       if (!val)
+               return 0;
+
+       if ((len != sizeof(*val)) || (fdt32_to_cpu(*val) == (uint32_t)-1))
+               return (uint32_t)-1;
+
+       return fdt32_to_cpu(*val);
+}
+
+/**
+ * overlay_get_target - retrieves the offset of a fragment's target
+ * @fdt: Base device tree blob
+ * @fdto: Device tree overlay blob
+ * @fragment: node offset of the fragment in the overlay
+ * @pathp: pointer which receives the path of the target (or NULL)
+ *
+ * overlay_get_target() retrieves the target offset in the base
+ * device tree of a fragment, no matter how the actual targeting is
+ * done (through a phandle or a path)
+ *
+ * returns:
+ *      the targeted node offset in the base device tree
+ *      Negative error code on error
+ */
+static int overlay_get_target(const void *fdt, const void *fdto,
+                             int fragment, char const **pathp)
+{
+       uint32_t phandle;
+       const char *path = NULL;
+       int path_len = 0, ret;
+
+       /* Try first to do a phandle based lookup */
+       phandle = overlay_get_target_phandle(fdto, fragment);
+       if (phandle == (uint32_t)-1)
+               return -FDT_ERR_BADPHANDLE;
+
+       /* no phandle, try path */
+       if (!phandle) {
+               /* And then a path based lookup */
+               path = fdt_getprop(fdto, fragment, "target-path", &path_len);
+               if (path)
+                       ret = fdt_path_offset(fdt, path);
+               else
+                       ret = path_len;
+       } else
+               ret = fdt_node_offset_by_phandle(fdt, phandle);
+
+       /*
+       * If we haven't found either a target or a
+       * target-path property in a node that contains a
+       * __overlay__ subnode (we wouldn't be called
+       * otherwise), consider it a improperly written
+       * overlay
+       */
+       if (ret < 0 && path_len == -FDT_ERR_NOTFOUND)
+               ret = -FDT_ERR_BADOVERLAY;
+
+       /* return on error */
+       if (ret < 0)
+               return ret;
+
+       /* return pointer to path (if available) */
+       if (pathp)
+               *pathp = path ? path : NULL;
+
+       return ret;
+}
+
+/**
+ * overlay_phandle_add_offset - Increases a phandle by an offset
+ * @fdt: Base device tree blob
+ * @node: Device tree overlay blob
+ * @name: Name of the property to modify (phandle or linux,phandle)
+ * @delta: offset to apply
+ *
+ * overlay_phandle_add_offset() increments a node phandle by a given
+ * offset.
+ *
+ * returns:
+ *      0 on success.
+ *      Negative error code on error
+ */
+static int overlay_phandle_add_offset(void *fdt, int node,
+                                     const char *name, uint32_t delta)
+{
+       const fdt32_t *val;
+       uint32_t adj_val;
+       int len;
+
+       val = fdt_getprop(fdt, node, name, &len);
+       if (!val)
+               return len;
+
+       if (len != sizeof(*val))
+               return -FDT_ERR_BADPHANDLE;
+
+       adj_val = fdt32_to_cpu(*val);
+       if ((adj_val + delta) < adj_val)
+               return -FDT_ERR_NOPHANDLES;
+
+       adj_val += delta;
+       if (adj_val == (uint32_t)-1)
+               return -FDT_ERR_NOPHANDLES;
+
+       return fdt_setprop_inplace_u32(fdt, node, name, adj_val);
+}
+
+/**
+ * overlay_adjust_node_phandles - Offsets the phandles of a node
+ * @fdto: Device tree overlay blob
+ * @node: Offset of the node we want to adjust
+ * @delta: Offset to shift the phandles of
+ *
+ * overlay_adjust_node_phandles() adds a constant to all the phandles
+ * of a given node. This is mainly use as part of the overlay
+ * application process, when we want to update all the overlay
+ * phandles to not conflict with the overlays of the base device tree.
+ *
+ * returns:
+ *      0 on success
+ *      Negative error code on failure
+ */
+static int overlay_adjust_node_phandles(void *fdto, int node,
+                                       uint32_t delta)
+{
+       int child;
+       int ret;
+
+       ret = overlay_phandle_add_offset(fdto, node, "phandle", delta);
+       if (ret && ret != -FDT_ERR_NOTFOUND)
+               return ret;
+
+       ret = overlay_phandle_add_offset(fdto, node, "linux,phandle", delta);
+       if (ret && ret != -FDT_ERR_NOTFOUND)
+               return ret;
+
+       fdt_for_each_subnode(child, fdto, node) {
+               ret = overlay_adjust_node_phandles(fdto, child, delta);
+               if (ret)
+                       return ret;
+       }
+
+       return 0;
+}
+
+/**
+ * overlay_adjust_local_phandles - Adjust the phandles of a whole overlay
+ * @fdto: Device tree overlay blob
+ * @delta: Offset to shift the phandles of
+ *
+ * overlay_adjust_local_phandles() adds a constant to all the
+ * phandles of an overlay. This is mainly use as part of the overlay
+ * application process, when we want to update all the overlay
+ * phandles to not conflict with the overlays of the base device tree.
+ *
+ * returns:
+ *      0 on success
+ *      Negative error code on failure
+ */
+static int overlay_adjust_local_phandles(void *fdto, uint32_t delta)
+{
+       /*
+        * Start adjusting the phandles from the overlay root
+        */
+       return overlay_adjust_node_phandles(fdto, 0, delta);
+}
+
+/**
+ * overlay_update_local_node_references - Adjust the overlay references
+ * @fdto: Device tree overlay blob
+ * @tree_node: Node offset of the node to operate on
+ * @fixup_node: Node offset of the matching local fixups node
+ * @delta: Offset to shift the phandles of
+ *
+ * overlay_update_local_nodes_references() update the phandles
+ * pointing to a node within the device tree overlay by adding a
+ * constant delta.
+ *
+ * This is mainly used as part of a device tree application process,
+ * where you want the device tree overlays phandles to not conflict
+ * with the ones from the base device tree before merging them.
+ *
+ * returns:
+ *      0 on success
+ *      Negative error code on failure
+ */
+static int overlay_update_local_node_references(void *fdto,
+                                               int tree_node,
+                                               int fixup_node,
+                                               uint32_t delta)
+{
+       int fixup_prop;
+       int fixup_child;
+       int ret;
+
+       fdt_for_each_property_offset(fixup_prop, fdto, fixup_node) {
+               const fdt32_t *fixup_val;
+               const char *tree_val;
+               const char *name;
+               int fixup_len;
+               int tree_len;
+               int i;
+
+               fixup_val = fdt_getprop_by_offset(fdto, fixup_prop,
+                                                 &name, &fixup_len);
+               if (!fixup_val)
+                       return fixup_len;
+
+               if (fixup_len % sizeof(uint32_t))
+                       return -FDT_ERR_BADOVERLAY;
+               fixup_len /= sizeof(uint32_t);
+
+               tree_val = fdt_getprop(fdto, tree_node, name, &tree_len);
+               if (!tree_val) {
+                       if (tree_len == -FDT_ERR_NOTFOUND)
+                               return -FDT_ERR_BADOVERLAY;
+
+                       return tree_len;
+               }
+
+               for (i = 0; i < fixup_len; i++) {
+                       fdt32_t adj_val;
+                       uint32_t poffset;
+
+                       poffset = fdt32_to_cpu(fixup_val[i]);
+
+                       /*
+                        * phandles to fixup can be unaligned.
+                        *
+                        * Use a memcpy for the architectures that do
+                        * not support unaligned accesses.
+                        */
+                       memcpy(&adj_val, tree_val + poffset, sizeof(adj_val));
+
+                       adj_val = cpu_to_fdt32(fdt32_to_cpu(adj_val) + delta);
+
+                       ret = fdt_setprop_inplace_namelen_partial(fdto,
+                                                                 tree_node,
+                                                                 name,
+                                                                 strlen(name),
+                                                                 poffset,
+                                                                 &adj_val,
+                                                                 
sizeof(adj_val));
+                       if (ret == -FDT_ERR_NOSPACE)
+                               return -FDT_ERR_BADOVERLAY;
+
+                       if (ret)
+                               return ret;
+               }
+       }
+
+       fdt_for_each_subnode(fixup_child, fdto, fixup_node) {
+               const char *fixup_child_name = fdt_get_name(fdto, fixup_child,
+                                                           NULL);
+               int tree_child;
+
+               tree_child = fdt_subnode_offset(fdto, tree_node,
+                                               fixup_child_name);
+               if (tree_child == -FDT_ERR_NOTFOUND)
+                       return -FDT_ERR_BADOVERLAY;
+               if (tree_child < 0)
+                       return tree_child;
+
+               ret = overlay_update_local_node_references(fdto,
+                                                          tree_child,
+                                                          fixup_child,
+                                                          delta);
+               if (ret)
+                       return ret;
+       }
+
+       return 0;
+}
+
+/**
+ * overlay_update_local_references - Adjust the overlay references
+ * @fdto: Device tree overlay blob
+ * @delta: Offset to shift the phandles of
+ *
+ * overlay_update_local_references() update all the phandles pointing
+ * to a node within the device tree overlay by adding a constant
+ * delta to not conflict with the base overlay.
+ *
+ * This is mainly used as part of a device tree application process,
+ * where you want the device tree overlays phandles to not conflict
+ * with the ones from the base device tree before merging them.
+ *
+ * returns:
+ *      0 on success
+ *      Negative error code on failure
+ */
+static int overlay_update_local_references(void *fdto, uint32_t delta)
+{
+       int fixups;
+
+       fixups = fdt_path_offset(fdto, "/__local_fixups__");
+       if (fixups < 0) {
+               /* There's no local phandles to adjust, bail out */
+               if (fixups == -FDT_ERR_NOTFOUND)
+                       return 0;
+
+               return fixups;
+       }
+
+       /*
+        * Update our local references from the root of the tree
+        */
+       return overlay_update_local_node_references(fdto, 0, fixups,
+                                                   delta);
+}
+
+/**
+ * overlay_fixup_one_phandle - Set an overlay phandle to the base one
+ * @fdt: Base Device Tree blob
+ * @fdto: Device tree overlay blob
+ * @symbols_off: Node offset of the symbols node in the base device tree
+ * @path: Path to a node holding a phandle in the overlay
+ * @path_len: number of path characters to consider
+ * @name: Name of the property holding the phandle reference in the overlay
+ * @name_len: number of name characters to consider
+ * @poffset: Offset within the overlay property where the phandle is stored
+ * @label: Label of the node referenced by the phandle
+ *
+ * overlay_fixup_one_phandle() resolves an overlay phandle pointing to
+ * a node in the base device tree.
+ *
+ * This is part of the device tree overlay application process, when
+ * you want all the phandles in the overlay to point to the actual
+ * base dt nodes.
+ *
+ * returns:
+ *      0 on success
+ *      Negative error code on failure
+ */
+static int overlay_fixup_one_phandle(void *fdt, void *fdto,
+                                    int symbols_off,
+                                    const char *path, uint32_t path_len,
+                                    const char *name, uint32_t name_len,
+                                    int poffset, const char *label)
+{
+       const char *symbol_path;
+       uint32_t phandle;
+       fdt32_t phandle_prop;
+       int symbol_off, fixup_off;
+       int prop_len;
+
+       if (symbols_off < 0)
+               return symbols_off;
+
+       symbol_path = fdt_getprop(fdt, symbols_off, label,
+                                 &prop_len);
+       if (!symbol_path)
+               return prop_len;
+
+       symbol_off = fdt_path_offset(fdt, symbol_path);
+       if (symbol_off < 0)
+               return symbol_off;
+
+       phandle = fdt_get_phandle(fdt, symbol_off);
+       if (!phandle)
+               return -FDT_ERR_NOTFOUND;
+
+       fixup_off = fdt_path_offset_namelen(fdto, path, path_len);
+       if (fixup_off == -FDT_ERR_NOTFOUND)
+               return -FDT_ERR_BADOVERLAY;
+       if (fixup_off < 0)
+               return fixup_off;
+
+       phandle_prop = cpu_to_fdt32(phandle);
+       return fdt_setprop_inplace_namelen_partial(fdto, fixup_off,
+                                                  name, name_len, poffset,
+                                                  &phandle_prop,
+                                                  sizeof(phandle_prop));
+};
+
+/**
+ * overlay_fixup_phandle - Set an overlay phandle to the base one
+ * @fdt: Base Device Tree blob
+ * @fdto: Device tree overlay blob
+ * @symbols_off: Node offset of the symbols node in the base device tree
+ * @property: Property offset in the overlay holding the list of fixups
+ *
+ * overlay_fixup_phandle() resolves all the overlay phandles pointed
+ * to in a __fixups__ property, and updates them to match the phandles
+ * in use in the base device tree.
+ *
+ * This is part of the device tree overlay application process, when
+ * you want all the phandles in the overlay to point to the actual
+ * base dt nodes.
+ *
+ * returns:
+ *      0 on success
+ *      Negative error code on failure
+ */
+static int overlay_fixup_phandle(void *fdt, void *fdto, int symbols_off,
+                                int property)
+{
+       const char *value;
+       const char *label;
+       int len;
+
+       value = fdt_getprop_by_offset(fdto, property,
+                                     &label, &len);
+       if (!value) {
+               if (len == -FDT_ERR_NOTFOUND)
+                       return -FDT_ERR_INTERNAL;
+
+               return len;
+       }
+
+       do {
+               const char *path, *name, *fixup_end;
+               const char *fixup_str = value;
+               uint32_t path_len, name_len;
+               uint32_t fixup_len;
+               char *sep, *endptr;
+               int poffset, ret;
+
+               fixup_end = memchr(value, '\0', len);
+               if (!fixup_end)
+                       return -FDT_ERR_BADOVERLAY;
+               fixup_len = fixup_end - fixup_str;
+
+               len -= fixup_len + 1;
+               value += fixup_len + 1;
+
+               path = fixup_str;
+               sep = memchr(fixup_str, ':', fixup_len);
+               if (!sep || *sep != ':')
+                       return -FDT_ERR_BADOVERLAY;
+
+               path_len = sep - path;
+               if (path_len == (fixup_len - 1))
+                       return -FDT_ERR_BADOVERLAY;
+
+               fixup_len -= path_len + 1;
+               name = sep + 1;
+               sep = memchr(name, ':', fixup_len);
+               if (!sep || *sep != ':')
+                       return -FDT_ERR_BADOVERLAY;
+
+               name_len = sep - name;
+               if (!name_len)
+                       return -FDT_ERR_BADOVERLAY;
+
+               poffset = strtoul(sep + 1, &endptr, 10);
+               if ((*endptr != '\0') || (endptr <= (sep + 1)))
+                       return -FDT_ERR_BADOVERLAY;
+
+               ret = overlay_fixup_one_phandle(fdt, fdto, symbols_off,
+                                               path, path_len, name, name_len,
+                                               poffset, label);
+               if (ret)
+                       return ret;
+       } while (len > 0);
+
+       return 0;
+}
+
+/**
+ * overlay_fixup_phandles - Resolve the overlay phandles to the base
+ *                          device tree
+ * @fdt: Base Device Tree blob
+ * @fdto: Device tree overlay blob
+ *
+ * overlay_fixup_phandles() resolves all the overlay phandles pointing
+ * to nodes in the base device tree.
+ *
+ * This is one of the steps of the device tree overlay application
+ * process, when you want all the phandles in the overlay to point to
+ * the actual base dt nodes.
+ *
+ * returns:
+ *      0 on success
+ *      Negative error code on failure
+ */
+static int overlay_fixup_phandles(void *fdt, void *fdto)
+{
+       int fixups_off, symbols_off;
+       int property;
+
+       /* We can have overlays without any fixups */
+       fixups_off = fdt_path_offset(fdto, "/__fixups__");
+       if (fixups_off == -FDT_ERR_NOTFOUND)
+               return 0; /* nothing to do */
+       if (fixups_off < 0)
+               return fixups_off;
+
+       /* And base DTs without symbols */
+       symbols_off = fdt_path_offset(fdt, "/__symbols__");
+       if ((symbols_off < 0 && (symbols_off != -FDT_ERR_NOTFOUND)))
+               return symbols_off;
+
+       fdt_for_each_property_offset(property, fdto, fixups_off) {
+               int ret;
+
+               ret = overlay_fixup_phandle(fdt, fdto, symbols_off, property);
+               if (ret)
+                       return ret;
+       }
+
+       return 0;
+}
+
+/**
+ * overlay_apply_node - Merges a node into the base device tree
+ * @fdt: Base Device Tree blob
+ * @target: Node offset in the base device tree to apply the fragment to
+ * @fdto: Device tree overlay blob
+ * @node: Node offset in the overlay holding the changes to merge
+ *
+ * overlay_apply_node() merges a node into a target base device tree
+ * node pointed.
+ *
+ * This is part of the final step in the device tree overlay
+ * application process, when all the phandles have been adjusted and
+ * resolved and you just have to merge overlay into the base device
+ * tree.
+ *
+ * returns:
+ *      0 on success
+ *      Negative error code on failure
+ */
+static int overlay_apply_node(void *fdt, int target,
+                             void *fdto, int node)
+{
+       int property;
+       int subnode;
+
+       fdt_for_each_property_offset(property, fdto, node) {
+               const char *name;
+               const void *prop;
+               int prop_len;
+               int ret;
+
+               prop = fdt_getprop_by_offset(fdto, property, &name,
+                                            &prop_len);
+               if (prop_len == -FDT_ERR_NOTFOUND)
+                       return -FDT_ERR_INTERNAL;
+               if (prop_len < 0)
+                       return prop_len;
+
+               ret = fdt_setprop(fdt, target, name, prop, prop_len);
+               if (ret)
+                       return ret;
+       }
+
+       fdt_for_each_subnode(subnode, fdto, node) {
+               const char *name = fdt_get_name(fdto, subnode, NULL);
+               int nnode;
+               int ret;
+
+               nnode = fdt_add_subnode(fdt, target, name);
+               if (nnode == -FDT_ERR_EXISTS) {
+                       nnode = fdt_subnode_offset(fdt, target, name);
+                       if (nnode == -FDT_ERR_NOTFOUND)
+                               return -FDT_ERR_INTERNAL;
+               }
+
+               if (nnode < 0)
+                       return nnode;
+
+               ret = overlay_apply_node(fdt, nnode, fdto, subnode);
+               if (ret)
+                       return ret;
+       }
+
+       return 0;
+}
+
+/**
+ * overlay_merge - Merge an overlay into its base device tree
+ * @fdt: Base Device Tree blob
+ * @fdto: Device tree overlay blob
+ *
+ * overlay_merge() merges an overlay into its base device tree.
+ *
+ * This is the next to last step in the device tree overlay application
+ * process, when all the phandles have been adjusted and resolved and
+ * you just have to merge overlay into the base device tree.
+ *
+ * returns:
+ *      0 on success
+ *      Negative error code on failure
+ */
+static int overlay_merge(void *fdt, void *fdto)
+{
+       int fragment;
+
+       fdt_for_each_subnode(fragment, fdto, 0) {
+               int overlay;
+               int target;
+               int ret;
+
+               /*
+                * Each fragments will have an __overlay__ node. If
+                * they don't, it's not supposed to be merged
+                */
+               overlay = fdt_subnode_offset(fdto, fragment, "__overlay__");
+               if (overlay == -FDT_ERR_NOTFOUND)
+                       continue;
+
+               if (overlay < 0)
+                       return overlay;
+
+               target = overlay_get_target(fdt, fdto, fragment, NULL);
+               if (target < 0)
+                       return target;
+
+               ret = overlay_apply_node(fdt, target, fdto, overlay);
+               if (ret)
+                       return ret;
+       }
+
+       return 0;
+}
+
+static int get_path_len(const void *fdt, int nodeoffset)
+{
+       int len = 0, namelen;
+       const char *name;
+
+       FDT_RO_PROBE(fdt);
+
+       for (;;) {
+               name = fdt_get_name(fdt, nodeoffset, &namelen);
+               if (!name)
+                       return namelen;
+
+               /* root? we're done */
+               if (namelen == 0)
+                       break;
+
+               nodeoffset = fdt_parent_offset(fdt, nodeoffset);
+               if (nodeoffset < 0)
+                       return nodeoffset;
+               len += namelen + 1;
+       }
+
+       /* in case of root pretend it's "/" */
+       if (len == 0)
+               len++;
+       return len;
+}
+
+/**
+ * overlay_symbol_update - Update the symbols of base tree after a merge
+ * @fdt: Base Device Tree blob
+ * @fdto: Device tree overlay blob
+ *
+ * overlay_symbol_update() updates the symbols of the base tree with the
+ * symbols of the applied overlay
+ *
+ * This is the last step in the device tree overlay application
+ * process, allowing the reference of overlay symbols by subsequent
+ * overlay operations.
+ *
+ * returns:
+ *      0 on success
+ *      Negative error code on failure
+ */
+static int overlay_symbol_update(void *fdt, void *fdto)
+{
+       int root_sym, ov_sym, prop, path_len, fragment, target;
+       int len, frag_name_len, ret, rel_path_len;
+       const char *s, *e;
+       const char *path;
+       const char *name;
+       const char *frag_name;
+       const char *rel_path;
+       const char *target_path;
+       char *buf;
+       void *p;
+
+       ov_sym = fdt_subnode_offset(fdto, 0, "__symbols__");
+
+       /* if no overlay symbols exist no problem */
+       if (ov_sym < 0)
+               return 0;
+
+       root_sym = fdt_subnode_offset(fdt, 0, "__symbols__");
+
+       /* it no root symbols exist we should create them */
+       if (root_sym == -FDT_ERR_NOTFOUND)
+               root_sym = fdt_add_subnode(fdt, 0, "__symbols__");
+
+       /* any error is fatal now */
+       if (root_sym < 0)
+               return root_sym;
+
+       /* iterate over each overlay symbol */
+       fdt_for_each_property_offset(prop, fdto, ov_sym) {
+               path = fdt_getprop_by_offset(fdto, prop, &name, &path_len);
+               if (!path)
+                       return path_len;
+
+               /* verify it's a string property (terminated by a single \0) */
+               if (path_len < 1 || memchr(path, '\0', path_len) != 
&path[path_len - 1])
+                       return -FDT_ERR_BADVALUE;
+
+               /* keep end marker to avoid strlen() */
+               e = path + path_len;
+
+               if (*path != '/')
+                       return -FDT_ERR_BADVALUE;
+
+               /* get fragment name first */
+               s = strchr(path + 1, '/');
+               if (!s) {
+                       /* Symbol refers to something that won't end
+                        * up in the target tree */
+                       continue;
+               }
+
+               frag_name = path + 1;
+               frag_name_len = s - path - 1;
+
+               /* verify format; safe since "s" lies in \0 terminated prop */
+               len = sizeof("/__overlay__/") - 1;
+               if ((e - s) > len && (memcmp(s, "/__overlay__/", len) == 0)) {
+                       /* /<fragment-name>/__overlay__/<relative-subnode-path> 
*/
+                       rel_path = s + len;
+                       rel_path_len = e - rel_path - 1;
+               } else if ((e - s) == len
+                          && (memcmp(s, "/__overlay__", len - 1) == 0)) {
+                       /* /<fragment-name>/__overlay__ */
+                       rel_path = "";
+                       rel_path_len = 0;
+               } else {
+                       /* Symbol refers to something that won't end
+                        * up in the target tree */
+                       continue;
+               }
+
+               /* find the fragment index in which the symbol lies */
+               ret = fdt_subnode_offset_namelen(fdto, 0, frag_name,
+                                              frag_name_len);
+               /* not found? */
+               if (ret < 0)
+                       return -FDT_ERR_BADOVERLAY;
+               fragment = ret;
+
+               /* an __overlay__ subnode must exist */
+               ret = fdt_subnode_offset(fdto, fragment, "__overlay__");
+               if (ret < 0)
+                       return -FDT_ERR_BADOVERLAY;
+
+               /* get the target of the fragment */
+               ret = overlay_get_target(fdt, fdto, fragment, &target_path);
+               if (ret < 0)
+                       return ret;
+               target = ret;
+
+               /* if we have a target path use */
+               if (!target_path) {
+                       ret = get_path_len(fdt, target);
+                       if (ret < 0)
+                               return ret;
+                       len = ret;
+               } else {
+                       len = strlen(target_path);
+               }
+
+               ret = fdt_setprop_placeholder(fdt, root_sym, name,
+                               len + (len > 1) + rel_path_len + 1, &p);
+               if (ret < 0)
+                       return ret;
+
+               if (!target_path) {
+                       /* again in case setprop_placeholder changed it */
+                       ret = overlay_get_target(fdt, fdto, fragment, 
&target_path);
+                       if (ret < 0)
+                               return ret;
+                       target = ret;
+               }
+
+               buf = p;
+               if (len > 1) { /* target is not root */
+                       if (!target_path) {
+                               ret = fdt_get_path(fdt, target, buf, len + 1);
+                               if (ret < 0)
+                                       return ret;
+                       } else
+                               memcpy(buf, target_path, len + 1);
+
+               } else
+                       len--;
+
+               buf[len] = '/';
+               memcpy(buf + len + 1, rel_path, rel_path_len);
+               buf[len + 1 + rel_path_len] = '\0';
+       }
+
+       return 0;
+}
+
+int fdt_overlay_apply(void *fdt, void *fdto)
+{
+       uint32_t delta;
+       int ret;
+
+       FDT_RO_PROBE(fdt);
+       FDT_RO_PROBE(fdto);
+
+       ret = fdt_find_max_phandle(fdt, &delta);
+       if (ret)
+               goto err;
+
+       ret = overlay_adjust_local_phandles(fdto, delta);
+       if (ret)
+               goto err;
+
+       ret = overlay_update_local_references(fdto, delta);
+       if (ret)
+               goto err;
+
+       ret = overlay_fixup_phandles(fdt, fdto);
+       if (ret)
+               goto err;
+
+       ret = overlay_merge(fdt, fdto);
+       if (ret)
+               goto err;
+
+       ret = overlay_symbol_update(fdt, fdto);
+       if (ret)
+               goto err;
+
+       /*
+        * The overlay has been damaged, erase its magic.
+        */
+       fdt_set_magic(fdto, ~0);
+
+       return 0;
+
+err:
+       /*
+        * The overlay might have been damaged, erase its magic.
+        */
+       fdt_set_magic(fdto, ~0);
+
+       /*
+        * The base device tree might have been damaged, erase its
+        * magic.
+        */
+       fdt_set_magic(fdt, ~0);
+
+       return ret;
+}
-- 
2.7.4