Building an out-of-tree linux kernel module with separate output directory

Question

I want to build an out of tree kernel module with the output directory being separate from my source directory? How would I do this? I'm willing to go any route. I'm okay with minimal changes to the kernel build system, I'm okay with copying source files (however I do not want to rebuild if I haven't made any changes to the source files and this doesn't work if I copy source files normally), and I'm okay with setting a parameter or something.

Answer 1

many many people face this problem, including me. To support build external module at separate output directory. I modify the kbuild:

firstly, modify src variable at scripts/Makefile.build and scripts/Makefile.clean

-src := $(obj)
+src := $(if $(KBUILD_EXTMOD_SRC),$(KBUILD_EXTMOD_SRC)$(patsubst $(KBUILD_EXTMOD)%,%,$(obj)),$(obj))

secondly, modify scripts/Makefile.modpost

-src := $(obj)
+src := $(if $(KBUILD_EXTMOD_SRC),$(KBUILD_EXTMOD_SRC),$(obj))

 # Include the module's Makefile to find KBUILD_EXTRA_SYMBOLS
-include $(if $(wildcard $(KBUILD_EXTMOD)/Kbuild), \
-             $(KBUILD_EXTMOD)/Kbuild, $(KBUILD_EXTMOD)/Makefile)
+include $(if $(wildcard $(src)/Kbuild), \
+             $(src)/Kbuild, $(src)/Makefile)

then build external module like this:

make -c  $(kernel_src) M=$(extmod_outpu_dir) KBUILD_EXTMOD_SRC=$(extmod_src_dir) modules