kbuild: announce removal of SUBDIRS if used

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 <yamada.masahiro@socionext.com>
Acked-by: Boris Brezillon <boris.brezillon@bootlin.com> # for scx200_docflash.c
Acked-by: Guenter Roeck <linux@roeck-us.net> # for scx200_wdt.c

Documentation/kbuild/kbuild.txt

@@ -81,12 +81,7 @@ KBUILD_EXTMOD
 --------------------------------------------------
 Set the directory to look for the kernel source when building external
 modules.
-The directory can be specified in several ways:
-1) Use "M=..." on the command line
-2) Environment variable KBUILD_EXTMOD
-3) Environment variable SUBDIRS
-The possibilities are listed in the order they take precedence.
-Using "M=..." will always override the others.
+Setting "M=..." takes precedence over KBUILD_EXTMOD.
 
 KBUILD_OUTPUT
 --------------------------------------------------

Makefile

@@ -186,6 +186,10 @@ endif
 # Old syntax make ... SUBDIRS=$PWD is still supported
 # Setting the environment variable KBUILD_EXTMOD take precedence
 ifdef SUBDIRS
+  $(warning ================= WARNING ================)
+  $(warning 'SUBDIRS' will be removed after Linux 5.3)
+  $(warning Please use 'M=' or 'KBUILD_EXTMOD' instead)
+  $(warning ==========================================)
   KBUILD_EXTMOD ?= $(SUBDIRS)
 endif
 

drivers/mtd/maps/scx200_docflash.c

@@ -216,10 +216,3 @@ static void __exit cleanup_scx200_docflash(void)
 
 module_init(init_scx200_docflash);
 module_exit(cleanup_scx200_docflash);
-
-/*
-    Local variables:
-        compile-command: "make -k -C ../../.. SUBDIRS=drivers/mtd/maps modules"
-        c-basic-offset: 8
-    End:
-*/

drivers/watchdog/scx200_wdt.c

@@ -262,10 +262,3 @@ static void __exit scx200_wdt_cleanup(void)
 
 module_init(scx200_wdt_init);
 module_exit(scx200_wdt_cleanup);
-
-/*
-    Local variables:
-	compile-command: "make -k -C ../.. SUBDIRS=drivers/char modules"
-	c-basic-offset: 8
-    End:
-*/

samples/connector/Makefile

@@ -14,4 +14,4 @@ HOSTCFLAGS_ucon.o += -I$(objtree)/usr/include
 all: modules
 
 modules clean:
-	$(MAKE) -C ../.. SUBDIRS=$(CURDIR) $@
+	$(MAKE) -C ../.. M=$(CURDIR) $@