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

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Message-ID: <20251121064256.GB571346@ax162>
Date: Thu, 20 Nov 2025 23:42:56 -0700
From: Nathan Chancellor <nathan@...nel.org>
To: Thomas De Schampheleire <thomas.de_schampheleire@...ia.com>
Cc: linux-kbuild@...r.kernel.org, Masahiro Yamada <masahiroy@...nel.org>,
	Nicolas Schier <nsc@...nel.org>, linux-kernel@...r.kernel.org,
	devicetree@...r.kernel.org, Rob Herring <robh@...nel.org>
Subject: Re: [PATCH] kbuild: fix compilation of dtb specified on command-line
 without make rule

+ Rob, as he reviewed e7e2941300d2

On Thu, Nov 20, 2025 at 03:06:43PM +0100, Thomas De Schampheleire wrote:
> 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.
> 
> Fixes: e7e2941300d2 ("kbuild: split device tree build rules into scripts/Makefile.dtbs")
> Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@...ia.com>

Huh, TIL that .c and .S files can be built in the same way...

  $ touch init/foo.c init/bar.S
  $ make init/foo.o init/bar.o
  $ ls init/.foo.o.cmd init/.bar.o.cmd
  init/.bar.o.cmd
  init/.foo.o.cmd

I do not necessarily love splitting up the conditional includes from
each other but I am not sure it can be avoided... Nicolas, are you okay
with me taking this as a fix or do you want to take it via kbuild-next?
If the latter:

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
>  scripts/Makefile.build | 9 +++++----
>  1 file changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> index d0ee33a487be..6e0f9e922c68 100644
> --- a/scripts/Makefile.build
> +++ b/scripts/Makefile.build
> @@ -527,10 +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
>  # ---------------------------------------------------------------------------
>  
> @@ -568,6 +564,11 @@ 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
> +
>  # 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
>