linux-kernel - [PATCH v2] kbuild: fix compilation of dtb specified on command-line without make rule

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Message-ID: <20251126100017.1162330-1-thomas.de_schampheleire@nokia.com>
Date: Wed, 26 Nov 2025 11:00:16 +0100
From: Thomas De Schampheleire <thomas.de_schampheleire@...ia.com>
To: linux-kbuild@...r.kernel.org
Cc: Thomas De Schampheleire <thomas.de_schampheleire@...ia.com>,
	Rob Herring <robh@...nel.org>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nicolas Schier <nsc@...nel.org>,
	linux-kernel@...r.kernel.org
Subject: [PATCH v2] kbuild: fix compilation of dtb specified on command-line without make rule

Since commit e7e2941300d2 ("kbuild: split device tree build rules into
scripts/Makefile.dtbs"), it is no longer possible to compile a device tree
blob that is not specified in a make rule
like:
    dtb-$(CONFIG_FOO) += foo.dtb

Before the mentioned commit, one could copy a dts file to e.g.
arch/arm64/boot/dts/ (or a new subdirectory) and then convert it to a dtb
file using:
    make ARCH=arm64 foo.dtb

In this scenario, both 'dtb-y' and 'dtb-' are empty, and the inclusion of
scripts/Makefile.dtbs relies on 'targets' to contain the MAKECMDGOALS. The
value of 'targets', however, is only final later in the code.

Move the conditional include of scripts/Makefile.dtbs down to where the
value of 'targets' is final. Since Makefile.dtbs updates 'always-y' which is
used as a prerequisite in the build rule, the build rule also needs to move
down.

Fixes: e7e2941300d2 ("kbuild: split device tree build rules into scripts/Makefile.dtbs")
Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@...ia.com>
---
v2: fix 'make dtbs' by moving the 'Build' rule along

 scripts/Makefile.build | 26 ++++++++++++++------------
 1 file changed, 14 insertions(+), 12 deletions(-)

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index d0ee33a487be..82a6e328ceee 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -527,18 +527,6 @@ ifneq ($(userprogs),)
 include $(srctree)/scripts/Makefile.userprogs
 endif
 
-ifneq ($(need-dtbslist)$(dtb-y)$(dtb-)$(filter %.dtb %.dtb.o %.dtbo.o,$(targets)),)
-include $(srctree)/scripts/Makefile.dtbs
-endif
-
-# Build
-# ---------------------------------------------------------------------------
-
-$(obj)/: $(if $(KBUILD_BUILTIN), $(targets-for-builtin)) \
-	 $(if $(KBUILD_MODULES), $(targets-for-modules)) \
-	 $(subdir-ym) $(always-y)
-	@:
-
 # Single targets
 # ---------------------------------------------------------------------------
 
@@ -568,6 +556,20 @@ FORCE:
 targets += $(filter-out $(single-subdir-goals), $(MAKECMDGOALS))
 targets := $(filter-out $(PHONY), $(targets))
 
+# Now that targets is fully known, include dtb rules if needed
+ifneq ($(need-dtbslist)$(dtb-y)$(dtb-)$(filter %.dtb %.dtb.o %.dtbo.o,$(targets)),)
+include $(srctree)/scripts/Makefile.dtbs
+endif
+
+# Build
+# Needs to be after the include of Makefile.dtbs, which updates always-y
+# ---------------------------------------------------------------------------
+
+$(obj)/: $(if $(KBUILD_BUILTIN), $(targets-for-builtin)) \
+	 $(if $(KBUILD_MODULES), $(targets-for-modules)) \
+	 $(subdir-ym) $(always-y)
+	@:
+
 # Read all saved command lines and dependencies for the $(targets) we
 # may be building above, using $(if_changed{,_dep}). As an
 # optimization, we don't need to read them if the target does not

base-commit: deab487e0f9b39ae4603e22d7d00908ebfc9753c
-- 
2.51.0