[PATCH v2] kbuild: show hint if subdir-y/m is used to visit module Makefile

[Masahiro Yamada]

Since commit ff9b45c55b26 ("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 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 <jan.kiszka@xxxxxxxxxxx>
Cc: Tom Stonecypher <thomas.edwardx.stonecypher@xxxxxxxxx>
Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
---

Changes in v2:
 - fix false-positive warnings for single targets

 Makefile               | 2 +-
 scripts/Makefile.build | 7 +++++++
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 0e37ad2f77bf..fac25e279da6 100644
--- a/Makefile
+++ b/Makefile
@@ -1783,7 +1783,7 @@ 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
 # ===========================================================================
diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index 37a1d2cd49d4..2f66ed388d1c 100644
--- a/scripts/Makefile.build
+++ b/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-)),)
-- 
2.17.1