[RFC PATCH 5/7] kbuild: fit: Add opt-in FIT_FACTOR_DTBS support using fdtfactor

[Simon Glass]

Hi Doug,

On 2026-10-20T15:35:18, Douglas Anderson <dianders at chromium.org> wrote:
> kbuild: fit: Add opt-in FIT_FACTOR_DTBS support using fdtfactor
>
> Add opt-in support for factoring the DTBs inside a compiled FIT image
> into shared base DTBs and per-board overlays using `fdtfactor` when
> `FIT_FACTOR_DTBS=1` is passed to make:

Please can you add a short paragraph at the start saying why this is
wanted (the bootloader needs a base DTB plus a per-board DTBO) and
where fdtfactor comes from? It is not in scripts/dtc, so someone
reading the git log won't know where the tool lives or which version
is needed. A link to the dtc series would help.

>
> - When `FIT_FACTOR_DTBS` is set, pass `-@ -L` to `dtc` for the DTBs
>   included in the FIT image so they retain `/__symbols__` and
>   `/__local_fixups__` metadata needed for factoring.
> - Run `$(FDTFACTOR) $(FDTFACTOR_ARGS) -f $@` (with `FDTFACTOR_ARGS`
>   defaulting to `--strip-local-fixups`) at the end of `cmd_fit` and
>   `cmd_fit_its` when `FIT_FACTOR_DTBS` is set.
>
> Assisted-by: Gemini:gemini-pro-prerelease
> Signed-off-by: Douglas Anderson <dianders at chromium.org>
>
>  scripts/Makefile.dtbs |  8 ++++++++
>  scripts/Makefile.lib  | 18 ++++++++++++------
>  2 files changed, 20 insertions(+), 6 deletions(-)

> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> @@ -425,6 +425,8 @@ quiet_cmd_uimage = UIMAGE  $@
>  MAKE_FIT := $(srctree)/scripts/make_fit.py
> +FDTFACTOR ?= fdtfactor
> +FDTFACTOR_ARGS ?= --strip-local-fixups

Patches 1-3 go to some trouble to build fdtget and fdtput in-tree, and
cmd_fit_its puts scripts/dtc on the PATH, but fdtfactor comes from the
host. So FIT_FACTOR_DTBS=1 uses whatever fdtfactor happens to be on
the PATH, and the build does not depend on it. Is the plan to sync
fdtfactor.c into scripts/dtc once it lands upstream? If so, it would
be worth saying so under the --- line.

> diff --git a/scripts/Makefile.dtbs b/scripts/Makefile.dtbs
> @@ -137,6 +137,14 @@ DTC_FLAGS += -Wnode_name_chars_strict \
> +# When FIT_FACTOR_DTBS is set, pass -@ -L to dtc so the built DTBs retain the
> +# /__symbols__ and /__local_fixups__ nodes required by fdtfactor.
> +ifneq ($(FIT_FACTOR_DTBS),)
> +ifneq ($(filter arch/$(SRCARCH)/boot/dts%, $(obj)),)
> +DTC_FLAGS += -@ -L
> +endif
> +endif

One thing I'd like to check is composite DTBs (the foo-dtbs :=
base.dtb bar.dtbo entries built by cmd_fdtoverlay). My understanding
is that fdt_overlay_apply() merges the overlay's __symbols__ into the
base but drops its __local_fixups__ after using them to renumber
phandles inside the overlay. So the __local_fixups__ in a composite
DTB only cover the base part, even though both inputs were built with
-L

This doesn't affect lga or trogdor, but the make_fit.py path packs
every DTB in dtbs-list, and qcom has lots of composite entries
(lemans-evk-camera.dtb, etc). If fdtfactor relies on __local_fixups__
to find phandle properties, it may silently produce a bad overlay for
these. Please can you check this? It might be enough to detect such
DTBs and skip or refuse them.

> diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
> @@ -486,7 +490,9 @@ quiet_cmd_fit_its = FIT     $@
> -		done
> +		done \
> +		$(if $(FIT_FACTOR_DTBS),; $(FDTFACTOR) $(FDTFACTOR_ARGS) \
> +			$(if $(findstring 1,$(KBUILD_VERBOSE)),-v) -f $@)

Just to check, does fdtfactor handle compressed DTBs in the FIT, i.e.
nodes with compression = 'lz4' as in sc7180-trogdor.its from patch 7?
If not, FIT_FACTOR_DTBS=1 with that .its will either fail or silently
skip the DTBs. Either way, it would be worth mentioning in the commit
message.

Regards,
Simon