kbuild: add 'private' to target-specific variables

Currently, Kbuild produces inconsistent results in some cases.

You can do an interesting experiment using the --shuffle option, which
is supported by GNU Make 4.4 or later.

Set CONFIG_KVM_INTEL=y and CONFIG_KVM_AMD=m (or vice versa), and repeat
incremental builds w/wo --shuffle=reverse.

  $ make
    [ snip ]
    CC      arch/x86/kvm/kvm-asm-offsets.s

  $ make --shuffle=reverse
    [ snip ]
    CC [M]  arch/x86/kvm/kvm-asm-offsets.s

  $ make
    [ snip ]
    CC      arch/x86/kvm/kvm-asm-offsets.s

arch/x86/kvm/kvm-asm-offsets.s is rebuilt every time w/wo the [M] marker.

arch/x86/kvm/kvm-asm-offsets.s is built as built-in when it is built as
a prerequisite of arch/x86/kvm/kvm-intel.o, which is built-in.

arch/x86/kvm/kvm-asm-offsets.s is built as modular when it is built as
a prerequisite of arch/x86/kvm/kvm-amd.o, which is a module.

Another odd example is single target builds.

When CONFIG_LKDTM=m, drivers/misc/lkdtm/rodata.o can be built as
built-in or modular, depending on how it is built.

  $ make drivers/misc/lkdtm/lkdtm.o
    [ snip ]
    CC [M]  drivers/misc/lkdtm/rodata.o

  $ make drivers/misc/lkdtm/rodata.o
    [ snip ]
    CC      drivers/misc/lkdtm/rodata.o

drivers/misc/lkdtm/rodata.o is built as modular when it is built as a
prerequisite of another, but built as built-in when it is a final
target.

The same thing happens to drivers/memory/emif-asm-offsets.s when
CONFIG_TI_EMIF_SRAM=m.

  $ make drivers/memory/ti-emif-sram.o
    [ snip ]
    CC [M]  drivers/memory/emif-asm-offsets.s

  $ make drivers/memory/emif-asm-offsets.s
    [ snip ]
    CC      drivers/memory/emif-asm-offsets.s

This is because the part-of-module=y flag defined for the modules is
inherited by its prerequisites.

Target-specific variables are likely intended only for local use.
This commit adds 'private' to them.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>

Author: masahir0y

Makefile

@@ -1254,8 +1254,8 @@ define filechk_version.h
 	echo \#define LINUX_VERSION_SUBLEVEL $(SUBLEVEL)
 endef
 
-$(version_h): PATCHLEVEL := $(or $(PATCHLEVEL), 0)
-$(version_h): SUBLEVEL := $(or $(SUBLEVEL), 0)
+$(version_h): private PATCHLEVEL := $(or $(PATCHLEVEL), 0)
+$(version_h): private SUBLEVEL := $(or $(SUBLEVEL), 0)
 $(version_h): FORCE
 	$(call filechk,version.h)
 
@@ -1503,7 +1503,7 @@ MRPROPER_FILES += include/config include/generated          \
 
 # clean - Delete most, but leave enough to build external modules
 #
-clean: rm-files := $(CLEAN_FILES)
+clean: private rm-files := $(CLEAN_FILES)
 
 PHONY += archclean vmlinuxclean
 
@@ -1515,7 +1515,7 @@ clean: archclean vmlinuxclean resolve_btfids_clean
 
 # mrproper - Delete all generated files, including .config
 #
-mrproper: rm-files := $(MRPROPER_FILES)
+mrproper: private rm-files := $(MRPROPER_FILES)
 mrproper-dirs      := $(addprefix _mrproper_,scripts)
 
 PHONY += $(mrproper-dirs) mrproper
@@ -1792,7 +1792,7 @@ compile_commands.json: $(extmod_prefix)compile_commands.json
 PHONY += compile_commands.json
 
 clean-dirs := $(KBUILD_EXTMOD)
-clean: rm-files := $(KBUILD_EXTMOD)/Module.symvers $(KBUILD_EXTMOD)/modules.nsdeps \
+clean: private rm-files := $(KBUILD_EXTMOD)/Module.symvers $(KBUILD_EXTMOD)/modules.nsdeps \
 	$(KBUILD_EXTMOD)/compile_commands.json
 
 PHONY += prepare

scripts/Makefile.build

@@ -216,7 +216,7 @@ endif # CONFIG_FTRACE_MCOUNT_USE_RECORDMCOUNT
 
 is-standard-object = $(if $(filter-out y%, $(OBJECT_FILES_NON_STANDARD_$(target-stem).o)$(OBJECT_FILES_NON_STANDARD)n),y)
 
-$(obj)/%.o: objtool-enabled = $(if $(is-standard-object),$(if $(delay-objtool),$(is-single-obj-m),y))
+$(obj)/%.o: private objtool-enabled = $(if $(is-standard-object),$(if $(delay-objtool),$(is-single-obj-m),y))
 
 ifneq ($(findstring 1, $(KBUILD_EXTRA_WARN)),)
 cmd_warn_shared_object = $(if $(word 2, $(modname-multi)),$(warning $(kbuild-file): $*.o is added to multiple modules: $(modname-multi)))
@@ -437,8 +437,8 @@ define rule_ld_multi_m
 	$(call cmd,gen_objtooldep)
 endef
 
-$(multi-obj-m): objtool-enabled := $(delay-objtool)
-$(multi-obj-m): part-of-module := y
+$(multi-obj-m): private objtool-enabled := $(delay-objtool)
+$(multi-obj-m): private part-of-module := y
 $(multi-obj-m): %.o: %.mod FORCE
 	$(call if_changed_rule,ld_multi_m)
 $(call multi_depend, $(multi-obj-m), .o, -objs -y -m)