[RFC PATCH 00/15] Add support for structured tags and v18 dtb version

Herve Codina
2026-02-10T17:34:13Z

Recently, I sent a RFC series related to support for metadata and addon
device-trees [1].

During the discussion the concept of structured tags and "unknown" tags
emerged as well as the need to have them handled as a prerequisite of
support for metadata and addon.

The conclusion was the need for a new dtb version (v18) with support
for:
- Structured tags and based on them, "unknown" tags.
Those structured tags allow to have an standardized definition of
tags with the capability of skipping a tag and its related data
when a "unknown" tag is incountered by a given version of libfdt,
dtc and tools. Those "unknown" tags are tags defined in future
versions. Even if they exact meaning is unknown for an 'old'
version, they structure is understood and the 'old' version can skip
them without any errors if allowed.

- Flags in the dtb header (dt_flags).
The goal of this field is to have a placeholder to specify the
type of dtb we are dealing with. For instance, addons dtb will set a
flag in this placeholder

- A last compatible version for writing purpose.
The goal of the new dtb header field (last_comp_version_w) is to
disable globally any modification. It works similarly to
last_comp_version but for modification. It can be used to avoid any
modification that could be done by an 'old' version and could lead
to inconsistencies between the modification itself and some
"unknown" tags.

This RFC series implements those features and leads to the v18 dtb
version.

First patches (patches from 1 to 8) are patches fixing issues or
preparation patches. IMHO, those patches could be taken even if other
patches in the series lead to discussions.

Patch 9 introduces the structured tags. The patch gives definitions
needed to handle those tags.

Patches 10, 11 and 12 handles "unknown" tags in fdtdump, dtc and libfdt.
This is the reading part implementation related to "unknown" tags in
tools and lib.

Patch 13 is a preparation commit for patch 14 and patch 14 itself is
handling modifications (writing part) when unknown tags are involved.

The last patch (patch 15) bumps the dtb version including changes that
cannot be moved out of the version bump without having a v18 without all
expected features.

Also, several tests are added as soon as the related feature is
supported. Those tests are part of the last commit adding the feature.

As already said, this current RFC series is a prerequisite to the
support for metadata and addons. The RFC series related to metadata and
addons [1] will be rebased on top of this prerequisite. Please, keep
that in mind for the review of this current RFC prerequisite series.

[1] https://lore.kernel.org/all/20260112142009.1006236-1-herve.codina@bootlin.com/

Best regards,
Hervé

Herve Codina (15):
dtc: Use a consistent type for basenamelen
fdtdump: Remove dtb version check
fdtdump: Return an error code on wrong tag value
libfdt: fdt_rw: Introduce fdt_downgrade_version()
libfdt: Introduce fdt_first_node()
libfdt: Don't assume that a FDT_BEGIN_NODE tag is available at offset
0
libfdt: fdt_check_full: Handle FDT_NOP when FDT_END is expected
tests: asm: Introduce treehdr_vers macro
Introduce structured tag value definition
fdtdump: Handle unknown tags
flattree: Handle unknown tags
libfdt: Handle unknown tags in fdt_get_next()
libfdt: Introduce fdt_ptr_offset_
libfdt: Handle unknown tags on dtb modifications
Introduce v18 dtb version

checks.c | 2 +-
dtc.h | 4 +-
fdtdump.c | 57 ++++++-
flattree.c | 102 ++++++++++--
libfdt/fdt.c | 110 ++++++++++++-
libfdt/fdt.h | 28 ++++
libfdt/fdt_check.c | 2 +-
libfdt/fdt_ro.c | 16 +-
libfdt/fdt_rw.c | 151 +++++++++++++++++-
libfdt/fdt_sw.c | 3 +
libfdt/libfdt.h | 7 +-
libfdt/libfdt_internal.h | 9 ++
pylibfdt/libfdt.i | 18 +++
tests/dumptrees.c | 5 +-
tests/pylibfdt_tests.py | 10 +-
tests/run_tests.sh | 115 ++++++++++++-
tests/testdata.h | 3 +
tests/testutils.c | 2 +-
tests/trees.S | 143 ++++++++++++++++-
tests/unknown_tags_can_skip.dtb.dts.expect | 19 +++
tests/unknown_tags_can_skip.dtb.expect | 26 +++
...own_tags_can_skip.fdtput.test.dtb.0.expect | 32 ++++
...own_tags_can_skip.fdtput.test.dtb.1.expect | 36 +++++
...own_tags_can_skip.fdtput.test.dtb.2.expect | 34 ++++
...own_tags_can_skip.fdtput.test.dtb.3.expect | 36 +++++
...own_tags_can_skip.fdtput.test.dtb.4.expect | 35 ++++
...own_tags_can_skip.fdtput.test.dtb.5.expect | 33 ++++
...own_tags_can_skip.fdtput.test.dtb.6.expect | 28 ++++
28 files changed, 1017 insertions(+), 49 deletions(-)
create mode 100644 tests/unknown_tags_can_skip.dtb.dts.expect
create mode 100644 tests/unknown_tags_can_skip.dtb.expect
create mode 100644 tests/unknown_tags_can_skip.fdtput.test.dtb.0.expect
create mode 100644 tests/unknown_tags_can_skip.fdtput.test.dtb.1.expect
create mode 100644 tests/unknown_tags_can_skip.fdtput.test.dtb.2.expect
create mode 100644 tests/unknown_tags_can_skip.fdtput.test.dtb.3.expect
create mode 100644 tests/unknown_tags_can_skip.fdtput.test.dtb.4.expect
create mode 100644 tests/unknown_tags_can_skip.fdtput.test.dtb.5.expect
create mode 100644 tests/unknown_tags_can_skip.fdtput.test.dtb.6.expect

--
2.52.0