Add FDT overlays support to ubldr

FDT overlays is de-facto standard for describing expansion boards like
Beaglebone capes or Raspberry Pi shields. The ides is to have basic
DTB for base board and overlays DTB for shields/capes and to construct
final DTB either using human-readable configuration or some
self-discovery mechanism. I believe this approach can also be expanded
to support dynamically loadable FPGA bitstreams on systems like
Zedboard/Zybo.

Overlaying process is simmilar to executable link process for
binaries: each DTB has "exported" symbols and "undefined" symbols, the
latter are resolved using information for the former obtained from
base DTB or one of the overlays applied earlier (more rare case).

This symbols information is not generated by standard dtc that FreeBSD
has in base system, patched[1] version required to produces
overlay-compatible blobs. So although DTB files generated by
buildkernel do not support overlays there are enough
vendor/community-provided DTB blobs ciruclating around to justify
committing this change to ubldr.

This commit introduces handler for "fdt_overlays" variable that can be
defined either as a loader env variable or U-Boot env variable.
fdt_overlays is comma-separated list of .dtbo files located in
/boot/dtb/ directory along with base .dtb. ubldr loads files and
applies them one-by-one to base .dtb and then passes result blob to
the kernel.

[1] https://github.com/RobertCNelson/dtc/commit/dd6a0533e846e8d5e690a618fa35cc15a6103efb

Differential Revision:	https://reviews.freebsd.org/D3180

6 files changed, 616 insertions, 3 deletions

diff --git a/sys/boot/fdt/Makefile b/sys/boot/fdt/Makefile
index 22d8da5..ce11dbb 100644
--- a/sys/boot/fdt/Makefile
+++ b/sys/boot/fdt/Makefile
@@ -10,7 +10,7 @@ SRCS+=		fdt.c fdt_ro.c fdt_wip.c fdt_sw.c fdt_rw.c fdt_strerror.c \
 		fdt_empty_tree.c
 
 # Loader's fdt commands extension sources.
-SRCS+=		fdt_loader_cmd.c
+SRCS+=		fdt_loader_cmd.c fdt_overlay.c
 
 CFLAGS+=	-I${.CURDIR}/../../contrib/libfdt/ -I${.CURDIR}/../common/
 
diff --git a/sys/boot/fdt/fdt_loader_cmd.c b/sys/boot/fdt/fdt_loader_cmd.c
index 8cd8ab5..6983a59 100644
--- a/sys/boot/fdt/fdt_loader_cmd.c
+++ b/sys/boot/fdt/fdt_loader_cmd.c
@@ -39,6 +39,7 @@ __FBSDID("$FreeBSD$");
 
 #include "bootstrap.h"
 #include "fdt_platform.h"
+#include "fdt_overlay.h"
 
 #ifdef DEBUG
 #define debugf(fmt, args...) do { printf("%s(): ", __func__);	\
@@ -276,6 +277,128 @@ fdt_load_dtb_file(const char * filename)
 	return (0);
 }
 
+static int
+fdt_load_dtb_overlay(const char * filename)
+{
+	struct preloaded_file *bfp, *oldbfp;
+	struct fdt_header header;
+	int err;
+
+	debugf("fdt_load_dtb_overlay(%s)\n", filename);
+
+	oldbfp = file_findfile(filename, "dtbo");
+
+	/* Attempt to load and validate a new dtb from a file. */
+	if ((bfp = file_loadraw(filename, "dtbo", 1)) == NULL) {
+		printf("failed to load file '%s'\n", filename);
+		return (1);
+	}
+
+	COPYOUT(bfp->f_addr, &header, sizeof(header));
+	err = fdt_check_header(&header);
+
+	if (err < 0) {
+		file_discard(bfp);
+		if (err == -FDT_ERR_BADVERSION)
+			printf("incompatible blob version: %d, should be: %d\n",
+			    fdt_version(fdtp), FDT_LAST_SUPPORTED_VERSION);
+
+		else
+			printf("error validating blob: %s\n",
+			    fdt_strerror(err));
+		return (1);
+	}
+
+	/* A new dtb was validated, discard any previous file. */
+	if (oldbfp)
+		file_discard(oldbfp);
+
+	return (0);
+}
+
+int
+fdt_load_dtb_overlays(const char * filenames)
+{
+	char *names;
+	char *name;
+	char *comaptr;
+
+	debugf("fdt_load_dtb_overlay(%s)\n", filenames);
+
+	names = strdup(filenames);
+	if (names == NULL)
+		return (1);
+	name = names;
+	do {
+		comaptr = strchr(name, ',');
+		if (comaptr)
+			*comaptr = '\0';
+		fdt_load_dtb_overlay(name);
+		name = comaptr + 1;
+	} while(comaptr);
+
+	free(names);
+	return (0);
+}
+
+void
+fdt_apply_overlays()
+{
+	struct preloaded_file *fp;
+	size_t overlays_size, max_overlay_size, new_fdtp_size;
+	void *new_fdtp;
+	void *overlay;
+	int rv;
+
+	if ((fdtp == NULL) || (fdtp_size == 0))
+		return;
+
+	overlays_size = 0;
+	max_overlay_size = 0;
+	for (fp = file_findfile(NULL, "dtbo"); fp != NULL; fp = fp->f_next) {
+		if (max_overlay_size < fp->f_size)
+			max_overlay_size = fp->f_size;
+		overlays_size += fp->f_size;
+	}
+
+	/* Nothing to apply */
+	if (overlays_size == 0)
+		return;
+
+	/* It's actually more than enough */
+	new_fdtp_size = fdtp_size + overlays_size;
+	new_fdtp = malloc(new_fdtp_size);
+	if (new_fdtp == NULL) {
+		printf("failed to allocate memory for DTB blob with overlays\n");
+		return;
+	}
+
+	overlay = malloc(max_overlay_size);
+	if (overlay == NULL) {
+		printf("failed to allocate memory for DTB blob with overlays\n");
+		free(new_fdtp);
+		return;
+	}
+
+	rv = fdt_open_into(fdtp, new_fdtp, new_fdtp_size);
+	if (rv != 0) {
+		printf("failed to open DTB blob for applying overlays\n");
+		return;
+	}
+
+	for (fp = file_findfile(NULL, "dtbo"); fp != NULL; fp = fp->f_next) {
+		printf("applying DTB overlay '%s'\n", fp->f_name);
+		COPYOUT(fp->f_addr, overlay, fp->f_size);
+		fdt_overlay_apply(new_fdtp, overlay, fp->f_size);
+	}
+
+	free(fdtp);
+	fdtp = new_fdtp;
+	fdtp_size = new_fdtp_size;
+
+	free(overlay);
+}
+
 int
 fdt_setup_fdtp()
 {
diff --git a/sys/boot/fdt/fdt_overlay.c b/sys/boot/fdt/fdt_overlay.c
new file mode 100644
index 0000000..a19f06e
--- /dev/null
+++ b/sys/boot/fdt/fdt_overlay.c
@@ -0,0 +1,438 @@
+/*-
+ * Copyright (c) 2015 Oleksandr Tymoshenko <gonzo@FreeBSD.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ */
+
+#include <sys/cdefs.h>
+__FBSDID("$FreeBSD$");
+
+#include <stand.h>
+#include <libfdt.h>
+
+#include "fdt_overlay.h"
+
+/*
+ * Get max phandle
+ */
+static uint32_t
+fdt_max_phandle(void *fdtp)
+{
+	int o, depth;
+	uint32_t max_phandle, phandle;
+
+	depth = 1;
+	o = fdt_path_offset(fdtp, "/");
+	max_phandle = fdt_get_phandle(fdtp, o);
+	for (depth = 0; (o >= 0) && (depth >= 0); o = fdt_next_node(fdtp, o, &depth)) {
+		phandle = fdt_get_phandle(fdtp, o);
+		if (max_phandle < phandle)
+			max_phandle = phandle;
+	}
+
+	return max_phandle;
+}
+
+/*
+ * Returns exact memory location specified by fixup in format
+ * /path/to/node:property:offset
+ */
+static void *
+fdt_get_fixup_location(void *fdtp, const char *fixup)
+{
+	char *path, *prop, *offsetp, *endp;
+	int prop_offset, o, proplen;
+	void  *result;
+
+	result = 0;
+
+	path = strdup(fixup);
+	prop = strchr(path, ':');
+	if (prop == NULL) {
+		printf("missing property part in \"%s\"\n", fixup);
+		result = NULL;
+		goto out;
+	}
+
+	*prop = 0;
+	prop++;
+
+	offsetp = strchr(prop, ':');
+	if (offsetp == NULL) {
+		printf("missing offset part in \"%s\"\n", fixup);
+		result = NULL;
+		goto out;
+	}
+
+	*offsetp = 0;
+	offsetp++;
+
+	prop_offset = strtoul(offsetp, &endp, 10);
+	if (*endp != '\0') {
+		printf("\"%s\" is not valid number\n", offsetp);
+		result = NULL;
+		goto out;
+	}
+
+	o = fdt_path_offset(fdtp, path);
+	if (o < 0) {
+		printf("path \"%s\" not found\n", path);
+		result = NULL;
+		goto out;
+	}
+
+	result = fdt_getprop_w(fdtp, o, prop, &proplen);
+	if (result == NULL){
+		printf("property \"%s\" not found in  \"%s\" node\n", prop, path);
+		result = NULL;
+		goto out;
+	}
+
+	if (proplen < prop_offset + sizeof(uint32_t)) {
+		printf("%s: property length is too small for fixup\n", fixup);
+		result = NULL;
+		goto out;
+	}
+
+	result = (char*)result + prop_offset;
+
+out:
+	free(path);
+	return (result);
+}
+
+/*
+ * Process one entry in __fixups__ { } node
+ * @fixups is property value, array of NUL-terminated strings
+ *   with fixup locations
+ * @fixups_len length of the fixups array in bytes
+ * @phandle is value for these locations
+ */
+static int
+fdt_do_one_fixup(void *fdtp, const char *fixups, int fixups_len, int phandle)
+{
+	void *fixup_pos;
+	uint32_t val;
+
+	val = cpu_to_fdt32(phandle);
+
+	while (fixups_len > 0) {
+		fixup_pos = fdt_get_fixup_location(fdtp, fixups);
+		if (fixup_pos != NULL)
+			memcpy(fixup_pos, &val, sizeof(val));
+
+		fixups_len -= strlen(fixups) + 1;
+		fixups += strlen(fixups) + 1;
+	}
+
+	return (0);
+}
+
+/*
+ * Increase u32 value at pos by offset
+ */
+static void
+fdt_increase_u32(void *pos, uint32_t offset)
+{
+	uint32_t val;
+
+	memcpy(&val, pos,  sizeof(val));
+	val = cpu_to_fdt32(fdt32_to_cpu(val) + offset);
+	memcpy(pos, &val, sizeof(val));
+}
+
+/*
+ * Process local fixups
+ * @fixups is property value, array of NUL-terminated strings
+ *   with fixup locations
+ * @fixups_len length of the fixups array in bytes
+ * @offset value these locations should be increased by
+ */
+static int
+fdt_do_local_fixup(void *fdtp, const char *fixups, int fixups_len, int offset)
+{
+	void *fixup_pos;
+
+	while (fixups_len > 0) {
+		fixup_pos = fdt_get_fixup_location(fdtp, fixups);
+		if (fixup_pos != NULL)
+			fdt_increase_u32(fixup_pos, offset);
+
+		fixups_len -= strlen(fixups) + 1;
+		fixups += strlen(fixups) + 1;
+	}
+
+	return (0);
+}
+
+/*
+ * Increase node phandle by phandle_offset
+ */
+static void
+fdt_increase_phandle(void *fdtp, int node_offset, uint32_t phandle_offset)
+{
+	int proplen;
+	void *phandle_pos, *node_pos;
+
+	node_pos = (char*)fdtp + node_offset;
+
+	phandle_pos = fdt_getprop_w(fdtp, node_offset, "phandle", &proplen);
+	if (phandle_pos)
+		fdt_increase_u32(phandle_pos, phandle_offset);
+	phandle_pos = fdt_getprop_w(fdtp, node_offset, "linux,phandle", &proplen);
+	if (phandle_pos)
+		fdt_increase_u32(phandle_pos, phandle_offset);
+}
+
+/*
+ * Increase all phandles by offset
+ */
+static void
+fdt_increase_phandles(void *fdtp, uint32_t offset)
+{
+	int o, depth;
+
+	o = fdt_path_offset(fdtp, "/");
+	for (depth = 0; (o >= 0) && (depth >= 0); o = fdt_next_node(fdtp, o, &depth)) {
+		fdt_increase_phandle(fdtp, o, offset);
+	}
+}
+
+/*
+ * Overlay one node defined by <overlay_fdtp, overlay_o> over <main_fdtp, target_o>
+ */
+static void
+fdt_overlay_node(void *main_fdtp, int target_o, void *overlay_fdtp, int overlay_o)
+{
+	int len, o, depth;
+	const char *name;
+	const void *val;
+	int target_subnode_o;
+
+	/* Overlay properties */
+	for (o = fdt_first_property_offset(overlay_fdtp, overlay_o);
+	    o >= 0; o = fdt_next_property_offset(overlay_fdtp, o)) {
+		val = fdt_getprop_by_offset(overlay_fdtp, o, &name, &len);
+		if (val)
+			fdt_setprop(main_fdtp, target_o, name, val, len);
+	}
+
+	/* Now overlay nodes */
+	o = overlay_o;
+        for (depth = 0; (o >= 0) && (depth >= 0);
+	    o = fdt_next_node(overlay_fdtp, o, &depth)) {
+		if (depth != 1)
+			continue;
+		/* Check if there is node with the same name */
+		name = fdt_get_name(overlay_fdtp, o, NULL);
+		target_subnode_o = fdt_subnode_offset(main_fdtp, target_o, name);
+		if (target_subnode_o < 0) {
+			/* create new subnode and run merge recursively */
+			target_subnode_o = fdt_add_subnode(main_fdtp, target_o, name);
+			if (target_subnode_o < 0) {
+				printf("failed to create subnode \"%s\": %d\n",
+				    name, target_subnode_o);
+				return;
+			}
+		}
+
+		fdt_overlay_node(main_fdtp, target_subnode_o,
+		    overlay_fdtp, o);
+	}
+}
+
+/*
+ * Apply one overlay fragment
+ */
+static void
+fdt_apply_fragment(void *main_fdtp, void *overlay_fdtp, int fragment_o)
+{
+	uint32_t target;
+	const char *target_path;
+	const void *val;
+	int target_node_o, overlay_node_o;
+
+	target_node_o = -1;
+	val = fdt_getprop(overlay_fdtp, fragment_o, "target", NULL);
+	if (val) {
+		memcpy(&target, val, sizeof(target));
+		target = fdt32_to_cpu(target);
+		target_node_o = fdt_node_offset_by_phandle(main_fdtp, target);
+		if (target_node_o < 0) {
+			printf("failed to find target %04x\n", target);
+			return;
+		}
+	}
+
+	if (target_node_o < 0) {
+		target_path = fdt_getprop(overlay_fdtp, fragment_o, "target-path", NULL);
+		if (target_path == NULL)
+			return;
+
+		target_node_o = fdt_path_offset(main_fdtp, target_path);
+		if (target_node_o < 0) {
+			printf("failed to find target-path %s\n", target_path);
+			return;
+		}
+	}
+
+	if (target_node_o < 0)
+		return;
+
+	overlay_node_o = fdt_subnode_offset(overlay_fdtp, fragment_o, "__overlay__");
+	if (overlay_node_o < 0) {
+		printf("missing __overlay__ sub-node\n");
+		return;
+	}
+
+	fdt_overlay_node(main_fdtp, target_node_o, overlay_fdtp, overlay_node_o);
+}
+
+/*
+ * Handle __fixups__ node in overlay DTB
+ */
+static int
+fdt_overlay_do_fixups(void *main_fdtp, void *overlay_fdtp)
+{
+	int main_symbols_o, symbol_o, overlay_fixups_o;
+	int fixup_prop_o;
+	int len;
+	const char *fixups, *name;
+	const char *symbol_path;
+	uint32_t phandle;
+
+	main_symbols_o = fdt_path_offset(main_fdtp, "/__symbols__");
+	overlay_fixups_o = fdt_path_offset(overlay_fdtp, "/__fixups__");
+
+	if (main_symbols_o < 0)
+		return (-1);
+	if (overlay_fixups_o < 0)
+		return (-1);
+
+	for (fixup_prop_o = fdt_first_property_offset(overlay_fdtp, overlay_fixups_o);
+	    fixup_prop_o >= 0;
+	    fixup_prop_o = fdt_next_property_offset(overlay_fdtp, fixup_prop_o)) {
+		fixups = fdt_getprop_by_offset(overlay_fdtp, fixup_prop_o, &name, &len);
+		symbol_path = fdt_getprop(main_fdtp, main_symbols_o, name, NULL);
+		if (symbol_path == NULL) {
+			printf("couldn't find \"%s\" symbol in main dtb\n", name);
+			return (-1);
+		}
+		symbol_o = fdt_path_offset(main_fdtp, symbol_path);
+		if (symbol_o < 0) {
+			printf("couldn't find \"%s\" path in main dtb\n", symbol_path);
+			return (-1);
+		}
+		phandle = fdt_get_phandle(main_fdtp, symbol_o);
+		if (fdt_do_one_fixup(overlay_fdtp, fixups, len, phandle) < 0)
+			return (-1);
+	}
+
+	return (0);
+}
+
+/*
+ * Handle __local_fixups__ node in overlay DTB
+ */
+static int
+fdt_overlay_do_local_fixups(void *main_fdtp, void *overlay_fdtp)
+{
+	int overlay_local_fixups_o;
+	int len;
+	const char *fixups;
+	uint32_t phandle_offset;
+
+	overlay_local_fixups_o = fdt_path_offset(overlay_fdtp, "/__local_fixups__");
+
+	if (overlay_local_fixups_o < 0)
+		return (-1);
+
+	phandle_offset = fdt_max_phandle(main_fdtp);
+	fdt_increase_phandles(overlay_fdtp, phandle_offset);
+	fixups = fdt_getprop_w(overlay_fdtp, overlay_local_fixups_o, "fixup", &len);
+	if (fixups) {
+		if (fdt_do_local_fixup(overlay_fdtp, fixups, len, phandle_offset) < 0)
+			return (-1);
+	}
+
+	return (0);
+}
+
+/*
+ * Apply all fragments to main DTB
+ */
+static int
+fdt_overlay_apply_fragments(void *main_fdtp, void *overlay_fdtp)
+{
+	int o, depth;
+
+	o = fdt_path_offset(overlay_fdtp, "/");
+	for (depth = 0; (o >= 0) && (depth >= 0); o = fdt_next_node(overlay_fdtp, o, &depth)) {
+		if (depth != 1)
+			continue;
+
+		fdt_apply_fragment(main_fdtp, overlay_fdtp, o);
+	}
+
+	return (0);
+}
+
+int
+fdt_overlay_apply(void *main_fdtp, void *overlay_fdtp, size_t overlay_length)
+{
+	void *overlay_copy;
+	int rv;
+
+	rv = 0;
+
+	/* We modify overlay in-place, so we need writable copy */
+	overlay_copy = malloc(overlay_length);
+	if (overlay_copy == NULL) {
+		printf("failed to allocate memory for overlay copy\n");
+		return (-1);
+	}
+
+	memcpy(overlay_copy, overlay_fdtp, overlay_length);
+
+	if (fdt_overlay_do_fixups(main_fdtp, overlay_copy) < 0) {
+		printf("failed to perform fixups in overlay\n");
+		rv = -1;
+		goto out;
+	}
+
+	if (fdt_overlay_do_local_fixups(main_fdtp, overlay_copy) < 0) {
+		printf("failed to perform local fixups in overlay\n");
+		rv = -1;
+		goto out;
+	}
+
+	if (fdt_overlay_apply_fragments(main_fdtp, overlay_copy) < 0) {
+		printf("failed to apply fragments\n");
+		rv = -1;
+	}
+
+out:
+	free(overlay_copy);
+
+	return (rv);
+}
diff --git a/sys/boot/fdt/fdt_overlay.h b/sys/boot/fdt/fdt_overlay.h
new file mode 100644
index 0000000..0f91343
--- /dev/null
+++ b/sys/boot/fdt/fdt_overlay.h
@@ -0,0 +1,34 @@
+/*-
+ * Copyright (c) 2015 Oleksandr Tymoshenko <gonzo@FreeBSD.org>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+ * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+ * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+ * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+ * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+ * SUCH DAMAGE.
+ *
+ * $FreeBSD$
+ */
+
+#ifndef FDT_OVERLAY_H
+#define FDT_OVERLAY_H
+
+int fdt_overlay_apply(void *main_fdtp, void *overlay_fdtp, size_t overlay_length);
+
+#endif /* FDT_OVERLAY_H */
diff --git a/sys/boot/fdt/fdt_platform.h b/sys/boot/fdt/fdt_platform.h
index cad7602..eae940a 100644
--- a/sys/boot/fdt/fdt_platform.h
+++ b/sys/boot/fdt/fdt_platform.h
@@ -43,8 +43,10 @@ void fdt_fixup_cpubusfreqs(unsigned long, unsigned long);
 void fdt_fixup_ethernet(const char *, char *, int);
 void fdt_fixup_memory(struct fdt_mem_region *, size_t);
 void fdt_fixup_stdout(const char *);
+void fdt_apply_overlays(void);
 int fdt_load_dtb_addr(struct fdt_header *);
 int fdt_load_dtb_file(const char *);
+int fdt_load_dtb_overlays(const char *);
 int fdt_setup_fdtp(void);
 
 /* The platform library needs to implement these functions */
diff --git a/sys/boot/uboot/fdt/uboot_fdt.c b/sys/boot/uboot/fdt/uboot_fdt.c
index 6b646f6..848d859 100644
--- a/sys/boot/uboot/fdt/uboot_fdt.c
+++ b/sys/boot/uboot/fdt/uboot_fdt.c
@@ -45,6 +45,7 @@ fdt_platform_load_dtb(void)
 	struct fdt_header *hdr;
 	const char *s;
 	char *p;
+	int rv;
 
 	/*
 	 * If the U-boot environment contains a variable giving the address of a
@@ -68,6 +69,8 @@ fdt_platform_load_dtb(void)
 		}
 	}
 
+	rv = 1;
+
 	/*
 	 * Try to get FDT filename first from loader env and then from u-boot env
 	 */
@@ -79,11 +82,21 @@ fdt_platform_load_dtb(void)
 	if (s != NULL && *s != '\0') {
 		if (fdt_load_dtb_file(s) == 0) {
 			printf("Loaded DTB from file '%s'.\n", s);
-			return (0);
+			rv = 0;
 		}
 	}
 
-	return (1);
+	if (rv == 0) {
+		s = getenv("fdt_overlays");
+		if (s == NULL)
+			s = ub_env_get("fdt_overlays");
+		if (s != NULL && *s != '\0') {
+			printf("Loading DTB overlays: '%s'\n", s);
+			fdt_load_dtb_overlays(s);
+		}
+	}
+
+	return (rv);
 }
 
 void
@@ -99,6 +112,9 @@ fdt_platform_fixups(void)
 	eth_no = 0;
 	ethstr = NULL;
 
+	/* Apply overlays before anything else */
+	fdt_apply_overlays();
+
 	/* Acquire sys_info */
 	si = ub_get_sys_info();