Re: [PATCH v3 2/2] kbuild: Support directory targets for building DTBs

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] kbuild: unconditional need-dtbslist=1 causes unnecessary dtb compilation and dtbs-list files during standard builds
--

commit 645916678e1f6dbf830062f35ddde56a42d6b0df
Author: Rob Herring (Arm) <robh@kernel.org>

kbuild: Support directory targets for building DTBs

This commit modifies the Kbuild system to allow directory targets for
building all device tree binaries (DTBs) within a directory. It updates
both dtbs and single-build targets to use directory rules.

> diff --git a/Makefile b/Makefile
> index c307ee9293bcd..75d290c2b7d88 100644
> --- a/Makefile
> +++ b/Makefile
[ ... ]
> @@ -2234,7 +2234,7 @@ prepare: outputmakefile
>  # Error messages still appears in the original language
>  PHONY += $(build-dir)
>  $(build-dir): $(single-build-prep)
> -	$(Q)$(MAKE) $(build)=$@ need-builtin=1 need-modorder=1 $(single-goals)
> +	$(Q)$(MAKE) $(build)=$@ need-builtin=1 need-modorder=1 need-dtbslist=1 $(single-goals)

[Severity: High]
Does adding need-dtbslist=1 unconditionally to the $(build-dir) recipe
cause unnecessary dtb compilation and empty dtbs-list file generation
during standard builds?

Because standard KBUILD_VMLINUX_OBJS targets (like built-in.a) depend
on '.' (which evaluates to $(build-dir)), 'make vmlinux' evaluates this
rule to recurse into the source tree. This exports need-dtbslist=1 into
MAKEFLAGS, which is then inherited by all Kbuild sub-makes.

> diff --git a/scripts/Makefile.dtbs b/scripts/Makefile.dtbs
> index c4e4663902844..c7ec2a2a308ef 100644
> --- a/scripts/Makefile.dtbs
> +++ b/scripts/Makefile.dtbs
> @@ -23,7 +23,9 @@ dtb-y           := $(addprefix $(obj)/, $(dtb-y))
>  multi-dtb-y     := $(addprefix $(obj)/, $(multi-dtb-y))
>  real-dtb-y      := $(addprefix $(obj)/, $(real-dtb-y))
>  
> +ifdef need-dtbslist
>  always-y        += $(dtb-y)
> +endif
>  targets         += $(real-dtb-y)

Because need-dtbslist=1 is globally inherited, it now unconditionally
appends all device trees to always-y in every visited directory.

Additionally, it triggers the generation of dtbs-list files across the
tree via existing code further down in scripts/Makefile.dtbs:

scripts/Makefile.dtbs:
    ifdef need-dtbslist
    subdir-dtbslist := $(addsuffix /dtbs-list, $(subdir-ym))
    dtb-y           += $(subdir-dtbslist)
    always-y        += $(obj)/dtbs-list
    endif

Could this unconditionally traverse arch/$(SRCARCH)/boot/dts/ and
compile all device trees, regressing build times, while cluttering
unrelated directories (like fs/ or mm/) with empty dtbs-list files?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20261007-dtb-dirs-v3-v3-0-b3ab32b64fa6@kernel.org?part=2