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SubjectMutli-directory module Makefiles

in drivers/net/ethernet/netronome/nfp there is a module which is built
from C sources in 4 directories. What is the best way to handle that?

Currently we just add all the objects in one Makefile:

nfp-objs := \
nfpcore/nfp6000_pcie.o \
nfpcore/nfp_cppcore.o \

However, this makes it impossible to build a single object in subdirs:

$ make drivers/net/ethernet/netronome/nfp/nfpcore/nfp_cppcore.o
scripts/ drivers/net/ethernet/netronome/nfp/nfpcore/Makefile: No such file or directory
make[2]: *** No rule to make target 'drivers/net/ethernet/netronome/nfp/nfpcore/Makefile'. Stop.

Documentation/kbuild/modules.txt also contains an unclear remark that
this is not recommended (or is it about the use of $(src)?):

--- 4.3 Several Subdirectories
To build the module complex.ko, we then need the following
kbuild file:

--> filename: Kbuild
obj-m := complex.o
complex-y := src/complex_main.o
complex-y += src/hal/hardwareif.o

ccflags-y := -I$(src)/include
ccflags-y += -I$(src)/src/hal/include

As you can see, kbuild knows how to handle object files located
in other directories. The trick is to specify the directory
relative to the kbuild file's location. **That being said, this
is NOT recommended practice.**

Making the include optional would work:

diff --git a/scripts/ b/scripts/
@@ -42,7 +42,7 @@ save-cflags := $(CFLAGS)
# The filename Kbuild has precedence over Makefile
kbuild-dir := $(if $(filter /%,$(src)),$(src),$(srctree)/$(src))
kbuild-file := $(if $(wildcard $(kbuild-dir)/Kbuild),$(kbuild-dir)/Kbuild,$(kbuild-dir)/Makefile)
-include $(kbuild-file)
+-include $(kbuild-file)

# If the save-* variables changed error out
Or we could create empty Makefiles in subdirectories... Is there a
better way of handling this? Which is preferred?

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