Surprising to me, Kbuild can handle a relative M= path while having in-kernel objects in a separate directory. In this case, the M= option actually defines: - the path to external module sources relative to $(srctree) - the path to external module objects relative to $(objtree) For example, make O=foo/bar M=baz ... will find the source files in baz, and create objects in foo/bar/baz. Currently, it works like that because of the presence of VPATH and --include-dir=$(abs_srctree), but this behavior is subtle and cannot be guaranteed in the future. I do not know if it was even officially supported ever. I do not recommend this usage. Reported-by: Will McVicker <willmcvicker@xxxxxxxxxx> Signed-off-by: Masahiro Yamada <masahiroy@xxxxxxxxxx> --- Makefile | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/Makefile b/Makefile index b0e4767735dc..e42a27447eae 100644 --- a/Makefile +++ b/Makefile @@ -1699,6 +1699,13 @@ else # KBUILD_EXTMOD # Install the modules built in the module directory # Assumes install directory is already created +ifdef building_out_of_srctree +ifneq ($(filter-out /%, $(KBUILD_EXTMOD)),) +$(warning The combination of out-of-tree build and relative M= path is not well supported.) +$(warning Please do not complain about any trouble happening under this circumstance.) +endif +endif + # We are always building only modules. KBUILD_BUILTIN := KBUILD_MODULES := 1 -- 2.27.0
