how to use KBUILD_EXTRA_SYMBOLS

[Boaz Harrosh <bharrosh@panasas.com>]

I have two out-of-tree modules that I compile with the M=
mechanics, one of the modules has imports from the other one.

I tried to follow modules.txt and put an KBUILD_EXTRA_SYMBOLS
when building. Somewhat like below:

	# derived.ko module Makefile
	# $(BASE_MODULE_DIR) must point to base.ko source directory
	KBUILD_BASE = +$(MAKE) -C $(KSRC) M=`pwd` KBUILD_OUTPUT=$(KBUILD_OUTPUT) ARCH=$(ARCH)

	derived:
		$(KBUILD_BASE) KBUILD_EXTRA_SYMBOLS=$(BASE_MODULE_DIR) modules

But doing this does work:
	# derived module Makefile
	# $(BASE_MODULE_DIR) must point to base.ko source directory

	KBUILD_BASE = +$(MAKE) -C $(KSRC) M=`pwd` KBUILD_OUTPUT=$(KBUILD_OUTPUT) ARCH=$(ARCH)

	derived:
		$(KBUILD_BASE) KBUILD_EXTMOD=$(BASE_MODULE_DIR) modules


The diff is s/KBUILD_EXTRA_SYMBOLS/KBUILD_EXTMOD.

It took me a long while and inspection the git-log of
Documentation/kbuild/modules.txt to figure that out. Perhaps an example
is do in documentation.

What am I doing wrong?

Thanks
Boaz