kbuild: Add new Kbuild variable KBUILD_EXTRA_SYMBOLS

This patch adds a new (Kbuild) Makefile variable KBUILD_EXTRA_SYMBOLS.
The space separated list of file names assigned to KBUILD_EXTRA_SYMBOLS
is used when calling scripts/mod/modpost during stage 2 of the Kbuild
process for non-kernel-tree modules.
Signed-off-by: Richard Hacker <lerichi@gmx.net>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Documentation/kbuild/modules.txt

@@ -486,7 +486,7 @@ Module.symvers contains a list of all exported symbols from a kernel build.
 	Sometimes, an external module uses exported symbols from another
 	external module. Kbuild needs to have full knowledge on all symbols
 	to avoid spitting out warnings about undefined symbols.
-	Two solutions exist to let kbuild know all symbols of more than
+	Three solutions exist to let kbuild know all symbols of more than
 	one external module.
 	The method with a top-level kbuild file is recommended but may be
 	impractical in certain situations.
@@ -523,6 +523,13 @@ Module.symvers contains a list of all exported symbols from a kernel build.
 		containing the sum of all symbols defined and not part of the
 		kernel.
 
+	Use make variable KBUILD_EXTRA_SYMBOLS in the Makefile
+		If it is impractical to copy Module.symvers from another
+		module, you can assign a space separated list of files to
+		KBUILD_EXTRA_SYMBOLS in your Makfile. These files will be
+		loaded by modpost during the initialisation of its symbol
+		tables.
+
 === 8. Tips & Tricks
 
 --- 8.1 Testing for CONFIG_FOO_BAR

scripts/Makefile.modpost

@@ -42,6 +42,13 @@ _modpost: __modpost
 
 include include/config/auto.conf
 include scripts/Kbuild.include
+
+ifneq ($(KBUILD_EXTMOD),)
+# Include the module's Makefile to find KBUILD_EXTRA_SYMBOLS
+include $(if $(wildcard $(KBUILD_EXTMOD)/Kbuild), \
+             $(KBUILD_EXTMOD)/Kbuild, $(KBUILD_EXTMOD)/Makefile)
+endif
+
 include scripts/Makefile.lib
 
 kernelsymfile := $(objtree)/Module.symvers
@@ -69,6 +76,7 @@ modpost = scripts/mod/modpost                    \
  $(if $(CONFIG_MODULE_SRCVERSION_ALL),-a,)       \
  $(if $(KBUILD_EXTMOD),-i,-o) $(kernelsymfile)   \
  $(if $(KBUILD_EXTMOD),-I $(modulesymfile))      \
+ $(if $(iKBUILD_EXTRA_SYMBOLS), $(patsubst %, -e %,$(EXTRA_SYMBOLS))) \
  $(if $(KBUILD_EXTMOD),-o $(modulesymfile))      \
  $(if $(CONFIG_DEBUG_SECTION_MISMATCH),,-S)      \
  $(if $(CONFIG_MARKERS),-K $(kernelmarkersfile)) \