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kbuild: add more comments for KBUILD_NOCMDDEP=1

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The cmd-check for KBUILD_NOCMDDEP=1 may not be clear until you see
commit c4d5ee1398 ("kbuild: make KBUILD_NOCMDDEP=1 handle empty
built-in.o").

When a phony target (i.e. FORCE) is the only prerequisite, Kbuild
uses a tricky way to detect that the target does not exist.

Add more comments.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>
This commit is contained in:
Masahiro Yamada 2022-12-29 18:15:01 +09:00
parent 92215e7a80
commit ee2162bd4f
1 changed files with 6 additions and 0 deletions
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6
scripts/Kbuild.include
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@ -170,9 +170,13 @@ cmd = @$(if $(cmd_$(1)),set -e; $($(quiet)log_print) $(delete-on-interrupt) $(cm
ifneq ($(KBUILD_NOCMDDEP),1)
# Check if both commands are the same including their order. Result is empty
# string if equal. User may override this check using make KBUILD_NOCMDDEP=1
# If the target does not exist, the *.cmd file should not be included so
# $(savedcmd_$@) gets empty. Then, target will be built even if $(newer-prereqs)
# happens to become empty.
cmd-check = $(filter-out $(subst $(space),$(space_escape),$(strip $(savedcmd_$@))), \
$(subst $(space),$(space_escape),$(strip $(cmd_$1))))
else
# We still need to detect missing targets.
cmd-check = $(if $(strip $(savedcmd_$@)),,1)
endif
@ -186,6 +190,8 @@ make-cmd = $(call escsq,$(subst $(pound),$$(pound),$(subst $$,$$$$,$(cmd_$(1))))
# Find any prerequisites that are newer than target or that do not exist.
# PHONY targets skipped in both cases.
# If there is no prerequisite other than phony targets, $(newer-prereqs) becomes
# empty even if the target does not exist. cmd-check saves this corner case.
newer-prereqs = $(filter-out $(PHONY),$?)
# It is a typical mistake to forget the FORCE prerequisite. Check it here so