kbuild: show hint if subdir-y/m is used to visit module Makefile

Since commit ff9b45c ("kbuild: modpost: read modules.order instead
of $(MODVERDIR)/*.mod"), a module is no longer built in the following
pattern:

  [Makefile]
  subdir-y := some-module

  [some-module/Makefile]
  obj-m := some-module.o

You cannot write Makefile this way in upstream because modules.order is
not correctly generated. subdir-y is used to descend to a sub-directory
that builds tools, device trees, etc.

For external modules, the modules order does not matter. So, the
Makefile above was known to work.

I believe the Makefile should be re-written as follows:

  [Makefile]
  obj-m := some-module/

  [some-module/Makefile]
  obj-m := some-module.o

However, people will have no idea if their Makefile suddenly stops
working. In fact, I received questions from multiple people.

Show a warning for a while if obj-m is specified in a Makefile visited
by subdir-y or subdir-m.

I touched the %/ rule to avoid false-positive warnings for the single
target.

Cc: Jan Kiszka <[email protected]>
Cc: Tom Stonecypher <[email protected]>
Signed-off-by: Masahiro Yamada <[email protected]>
Tested-by: Jan Kiszka <[email protected]>

Makefile

@@ -1788,7 +1788,7 @@ PHONY += /
 /: ./
 
 %/: prepare FORCE
-	$(Q)$(MAKE) KBUILD_MODULES=1 $(build)=$(build-dir)
+	$(Q)$(MAKE) KBUILD_MODULES=1 $(build)=$(build-dir) need-modorder=1
 
 # FIXME Should go into a make.lib or something
 # ===========================================================================

scripts/Makefile.build

@@ -52,6 +52,13 @@ ifndef obj
 $(warning kbuild: Makefile.build is included improperly)
 endif
 
+ifeq ($(MAKECMDGOALS)$(need-modorder),)
+ifneq ($(obj-m),)
+$(warning $(patsubst %.o,'%.ko',$(obj-m)) will not be built even though obj-m is specified.)
+$(warning You cannot use subdir-y/m to visit a module Makefile. Use obj-y/m instead.)
+endif
+endif
+
 # ===========================================================================
 
 ifneq ($(strip $(lib-y) $(lib-m) $(lib-)),)