On Wed, 2018-11-21 at 00:04 +0900, Masahiro Yamada wrote:

SUBDIRS has been kept as a backward compatibility since
commit ("[PATCH] kbuild: external module support") in 2002.

We do not need multiple ways to do the same thing, so I will remove
SUBDIRS after the Linux 5.3 release. I cleaned up in-tree code, and
updated the document so that nobody would try to use it.

Meanwhile, display the following warning if SUBDIRS is used.

Makefile:189: ================= WARNING ================
Makefile:190: 'SUBDIRS' will be removed after Linux 5.3
Makefile:191: Please use 'M=' or 'KBUILD_EXTMOD' instead
Makefile:192: ==========================================

Signed-off-by: Masahiro Yamada <redacted>

No, please don't do this. This is effectively a kernel←→user ABI.

The instructions for building a kernel module have been this, for
*ages*:

 echo 'obj-m := mymod.o' > Makefile
 make -C /lib/modules/`uname -r`/build SUBDIRS=`pwd`

People have muscle memory, they have it encoded into various of their
own makefiles and build scripts. Please don't make it stop working
unless there's actually a really good reason to do so.