Re: [PATCH v5 7/8] libfdt: Add overlay application function

[David Gibson <david-xT8FGy+AXnRB3Ne2BGzF6laj5H9X9Tb+@public.gmane.org>]

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On Fri, Jul 29, 2016 at 11:55:50AM +0200, Maxime Ripard wrote:
> The device tree overlays are a good way to deal with user-modifyable
> boards or boards with some kind of an expansion mechanism where we can
> easily plug new board in (like the BBB, the Raspberry Pi or the CHIP).
> 
> Add a new function to merge overlays with a base device tree.
> 
> Signed-off-by: Maxime Ripard <maxime.ripard-wi1+55ScJUtKEb57/3fJTNBPR1lH4CV8@public.gmane.org>

Sorry it's taken me so long to review this properly.

As I mentioned before, the only remaining concern I have is reporting
errors as sensibly as we can.  So here's my thoughts on that.  This is
all negotiable if you have reasons to suggest something else, of
course - especially the names and wordings.

I think you should add these error codes:

FDT_ERR_BADOVERLAY
	This indicates an incorrectly formatted overlay.  That means
	something wrong in the node or property contents which means
	the overlay can't properly be interpreted.  An error in the
	encoding *of* the nodes or properties would (still) be
	BADSTRUCTURE.  This should always be an error that can be
	detected (at least theoretically) in the overlay *alone*
	without reference to the base tree.  i.e. it implies that this
	overlay is bad in a way which means it couldn't apply to *any*
	base tree.

FDT_ERR_NOPHANDLES
	Means we ran out / overran the valid phandles while adjusting
	them during overlay application.

And I suggest an extension to the meaning of one existing code:

FDT_ERR_BADPHANDLE
	Should be expanded to cover encountering a badly formatted
	phandle property (e.g. wrong length) as well as passing a bad
	phandle value (0 or -1).


> ---
>  libfdt/Makefile.libfdt |   2 +-
>  libfdt/fdt_overlay.c   | 641 +++++++++++++++++++++++++++++++++++++++++++++++++
>  libfdt/libfdt.h        |  31 +++
>  libfdt/libfdt_env.h    |   1 +
>  4 files changed, 674 insertions(+), 1 deletion(-)
>  create mode 100644 libfdt/fdt_overlay.c
> 
> diff --git a/libfdt/Makefile.libfdt b/libfdt/Makefile.libfdt
> index 09c322ed82ba..098b3f36e668 100644
> --- a/libfdt/Makefile.libfdt
> +++ b/libfdt/Makefile.libfdt
> @@ -7,5 +7,5 @@ LIBFDT_soname = libfdt.$(SHAREDLIB_EXT).1
>  LIBFDT_INCLUDES = fdt.h libfdt.h libfdt_env.h
>  LIBFDT_VERSION = version.lds
>  LIBFDT_SRCS = fdt.c fdt_ro.c fdt_wip.c fdt_sw.c fdt_rw.c fdt_strerror.c fdt_empty_tree.c \
> -	fdt_addresses.c
> +	fdt_addresses.c fdt_overlay.c
>  LIBFDT_OBJS = $(LIBFDT_SRCS:%.c=%.o)
> diff --git a/libfdt/fdt_overlay.c b/libfdt/fdt_overlay.c
> new file mode 100644
> index 000000000000..ab48488b4b72
> --- /dev/null
> +++ b/libfdt/fdt_overlay.c
> @@ -0,0 +1,641 @@
> +#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 uint32_t *val;
> +	int len;

It might be easier to handle errors if you fold this into its caller.

> +
> +	val = fdt_getprop(fdto, fragment, "target", &len);
> +	if (!val)
> +		return 0;

> +	if ((len != sizeof(*val)) || (*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
> + *
> + * overlay_get_target() retrieves the target offset in the base
> + * device tree of a fragment, no matter how the actual targetting is
> + * done (through a phandle or a path)
> + *
> + * returns:
> + *      the targetted 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)
> +{
> +	uint32_t phandle;
> +	const char *path;
> +
> +	/* Try first to do a phandle based lookup */
> +	phandle = overlay_get_target_phandle(fdto, fragment);
> +	if (phandle == (uint32_t)-1)
> +		return -FDT_ERR_BADPHANDLE;

BADPHANDLE is good here, but see the expansion to its meaning above.

> +
> +	if (phandle)
> +		return fdt_node_offset_by_phandle(fdt, phandle);
> +
> +	/* And then a path based lookup */
> +	path = fdt_getprop(fdto, fragment, "target-path", NULL);
> +	if (!path)
> +		return -FDT_ERR_NOTFOUND;
> +
> +	return fdt_path_offset(fdt, path);
> +}
> +
> +/**
> + * 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 uint32_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_BADSTRUCTURE;

Should be BADPHANDLE.

> +	adj_val = fdt32_to_cpu(*val);
> +	if ((adj_val + delta) < adj_val)
> +		return -FDT_ERR_BADPHANDLE;

Should be NOPHANDLES.

> +	adj_val += delta;
> +	if (adj_val == (uint32_t)-1)
> +		return -FDT_ERR_BADPHANDLE;

Should be 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;
> +	}

I'm wondering if it might be an idea to add a core set_phandle()
function, which paired with get_phandle() would check that 'phandle'
and 'linux,phandle' are in sync.  But that's not within scope for this
patch.


> +	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 uint32_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_BADSTRUCTURE;


Should be BADOVERLAY.


> +
> +		tree_val = fdt_getprop(fdto, tree_node, name, &tree_len);
> +		if (!tree_val)
> +			return tree_len;

A NOTFOUND returned from fdt_getprop() should become BADOVERLAY (the
overlay has a local fixup that doesn't resolve).

> +		for (i = 0; i < (fixup_len / sizeof(uint32_t)); i++) {
> +			uint32_t adj_val, index;
> +
> +			index = 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 + index, sizeof(adj_val));
> +
> +			adj_val = fdt32_to_cpu(adj_val);
> +			adj_val += delta;

An overflow here (adjval + delta < adjval, or adjval + delta == -1)
should trigger BADOVERLAY - it means we have a local fixup applying to
something that isn't a valid phandle (or we would have hit an error
while adjusting the phandle properties themselves).

> +			adj_val = cpu_to_fdt32(adj_val);
> +
> +			ret = fdt_setprop_inplace_namelen_partial(fdto,
> +								  tree_node,
> +								  name,
> +								  strlen(name),
> +								  index,
> +								  &adj_val,
> +								  sizeof(adj_val));
> +			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 < 0)
> +			return tree_child;

Again a NOTFOUND should become BADOVERLAY, because the overlay is
attempting to fixup something that doesn't exist.

> +
> +		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
> + * @index: Index in 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 index, const char *label)
> +{
> +	const char *symbol_path;
> +	uint32_t phandle;
> +	int symbol_off, fixup_off;
> +	int prop_len;
> +
> +	symbol_path = fdt_getprop(fdt, symbols_off, label,
> +				  &prop_len);
> +	if (!symbol_path)
> +		return prop_len;

So here, applying an overlay to a base tree it doesn't belong with
will return NOTFOUND.  That seems reasonable to me.

> +	symbol_off = fdt_path_offset(fdt, symbol_path);
> +	if (symbol_off < 0)
> +		return symbol_off;

And here.

> +	phandle = fdt_get_phandle(fdt, symbol_off);
> +	if (!phandle)
> +		return -FDT_ERR_NOTFOUND;

Should return BADPHANDLE if phandle == -1 here.

For phandle == 0, NOTFOUND is probably ok for now.  I wonder if we
should actually attempt to allocate and add a phandle to the base tree
in this case.

> +
> +	fixup_off = fdt_path_offset_namelen(fdto, path, path_len);
> +	if (fixup_off < 0)
> +		return fixup_off;

Here, a NOTFOUND should become BADOVERLAY.

> +
> +	phandle = cpu_to_fdt32(phandle);
> +	return fdt_setprop_inplace_namelen_partial(fdto, fixup_off,
> +						   name, name_len, index,
> +						   &phandle, sizeof(phandle));

And here both NOTFOUND and NOSPACE should become BADOVERLAY.

> +};
> +
> +/**
> + * 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 index, ret;
> +
> +		fixup_end = memchr(value, '\0', len);
> +		if (!fixup_end)
> +			return -FDT_ERR_BADFIXUP;

BADOVERLAY

> +		fixup_len = fixup_end - fixup_str;
> +
> +		path = fixup_str;
> +		sep = memchr(fixup_str, ':', fixup_len);
> +		if (!sep || *sep != ':')
> +			return -FDT_ERR_BADFIXUP;

BADOVERLAY

> +		path_len = sep - path;
> +		if (path_len == (fixup_len - 1))
> +			return -FDT_ERR_BADFIXUP;

BADOVERLAY

> +		fixup_len -= path_len + 1;
> +		name = sep + 1;
> +		sep = memchr(name, ':', fixup_len);
> +		if (!sep || *sep != ':')
> +			return -FDT_ERR_BADFIXUP;

BADOVERLAY

> +		name_len = sep - name;
> +		if (!name_len)
> +			return -FDT_ERR_BADFIXUP;

BADOVERLAY

> +		index = strtoul(sep + 1, &endptr, 10);
> +		if ((*endptr != '\0') || (endptr <= (sep + 1)))
> +			return -FDT_ERR_BADFIXUP;

BADOVERLAY

> +		len -= fixup_len + 1;
> +		value += fixup_len + 1;
> +
> +		ret = overlay_fixup_one_phandle(fdt, fdto, symbols_off,
> +						path, path_len, name, name_len,
> +						index, label);
> +		if (ret)
> +			return ret;
> +	} while (len > 0);

I can vaguely imagine it might be useful for some overlay generating
tools to be able to insert empty fixup properties so they don't they
don't need a special case for (# places to fix) == 0.  Since
supporting that (as a no-op) is just changing this do while to a
while, we might as well throw it in.

> +	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;
> +
> +	symbols_off = fdt_path_offset(fdt, "/__symbols__");
> +	fixups_off = fdt_path_offset(fdto, "/__fixups__");

Need an error check for both symbols_off and fixups_off here.

> +	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 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_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);
> +		if (target < 0)
> +			return target;
> +
> +		ret = overlay_apply_node(fdt, target, fdto, overlay);
> +		if (ret)
> +			return ret;
> +	}
> +
> +	return 0;
> +}
> +
> +int fdt_overlay_apply(void *fdt, void *fdto)
> +{
> +	uint32_t delta = fdt_get_max_phandle(fdt);
> +	int ret;
> +
> +	FDT_CHECK_HEADER(fdt);
> +	FDT_CHECK_HEADER(fdto);
> +
> +	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;
> +
> +	/*
> +	 * 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;
> +}
> diff --git a/libfdt/libfdt.h b/libfdt/libfdt.h
> index f49b45c29006..9892d06c06bc 100644
> --- a/libfdt/libfdt.h
> +++ b/libfdt/libfdt.h
> @@ -1756,6 +1756,37 @@ int fdt_add_subnode(void *fdt, int parentoffset, const char *name);
>   */
>  int fdt_del_node(void *fdt, int nodeoffset);
>  
> +/**
> + * fdt_overlay_apply - Applies a DT overlay on a base DT
> + * @fdt: pointer to the base device tree blob
> + * @fdto: pointer to the device tree overlay blob
> + *
> + * fdt_overlay_apply() will apply the given device tree overlay on the
> + * given base device tree.
> + *
> + * Expect the base device tree to be modified, even if the function
> + * returns an error.
> + *
> + * returns:
> + *	0, on success
> + *	-FDT_ERR_NOSPACE, there's not enough space in the base device tree
> + *	-FDT_ERR_NOTFOUND, the overlay points to some inexistant nodes or
> + *		properties in the base DT
> + *	-FDT_ERR_BADPHANDLE, the phandles in the overlay do not have the right
> + *		magic
> + *	-FDT_ERR_INTERNAL,
> + *	-FDT_ERR_BADLAYOUT,
> + *	-FDT_ERR_BADMAGIC,
> + *	-FDT_ERR_BADOFFSET,
> + *	-FDT_ERR_BADPATH,
> + *	-FDT_ERR_BADVERSION,
> + *	-FDT_ERR_BADSTRUCTURE,
> + *	-FDT_ERR_BADSTATE,
> + *	-FDT_ERR_BADFIXUP,
> + *	-FDT_ERR_TRUNCATED, standard meanings
> + */
> +int fdt_overlay_apply(void *fdt, void *fdto);
> +
>  /**********************************************************************/
>  /* Debugging / informational functions                                */
>  /**********************************************************************/
> diff --git a/libfdt/libfdt_env.h b/libfdt/libfdt_env.h
> index 9dea97dfff81..99f936dacc60 100644
> --- a/libfdt/libfdt_env.h
> +++ b/libfdt/libfdt_env.h
> @@ -54,6 +54,7 @@
>  
>  #include <stddef.h>
>  #include <stdint.h>
> +#include <stdlib.h>
>  #include <string.h>
>  
>  #ifdef __CHECKER__

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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