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

[Douglas Anderson]

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:

- 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@xxxxxxxxxxxx>
---
You can see the fdtfactor post at:
 http://lore.kernel.org/r/20261015182221.2931742-1-dianders@xxxxxxxxxxxx

This patch obviously could only land if some form of that series
lands. Presumably we'd also want to import fdtfactor into the local
kernel copy of the devicetree compiler before this patch.

If people hate this patch, we could certainly just run `fdtfactor`
after the kernel build finishes.

 scripts/Makefile.dtbs |  8 ++++++++
 scripts/Makefile.lib  | 18 ++++++++++++------
 2 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/scripts/Makefile.dtbs b/scripts/Makefile.dtbs
index 1bc42b30cc52..4b8d7039e45d 100644
--- a/scripts/Makefile.dtbs
+++ b/scripts/Makefile.dtbs
@@ -137,6 +137,14 @@ DTC_FLAGS += -Wnode_name_chars_strict \
              -Wunique_unit_address
 endif
 
+# 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
+
 DTC_FLAGS += $(DTC_FLAGS_$(target-stem))
 
 # Set -@ if the target is a base DTB that overlay is applied onto
diff --git a/scripts/Makefile.lib b/scripts/Makefile.lib
index 91880648fe21..bcb2b0e749a3 100644
--- 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
 
 # Use this to override the compression algorithm
 FIT_COMPRESSION ?= gzip
@@ -434,7 +436,9 @@ quiet_cmd_fit = FIT     $@
 		--name '$(UIMAGE_NAME)' $(FIT_EXTRA_ARGS) \
 		$(if $(findstring 1,$(KBUILD_VERBOSE)),-v) \
 		$(if $(FIT_DECOMPOSE_DTBS),--decompose-dtbs) \
-		--compress $(FIT_COMPRESSION) -k $< @$(word 2,$^)
+		--compress $(FIT_COMPRESSION) -k $< @$(word 2,$^) \
+		$(if $(FIT_FACTOR_DTBS),&& $(FDTFACTOR) $(FDTFACTOR_ARGS) \
+			$(if $(findstring 1,$(KBUILD_VERBOSE)),-v) -f $@)
 
 # If FIT_ITS is specified on the command line (either relative to $(srctree)
 # or relative to arch/$(SRCARCH)/boot/dts/), locate that .its script and
@@ -465,10 +469,10 @@ $(obj)/image.fit: $(FIT_ITS_IMAGES) $(FIT_ITS_DTBS) $(FIT_ITS_FILE)
 endif
 endif
 
-# Build the FIT image from the .its script with mkimage and populate any
-# missing 'compatible' property on /configurations/conf-* nodes from the
-# corresponding uncompressed .dtb's root '/compatible' property using
-# fdtget/fdtput.
+# Build the FIT image from the .its script with mkimage, populate any missing
+# 'compatible' property on /configurations/conf-* nodes from the corresponding
+# uncompressed .dtb's root '/compatible' property using fdtget/fdtput, and
+# optionally factor the DTBs with fdtfactor when FIT_FACTOR_DTBS is set.
 quiet_cmd_fit_its = FIT     $@
       cmd_fit_its = \
 		PATH="$(abspath $(objtree))/scripts/dtc:$(PATH)"; \
@@ -486,7 +490,9 @@ quiet_cmd_fit_its = FIT     $@
 			dtb="$(objtree)/$${inc%.dtb*}.dtb"; \
 			compat=$$(fdtget -ts "$$dtb" / compatible); \
 			fdtput -ts $@ $$conf compatible $$compat; \
-		done
+		done \
+		$(if $(FIT_FACTOR_DTBS),; $(FDTFACTOR) $(FDTFACTOR_ARGS) \
+			$(if $(findstring 1,$(KBUILD_VERBOSE)),-v) -f $@)
 
 # XZ
 # ---------------------------------------------------------------------------
-- 
2.56.0.212.g129ede20d9-goog