Re: [PATCH v2 10/10] Introduce v18 dtb version

[Frank Li <Frank.li@nxp.com>]

On Thu, Apr 09, 2026 at 01:54:26PM +0200, Herve Codina wrote:
> This v18 version will add support for
>   - Structured tags.
>     Those tags value definition will allow "old" libfdt, dtc and other
>     tools to skip unknown tags if encountered in future dtb version.
>
>     libfdt, dtc and other tools implementing version v18 will be able to
>     skip unknown tags available in dtbd generated with later version of
>     dtc.
>
>   - dt_flags header field.
>     For now this flag field is set to 0. It is a placeholder for future
>     dtb version and could be used to store some dtb related information
>     such as the kind of dtb. For instance, the future addons format will
>     use this field to clearly identify that the dtb is an addon dtb.
>
>   - last_comp_version_w header field.
>     This field is similar to last_comp_version but for writing.
>     It contains the lowest version of the devicetree data structure with
>     which the version used can safely perform modifications (taking into
>     account following rules related to unknown tags).
>     If this lowest version is greater than the last known supported
>     version, modification are simply forbidden and lead to a
>     FDT_ERR_BADVERSION error.
>
> For modification, when an unknown tag that can be skipped is involved
> and last_comp_version_w allows modifications, the following rules
> apply:
>   - When a property is modified, tags related to this property are
>     removed and the dtb version is downgraded.
>
>   - When a property is removed, tags related to this property are
>     obviously removed. The dtb version is kept unchanged.
>
>   - When a property or a node is added, obviously no unknown tags are
>     added and the dtb version is kept unchanged.
>
>   - When a node is removed, tags related to this node are obviously
>     removed. The dtb version is kept unchanged.
>
>   - Adding, removing or modifying a property is not considered as a node
>     modification and so, those operations have no impacts on unknown
>     tags related to the node. Those node related tags are kept
>     unchanged.
>
>   - The only modification considered as a node modification is setting
>     its name. We consider that this operation has no impact on tags
>     related to the node. Here also, those node related tags and the dtb
>     version are kept unchanged.
>
>   - Global (dtb related) unknown tags are kept unchanged regardless the
>     modification done.
>
> In all cases, if unknown tags are not involved in a modification, the
> dtb version is not downgraded when the modification is made.
>
> It is worth noting that with this v18 version, the dtb version is not
> downgraded for any modification but only when unknown tags are removed
> due to a property modification. In v17 or older version any modification
> led to a dtb version downgrade.
>
> Signed-off-by: Herve Codina <herve.codina@bootlin.com>
> ---

Reviewed-by: Frank Li <Frank.Li@nxp.com>

>  dtc.h                                         |  2 +-
>  fdtdump.c                                     |  8 +++-
>  flattree.c                                    | 37 +++++++++++++++----
>  libfdt/fdt.h                                  |  5 +++
>  libfdt/fdt_rw.c                               | 20 +++++++---
>  libfdt/fdt_sw.c                               |  3 ++
>  libfdt/libfdt.h                               |  7 +++-
>  pylibfdt/libfdt.i                             | 18 +++++++++
>  tests/dumptrees.c                             |  3 +-
>  tests/pylibfdt_tests.py                       | 10 +++--
>  tests/run_tests.sh                            | 26 +++++++++----
>  tests/testdata.h                              |  1 +
>  tests/testutils.c                             |  2 +-
>  tests/trees.S                                 | 29 +++++++++++++--
>  ...own_tags_can_skip.fdtput.test.dtb.0.expect |  1 +
>  ...own_tags_can_skip.fdtput.test.dtb.1.expect |  3 +-
>  ...own_tags_can_skip.fdtput.test.dtb.2.expect |  3 +-
>  ...own_tags_can_skip.fdtput.test.dtb.3.expect |  3 +-
>  ...own_tags_can_skip.fdtput.test.dtb.4.expect |  3 +-
>  ...own_tags_can_skip.fdtput.test.dtb.5.expect |  3 +-
>  ...own_tags_can_skip.fdtput.test.dtb.6.expect |  3 +-
>  21 files changed, 152 insertions(+), 38 deletions(-)
>
> diff --git a/dtc.h b/dtc.h
> index 473552e..f0c2cde 100644
> --- a/dtc.h
> +++ b/dtc.h
> @@ -29,7 +29,7 @@
>  #define debug(...)
>  #endif
>
> -#define DEFAULT_FDT_VERSION	17
> +#define DEFAULT_FDT_VERSION	18
>
>  /*
>   * Command line options
> diff --git a/fdtdump.c b/fdtdump.c
> index 7a8b278..5994e1d 100644
> --- a/fdtdump.c
> +++ b/fdtdump.c
> @@ -18,7 +18,7 @@
>  #include "util.h"
>
>  #define FDT_MAGIC_SIZE	4
> -#define MAX_VERSION 17U
> +#define MAX_VERSION 18U
>
>  #define ALIGN(x, a)	(((x) + ((a) - 1)) & ~((a) - 1))
>  #define PALIGN(p, a)	((void *)(ALIGN((uintptr_t)(p), (a))))
> @@ -86,6 +86,12 @@ static void dump_blob(void *blob, bool debug, int dump_unknown)
>  	if (version >= 17)
>  		printf("// size_dt_struct:\t0x%"PRIx32"\n",
>  		       fdt32_to_cpu(bph->size_dt_struct));
> +	if (version >= 18) {
> +		printf("// dt_flags:\t\t0x%"PRIx32"\n",
> +		       fdt32_to_cpu(bph->dt_flags));
> +		printf("// last_comp_version_w:\t%"PRIu32"\n",
> +		       fdt32_to_cpu(bph->last_comp_version_w));
> +	}
>  	printf("\n");
>
>  	for (i = 0; ; i++) {
> diff --git a/flattree.c b/flattree.c
> index 88dbfa7..ff57fc5 100644
> --- a/flattree.c
> +++ b/flattree.c
> @@ -13,23 +13,29 @@
>  #define FTF_STRTABSIZE	0x10
>  #define FTF_STRUCTSIZE	0x20
>  #define FTF_NOPS	0x40
> +#define FTF_DTFLAGS	0x80
> +#define FTF_LCVERSW	0x100
>
>  static struct version_info {
>  	int version;
>  	int last_comp_version;
> +	int last_comp_version_w;
>  	int hdr_size;
>  	int flags;
>  } version_table[] = {
> -	{1, 1, FDT_V1_SIZE,
> +	{1, 1, 0, FDT_V1_SIZE,
>  	 FTF_FULLPATH|FTF_VARALIGN|FTF_NAMEPROPS},
> -	{2, 1, FDT_V2_SIZE,
> +	{2, 1, 0, FDT_V2_SIZE,
>  	 FTF_FULLPATH|FTF_VARALIGN|FTF_NAMEPROPS|FTF_BOOTCPUID},
> -	{3, 1, FDT_V3_SIZE,
> +	{3, 1, 0, FDT_V3_SIZE,
>  	 FTF_FULLPATH|FTF_VARALIGN|FTF_NAMEPROPS|FTF_BOOTCPUID|FTF_STRTABSIZE},
> -	{16, 16, FDT_V3_SIZE,
> +	{16, 16, 0, FDT_V3_SIZE,
>  	 FTF_BOOTCPUID|FTF_STRTABSIZE|FTF_NOPS},
> -	{17, 16, FDT_V17_SIZE,
> +	{17, 16, 0, FDT_V17_SIZE,
>  	 FTF_BOOTCPUID|FTF_STRTABSIZE|FTF_STRUCTSIZE|FTF_NOPS},
> +	{18, 16, 17, FDT_V18_SIZE,
> +	 FTF_BOOTCPUID|FTF_STRTABSIZE|FTF_STRUCTSIZE|FTF_NOPS|FTF_DTFLAGS|
> +	 FTF_LCVERSW},
>  };
>
>  struct emitter {
> @@ -314,7 +320,7 @@ static struct data flatten_reserve_list(struct reserve_info *reservelist,
>  static void make_fdt_header(struct fdt_header *fdt,
>  			    struct version_info *vi,
>  			    int reservesize, int dtsize, int strsize,
> -			    int boot_cpuid_phys)
> +			    int boot_cpuid_phys, uint32_t dt_flags)
>  {
>  	int reserve_off;
>
> @@ -341,6 +347,10 @@ static void make_fdt_header(struct fdt_header *fdt,
>  		fdt->size_dt_strings = cpu_to_fdt32(strsize);
>  	if (vi->flags & FTF_STRUCTSIZE)
>  		fdt->size_dt_struct = cpu_to_fdt32(dtsize);
> +	if (vi->flags & FTF_DTFLAGS)
> +		fdt->dt_flags = cpu_to_fdt32(dt_flags);
> +	if (vi->flags & FTF_LCVERSW)
> +		fdt->last_comp_version_w = cpu_to_fdt32(vi->last_comp_version_w);
>  }
>
>  void dt_to_blob(FILE *f, struct dt_info *dti, int version)
> @@ -368,7 +378,7 @@ void dt_to_blob(FILE *f, struct dt_info *dti, int version)
>
>  	/* Make header */
>  	make_fdt_header(&fdt, vi, reservebuf.len, dtbuf.len, strbuf.len,
> -			dti->boot_cpuid_phys);
> +			dti->boot_cpuid_phys, 0);
>
>  	/*
>  	 * If the user asked for more space than is used, adjust the totalsize.
> @@ -499,6 +509,16 @@ void dt_to_asm(FILE *f, struct dt_info *dti, int version)
>  			symprefix, symprefix);
>  	}
>
> +	if (vi->flags & FTF_DTFLAGS) {
> +		fprintf(f, "\t/* dt_flags */\n");
> +		asm_emit_cell(f, 0);
> +	}
> +
> +	if (vi->flags & FTF_LCVERSW) {
> +		fprintf(f, "\t/* last_comp_version_w */\n");
> +		asm_emit_cell(f, vi->last_comp_version_w);
> +	}
> +
>  	/*
>  	 * Reserve map entries.
>  	 * Align the reserve map to a doubleword boundary.
> @@ -955,6 +975,9 @@ struct dt_info *dt_from_blob(const char *fname)
>  		flags |= FTF_NOPS;
>  	}
>
> +	if (version >= 18)
> +		flags |= FTF_DTFLAGS | FTF_LCVERSW;
> +
>  	inbuf_init(&memresvbuf,
>  		   blob + off_mem_rsvmap, blob + totalsize);
>  	inbuf_init(&dtbuf, blob + off_dt, blob + totalsize);
> diff --git a/libfdt/fdt.h b/libfdt/fdt.h
> index e6f75e7..12d358d 100644
> --- a/libfdt/fdt.h
> +++ b/libfdt/fdt.h
> @@ -26,6 +26,10 @@ struct fdt_header {
>
>  	/* version 17 fields below */
>  	fdt32_t size_dt_struct;		 /* size of the structure block */
> +
> +	/* version 18 fields below */
> +	fdt32_t dt_flags;		/* Ored value of FDT_FLAG_XXXX */
> +	fdt32_t last_comp_version_w;	/* last compatible version for writing */
>  };
>
>  struct fdt_reserve_entry {
> @@ -85,5 +89,6 @@ struct fdt_property {
>  #define FDT_V3_SIZE	(FDT_V2_SIZE + sizeof(fdt32_t))
>  #define FDT_V16_SIZE	FDT_V3_SIZE
>  #define FDT_V17_SIZE	(FDT_V16_SIZE + sizeof(fdt32_t))
> +#define FDT_V18_SIZE	(FDT_V17_SIZE + 2 * sizeof(fdt32_t))
>
>  #endif /* FDT_H */
> diff --git a/libfdt/fdt_rw.c b/libfdt/fdt_rw.c
> index a8f53b4..0a4a03f 100644
> --- a/libfdt/fdt_rw.c
> +++ b/libfdt/fdt_rw.c
> @@ -34,13 +34,17 @@ static int fdt_rw_probe_(void *fdt)
>  		return 0;
>  	FDT_RO_PROBE(fdt);
>
> -	if (!can_assume(LATEST) && fdt_version(fdt) < 17)
> -		return -FDT_ERR_BADVERSION;
> +	if (!can_assume(LATEST)) {
> +		if (fdt_version(fdt) < 17)
> +			return -FDT_ERR_BADVERSION;
> +		else if (fdt_version(fdt) >= 18 &&
> +			 fdt_last_comp_version_w(fdt) > FDT_LAST_SUPPORTED_VERSION)
> +			return -FDT_ERR_BADVERSION;
> +	}
>  	if (fdt_blocks_misordered_(fdt, sizeof(struct fdt_reserve_entry),
>  				   fdt_size_dt_struct(fdt)))
>  		return -FDT_ERR_BADLAYOUT;
>
> -	fdt_downgrade_version(fdt);
>  	return 0;
>  }
>
> @@ -582,7 +586,11 @@ int fdt_open_into(const void *fdt, void *buf, int bufsize)
>  		err = fdt_move(fdt, buf, bufsize);
>  		if (err)
>  			return err;
> -		fdt_set_version(buf, 17);
> +		if (can_assume(LATEST) || fdt_version(fdt) < 18) {
> +			fdt_set_version(buf, 18);
> +			fdt_set_dt_flags(buf, 0);
> +			fdt_set_last_comp_version_w(buf, 17);
> +		}
>  		fdt_set_size_dt_struct(buf, struct_size);
>  		fdt_set_totalsize(buf, bufsize);
>  		return 0;
> @@ -611,8 +619,10 @@ int fdt_open_into(const void *fdt, void *buf, int bufsize)
>
>  	fdt_set_magic(buf, FDT_MAGIC);
>  	fdt_set_totalsize(buf, bufsize);
> -	fdt_set_version(buf, 17);
> +	fdt_set_version(buf, 18);
>  	fdt_set_last_comp_version(buf, 16);
> +	fdt_set_dt_flags(buf, 0);
> +	fdt_set_last_comp_version_w(buf, 17);
>  	fdt_set_boot_cpuid_phys(buf, fdt_boot_cpuid_phys(fdt));
>
>  	return 0;
> diff --git a/libfdt/fdt_sw.c b/libfdt/fdt_sw.c
> index 4c569ee..10da0d6 100644
> --- a/libfdt/fdt_sw.c
> +++ b/libfdt/fdt_sw.c
> @@ -137,6 +137,9 @@ int fdt_create_with_flags(void *buf, int bufsize, uint32_t flags)
>  	fdt_set_off_dt_struct(fdt, fdt_off_mem_rsvmap(fdt));
>  	fdt_set_off_dt_strings(fdt, 0);
>
> +	fdt_set_dt_flags(fdt, 0);
> +	fdt_set_last_comp_version_w(fdt, FDT_LAST_COMPATIBLE_VERSION_W);
> +
>  	return 0;
>  }
>
> diff --git a/libfdt/libfdt.h b/libfdt/libfdt.h
> index 7a10f66..71c7de5 100644
> --- a/libfdt/libfdt.h
> +++ b/libfdt/libfdt.h
> @@ -15,7 +15,8 @@ extern "C" {
>
>  #define FDT_FIRST_SUPPORTED_VERSION	0x02
>  #define FDT_LAST_COMPATIBLE_VERSION	0x10
> -#define FDT_LAST_SUPPORTED_VERSION	0x11
> +#define FDT_LAST_COMPATIBLE_VERSION_W	0x11
> +#define FDT_LAST_SUPPORTED_VERSION	0x12
>
>  /* Error codes: informative error codes */
>  #define FDT_ERR_NOTFOUND	1
> @@ -284,6 +285,8 @@ int fdt_next_subnode(const void *fdt, int offset);
>  #define fdt_boot_cpuid_phys(fdt)	(fdt_get_header(fdt, boot_cpuid_phys))
>  #define fdt_size_dt_strings(fdt)	(fdt_get_header(fdt, size_dt_strings))
>  #define fdt_size_dt_struct(fdt)		(fdt_get_header(fdt, size_dt_struct))
> +#define fdt_dt_flags(fdt)		(fdt_get_header(fdt, dt_flags))
> +#define fdt_last_comp_version_w(fdt)	(fdt_get_header(fdt, last_comp_version_w))
>
>  #define fdt_set_hdr_(name) \
>  	static inline void fdt_set_##name(void *fdt, uint32_t val) \
> @@ -301,6 +304,8 @@ fdt_set_hdr_(last_comp_version)
>  fdt_set_hdr_(boot_cpuid_phys)
>  fdt_set_hdr_(size_dt_strings)
>  fdt_set_hdr_(size_dt_struct)
> +fdt_set_hdr_(dt_flags)
> +fdt_set_hdr_(last_comp_version_w)
>  #undef fdt_set_hdr_
>
>  /**
> diff --git a/pylibfdt/libfdt.i b/pylibfdt/libfdt.i
> index 1f9c047..90966bd 100644
> --- a/pylibfdt/libfdt.i
> +++ b/pylibfdt/libfdt.i
> @@ -281,6 +281,22 @@ class FdtRo(object):
>          """
>          return fdt_size_dt_struct(self._fdt)
>
> +    def dt_flags(self):
> +        """Return flags from the header
> +
> +        Returns:
> +            flags value from the header
> +        """
> +        return fdt_dt_flags(self._fdt)
> +
> +    def last_comp_version_w(self):
> +        """Return the last compatible version for writing of the device tree
> +
> +        Returns:
> +            Last compatible version number for writing of the device tree
> +        """
> +        return fdt_last_comp_version_w(self._fdt)
> +
>      def num_mem_rsv(self, quiet=()):
>          """Return the number of memory reserve-map records
>
> @@ -1215,6 +1231,8 @@ uint32_t fdt_last_comp_version(const void *fdt);
>  uint32_t fdt_boot_cpuid_phys(const void *fdt);
>  uint32_t fdt_size_dt_strings(const void *fdt);
>  uint32_t fdt_size_dt_struct(const void *fdt);
> +uint32_t fdt_dt_flags(const void *fdt);
> +uint32_t fdt_last_comp_version_w(const void *fdt);
>
>  int fdt_property_string(void *fdt, const char *name, const char *val);
>  int fdt_property_cell(void *fdt, const char *name, uint32_t val);
> diff --git a/tests/dumptrees.c b/tests/dumptrees.c
> index 4732fff..c05d216 100644
> --- a/tests/dumptrees.c
> +++ b/tests/dumptrees.c
> @@ -27,7 +27,8 @@ static struct {
>  	TREE(two_roots),
>  	TREE(named_root),
>  	TREE(unknown_tags_can_skip),
> -	TREE(unknown_tags_no_skip)
> +	TREE(unknown_tags_no_skip),
> +	TREE(last_comp_version_w_future)
>  };
>
>  #define NUM_TREES	(sizeof(trees) / sizeof(trees[0]))
> diff --git a/tests/pylibfdt_tests.py b/tests/pylibfdt_tests.py
> index a4f73ed..72c98c5 100644
> --- a/tests/pylibfdt_tests.py
> +++ b/tests/pylibfdt_tests.py
> @@ -285,14 +285,16 @@ class PyLibfdtBasicTests(unittest.TestCase):
>          """Test that we can access the header values"""
>          self.assertEqual(self.fdt.magic(), 0xd00dfeed)
>          self.assertEqual(self.fdt.totalsize(), len(self.fdt._fdt))
> -        self.assertEqual(self.fdt.off_dt_struct(), 88)
> -        self.assertEqual(self.fdt.off_dt_strings(), 652)
> -        self.assertEqual(self.fdt.off_mem_rsvmap(), 40)
> -        self.assertEqual(self.fdt.version(), 17)
> +        self.assertEqual(self.fdt.off_dt_struct(), 96)
> +        self.assertEqual(self.fdt.off_dt_strings(), 660)
> +        self.assertEqual(self.fdt.off_mem_rsvmap(), 48)
> +        self.assertEqual(self.fdt.version(), 18)
>          self.assertEqual(self.fdt.last_comp_version(), 16)
>          self.assertEqual(self.fdt.boot_cpuid_phys(), 0)
>          self.assertEqual(self.fdt.size_dt_strings(), 105)
>          self.assertEqual(self.fdt.size_dt_struct(), 564)
> +        self.assertEqual(self.fdt.dt_flags(), 0)
> +        self.assertEqual(self.fdt.last_comp_version_w(), 17)
>
>      def testPack(self):
>          """Test that we can pack the tree after deleting something"""
> diff --git a/tests/run_tests.sh b/tests/run_tests.sh
> index 1ba937d..90ba558 100755
> --- a/tests/run_tests.sh
> +++ b/tests/run_tests.sh
> @@ -576,7 +576,8 @@ libfdt_tests () {
>      run_test dtbs_equal_ordered cell-overflow.test.dtb cell-overflow-results.test.dtb
>
>      # check full tests
> -    for good in test_tree1.dtb unknown_tags_can_skip.dtb; do
> +    for good in test_tree1.dtb unknown_tags_can_skip.dtb \
> +		last_comp_version_w_future.dtb; do
>  	run_test check_full $good
>      done
>      for bad in truncated_property.dtb truncated_string.dtb \
> @@ -1044,32 +1045,41 @@ fdtput_tests () {
>
>      # TODO: Add tests for verbose mode?
>
> +    # Not allowed to modify a dtb due to last_comp_version_w
> +    # FDT_ERR_BADVERSION error code is returned
> +    dtb=last_comp_version_w_future.fdtput.test.dtb
> +    cp last_comp_version_w_future.dtb $dtb
> +    run_wrap_error_test $DTPUT $dtb /subnode prop-int -tu 123
> +    run_wrap_error_test $DTPUT $dtb -d /subnode prop-int
> +    run_wrap_error_test $DTPUT $dtb -c /new-node
> +    run_wrap_error_test $DTPUT $dtb -r /subnode
> +
>      # Modify a dtb containing some "unknown" tags that can be skipped
>      dtb=unknown_tags_can_skip.fdtput.test.dtb
>      cp unknown_tags_can_skip.dtb $dtb
>      base_run_test wrap_fdtdump $dtb $dtb.0.out
>      # Remove unneeded header fields (keep those related to versions)
> -    sed -i '/^\/.*\(magic\|off\|size\|cpu\)/d' $dtb.0.out
> +    sed -i '/^\/.*\(magic\|off\|size\|cpu\|flags\)/d' $dtb.0.out
>      base_run_test check_diff $dtb.0.out "$SRCDIR/$dtb.0.expect"
>
>      run_fdtput_test "vwxyz" $dtb / prop-str -ts "vwxyz"
>      base_run_test wrap_fdtdump $dtb $dtb.1.out
>      # Remove unneeded header fields (keep those related to versions)
> -    sed -i '/^\/.*\(magic\|off\|size\|cpu\)/d' $dtb.1.out
> +    sed -i '/^\/.*\(magic\|off\|size\|cpu\|flags\)/d' $dtb.1.out
>      base_run_test check_diff $dtb.1.out "$SRCDIR/$dtb.1.expect"
>
>      cp unknown_tags_can_skip.dtb $dtb
>      run_wrap_test $DTPUT $dtb -c /tst-fdtput
>      base_run_test wrap_fdtdump $dtb $dtb.2.out
>      # Remove unneeded header fields (keep those related to versions)
> -    sed -i '/^\/.*\(magic\|off\|size\|cpu\)/d' $dtb.2.out
> +    sed -i '/^\/.*\(magic\|off\|size\|cpu\|flags\)/d' $dtb.2.out
>      base_run_test check_diff $dtb.2.out "$SRCDIR/$dtb.2.expect"
>      run_wrap_test $DTPUT $dtb -c /tst-fdtput/n1 /tst-fdtput/n2 /tst-fdtput/n3
>      run_wrap_test $DTPUT $dtb -r /tst-fdtput/n1 /tst-fdtput/n3
>      run_fdtget_test "n2" $dtb -l  /tst-fdtput
>      base_run_test wrap_fdtdump $dtb $dtb.3.out
>      # Remove unneeded header fields (keep those related to versions)
> -    sed -i '/^\/.*\(magic\|off\|size\|cpu\)/d' $dtb.3.out
> +    sed -i '/^\/.*\(magic\|off\|size\|cpu\|flags\)/d' $dtb.3.out
>      base_run_test check_diff $dtb.3.out "$SRCDIR/$dtb.3.expect"
>
>      cp unknown_tags_can_skip.dtb $dtb
> @@ -1077,21 +1087,21 @@ fdtput_tests () {
>      run_fdtget_test "prop-int" $dtb -p  /
>      base_run_test wrap_fdtdump $dtb $dtb.4.out
>      # Remove unneeded header fields (keep those related to versions)
> -    sed -i '/^\/.*\(magic\|off\|size\|cpu\)/d' $dtb.4.out
> +    sed -i '/^\/.*\(magic\|off\|size\|cpu\|flags\)/d' $dtb.4.out
>      base_run_test check_diff $dtb.4.out "$SRCDIR/$dtb.4.expect"
>
>      cp unknown_tags_can_skip.dtb $dtb
>      run_wrap_test $DTPUT $dtb /subnode2 prop-tst-fdtput -ts "Test fdtput"
>      base_run_test wrap_fdtdump $dtb $dtb.5.out
>      # Remove unneeded header fields (keep those related to versions)
> -    sed -i '/^\/.*\(magic\|off\|size\|cpu\)/d' $dtb.5.out
> +    sed -i '/^\/.*\(magic\|off\|size\|cpu\|flags\)/d' $dtb.5.out
>      base_run_test check_diff $dtb.5.out "$SRCDIR/$dtb.5.expect"
>
>      cp unknown_tags_can_skip.dtb $dtb
>      run_wrap_test $DTPUT $dtb -r /subnode2/subsubnode
>      base_run_test wrap_fdtdump $dtb $dtb.6.out
>      # Remove unneeded header fields (keep those related to versions)
> -    sed -i '/^\/.*\(magic\|off\|size\|cpu\)/d' $dtb.6.out
> +    sed -i '/^\/.*\(magic\|off\|size\|cpu\|flags\)/d' $dtb.6.out
>      base_run_test check_diff $dtb.6.out "$SRCDIR/$dtb.6.expect"
>  }
>
> diff --git a/tests/testdata.h b/tests/testdata.h
> index aef04ab..26c7c18 100644
> --- a/tests/testdata.h
> +++ b/tests/testdata.h
> @@ -59,4 +59,5 @@ extern struct fdt_header two_roots;
>  extern struct fdt_header named_root;
>  extern struct fdt_header unknown_tags_can_skip;
>  extern struct fdt_header unknown_tags_no_skip;
> +extern struct fdt_header last_comp_version_w_future;
>  #endif /* ! __ASSEMBLER__ */
> diff --git a/tests/testutils.c b/tests/testutils.c
> index 54da2e4..728d89c 100644
> --- a/tests/testutils.c
> +++ b/tests/testutils.c
> @@ -344,7 +344,7 @@ void *open_blob_rw(const void *blob)
>  {
>  	int err;
>  	void *buf;
> -	int newsize = fdt_totalsize(blob) + 8;
> +	int newsize = fdt_totalsize(blob) + 8 + 2 * 4;
>
>  	buf = xmalloc(newsize);
>  	err = fdt_open_into(blob, buf, newsize);
> diff --git a/tests/trees.S b/tests/trees.S
> index ef9a175..9215f3e 100644
> --- a/tests/trees.S
> +++ b/tests/trees.S
> @@ -8,7 +8,7 @@
>  	.byte	(\val) & 0xff
>  	.endm
>
> -	.macro	treehdr_vers	tree vers last_comp_vers
> +	.macro	treehdr_vers	tree vers last_comp_vers last_comp_vers_w
>  	.balign	8
>  	.globl	\tree
>  \tree :
> @@ -22,10 +22,12 @@
>  	fdtlong	0
>  	fdtlong	(\tree\()_strings_end - \tree\()_strings)
>  	fdtlong	(\tree\()_struct_end - \tree\()_struct)
> +	fdtlong	0
> +	fdtlong	\last_comp_vers_w
>  	.endm
>
>  	.macro	treehdr	tree
> -	treehdr_vers \tree 0x11 0x10
> +	treehdr_vers \tree 0x12 0x10 0x11
>  	.endm
>
>  	.macro	rsvmape	addrh, addrl, lenh, lenl
> @@ -334,7 +336,7 @@ named_root_end:
>  	 * Use a really future dtb version to check version downgrade on
>  	 * modification.
>  	 */
> -	treehdr_vers	unknown_tags_can_skip 0xffffffff 0x10
> +	treehdr_vers	unknown_tags_can_skip 0xffffffff 0x10 0x11
>  	empty_rsvmap	unknown_tags_can_skip
>
>  unknown_tags_can_skip_struct:
> @@ -444,3 +446,24 @@ unknown_tags_no_skip_strings:
>  unknown_tags_no_skip_strings_end:
>
>  unknown_tags_no_skip_end:
> +
> +
> +	/* Tree with last_comp_version_w set to avoid any modifications */
> +	treehdr_vers	last_comp_version_w_future 0xffffffff 0x10 0xffffffff
> +	empty_rsvmap	last_comp_version_w_future
> +
> +last_comp_version_w_future_struct:
> +	beginn	""
> +		propu64	last_comp_version_w_future, prop_int, 1, 2
> +		beginn	"subnode"
> +			propu64	last_comp_version_w_future, prop_int, 1, 2
> +		endn
> +	endn
> +	fdtlong	FDT_END
> +last_comp_version_w_future_struct_end:
> +
> +last_comp_version_w_future_strings:
> +	string	last_comp_version_w_future, prop_int, "prop-int"
> +last_comp_version_w_future_strings_end:
> +
> +last_comp_version_w_future_end:
> diff --git a/tests/unknown_tags_can_skip.fdtput.test.dtb.0.expect b/tests/unknown_tags_can_skip.fdtput.test.dtb.0.expect
> index 3cdf448..0d72e1f 100644
> --- a/tests/unknown_tags_can_skip.fdtput.test.dtb.0.expect
> +++ b/tests/unknown_tags_can_skip.fdtput.test.dtb.0.expect
> @@ -1,6 +1,7 @@
>  /dts-v1/;
>  // version:		4294967295
>  // last_comp_version:	16
> +// last_comp_version_w:	17
>
>  // Unknown tag ignored: 0xd0000000, data len 4 00000001
>  / {
> diff --git a/tests/unknown_tags_can_skip.fdtput.test.dtb.1.expect b/tests/unknown_tags_can_skip.fdtput.test.dtb.1.expect
> index 71f2d1c..d355d5a 100644
> --- a/tests/unknown_tags_can_skip.fdtput.test.dtb.1.expect
> +++ b/tests/unknown_tags_can_skip.fdtput.test.dtb.1.expect
> @@ -1,6 +1,7 @@
>  /dts-v1/;
> -// version:		17
> +// version:		18
>  // last_comp_version:	16
> +// last_comp_version_w:	17
>
>  // Unknown tag ignored: 0xd0000000, data len 4 00000001
>  / {
> diff --git a/tests/unknown_tags_can_skip.fdtput.test.dtb.2.expect b/tests/unknown_tags_can_skip.fdtput.test.dtb.2.expect
> index bd3a13b..69b4465 100644
> --- a/tests/unknown_tags_can_skip.fdtput.test.dtb.2.expect
> +++ b/tests/unknown_tags_can_skip.fdtput.test.dtb.2.expect
> @@ -1,6 +1,7 @@
>  /dts-v1/;
> -// version:		17
> +// version:		4294967295
>  // last_comp_version:	16
> +// last_comp_version_w:	17
>
>  // Unknown tag ignored: 0xd0000000, data len 4 00000001
>  / {
> diff --git a/tests/unknown_tags_can_skip.fdtput.test.dtb.3.expect b/tests/unknown_tags_can_skip.fdtput.test.dtb.3.expect
> index 237eb95..bbdd1c6 100644
> --- a/tests/unknown_tags_can_skip.fdtput.test.dtb.3.expect
> +++ b/tests/unknown_tags_can_skip.fdtput.test.dtb.3.expect
> @@ -1,6 +1,7 @@
>  /dts-v1/;
> -// version:		17
> +// version:		4294967295
>  // last_comp_version:	16
> +// last_comp_version_w:	17
>
>  // Unknown tag ignored: 0xd0000000, data len 4 00000001
>  / {
> diff --git a/tests/unknown_tags_can_skip.fdtput.test.dtb.4.expect b/tests/unknown_tags_can_skip.fdtput.test.dtb.4.expect
> index 8473040..98b0252 100644
> --- a/tests/unknown_tags_can_skip.fdtput.test.dtb.4.expect
> +++ b/tests/unknown_tags_can_skip.fdtput.test.dtb.4.expect
> @@ -1,6 +1,7 @@
>  /dts-v1/;
> -// version:		17
> +// version:		4294967295
>  // last_comp_version:	16
> +// last_comp_version_w:	17
>
>  // Unknown tag ignored: 0xd0000000, data len 4 00000001
>  / {
> diff --git a/tests/unknown_tags_can_skip.fdtput.test.dtb.5.expect b/tests/unknown_tags_can_skip.fdtput.test.dtb.5.expect
> index f7806f4..9738793 100644
> --- a/tests/unknown_tags_can_skip.fdtput.test.dtb.5.expect
> +++ b/tests/unknown_tags_can_skip.fdtput.test.dtb.5.expect
> @@ -1,6 +1,7 @@
>  /dts-v1/;
> -// version:		17
> +// version:		4294967295
>  // last_comp_version:	16
> +// last_comp_version_w:	17
>
>  // Unknown tag ignored: 0xd0000000, data len 4 00000001
>  / {
> diff --git a/tests/unknown_tags_can_skip.fdtput.test.dtb.6.expect b/tests/unknown_tags_can_skip.fdtput.test.dtb.6.expect
> index 029f3b4..22449ae 100644
> --- a/tests/unknown_tags_can_skip.fdtput.test.dtb.6.expect
> +++ b/tests/unknown_tags_can_skip.fdtput.test.dtb.6.expect
> @@ -1,6 +1,7 @@
>  /dts-v1/;
> -// version:		17
> +// version:		4294967295
>  // last_comp_version:	16
> +// last_comp_version_w:	17
>
>  // Unknown tag ignored: 0xd0000000, data len 4 00000001
>  / {
> --
> 2.53.0
>