ANDROID: Kbuild: Add support for KBUILD_MIXED_TREE

When KBUILD_MIXED_TREE points to the output folder of another kernel's
build output, Kbuild can compile a complete kernel tree's modules
against that other kernel tree's vmlinux. This is useful when two
kernel trees exist: a "Generic Kernel Image" tree and a "device kernel"
tree.  Both trees are complete kernel source trees, and the "Generic
Kernel Image" should provide the kernel Image and device kernel tree
provides device driver modules.

To accomplish this, references to vmlinux.symvers in the device kernel
should point to the generic kernel's vmlinux.symvers and the device
kernel should skip compilation of built-in files.

Bug: 178469391
Change-Id: I614f3e87519236c4e2c5da74937cb0ecd98a278a
Signed-off-by: Elliot Berman <eberman@codeaurora.org>

Makefile

@@ -131,6 +131,20 @@ $(if $(word 2, $(KBUILD_EXTMOD)), \
 
 export KBUILD_EXTMOD
 
+# ANDROID: set up mixed-build support. mixed-build allows device kernel modules
+# to be compiled against a GKI kernel. This approach still uses the headers and
+# Kbuild from device kernel, so care must be taken to ensure that those headers match.
+ifdef KBUILD_MIXED_TREE
+# Need vmlinux.symvers for modpost and System.map for depmod, check whether they exist in KBUILD_MIXED_TREE
+required_mixed_files=vmlinux.symvers System.map
+$(if $(filter-out $(words $(required_mixed_files)), \
+		$(words $(wildcard $(add-prefix $(KBUILD_MIXED_TREE)/,$(required_mixed_files))))),,\
+	$(error KBUILD_MIXED_TREE=$(KBUILD_MIXED_TREE) doesn't contain $(required_mixed_files)))
+endif
+
+mixed-build-prefix = $(if $(KBUILD_MIXED_TREE),$(KBUILD_MIXED_TREE)/)
+export KBUILD_MIXED_TREE
+
 # Kbuild will save output files in the current working directory.
 # This does not need to match to the root of the kernel source tree.
 #
@@ -655,11 +669,13 @@ drivers-y	+= virt/
 libs-y		:= lib/
 endif # KBUILD_EXTMOD
 
+ifndef KBUILD_MIXED_TREE
 # The all: target is the default when no target is given on the
 # command line.
 # This allow a user to issue only 'make' to build a kernel including modules
 # Defaults to vmlinux, but the arch makefile usually adds further targets
 all: vmlinux
+endif
 
 CFLAGS_GCOV	:= -fprofile-arcs -ftest-coverage \
 	$(call cc-option,-fno-tree-loop-im) \
@@ -1232,8 +1248,10 @@ cmd_link-vmlinux =                                                 \
 	$(CONFIG_SHELL) $< "$(LD)" "$(KBUILD_LDFLAGS)" "$(LDFLAGS_vmlinux)";    \
 	$(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true)
 
+ifndef KBUILD_MIXED_TREE
 vmlinux: scripts/link-vmlinux.sh autoksyms_recursive $(vmlinux-deps) FORCE
 	+$(call if_changed,link-vmlinux)
+endif
 
 targets := vmlinux
 
@@ -1454,7 +1472,9 @@ endif
 # using awk while concatenating to the final file.
 
 PHONY += modules
-modules: $(if $(KBUILD_BUILTIN),vmlinux) modules_check modules_prepare
+# if KBUILD_BUILTIN && !KBUILD_MIXED_TREE, depend on vmlinux
+modules: $(if $(KBUILD_BUILTIN), $(if $(KBUILD_MIXED_TREE),,vmlinux))
+modules: modules_check modules_prepare
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost
 
 PHONY += modules_check
@@ -1488,8 +1508,8 @@ _modinst_:
 		ln -s $(CURDIR) $(MODLIB)/build ; \
 	fi
 	@sed 's:^:kernel/:' modules.order > $(MODLIB)/modules.order
-	@cp -f modules.builtin $(MODLIB)/
+	@cp -f $(mixed-build-prefix)modules.builtin $(MODLIB)/
-	@cp -f $(objtree)/modules.builtin.modinfo $(MODLIB)/
+	@cp -f $(or $(mixed-build-prefix),$(objtree)/)modules.builtin.modinfo $(MODLIB)/
 	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modinst
 
 # This depmod is only for convenience to give the initial
@@ -1868,7 +1888,7 @@ descend: $(build-dirs)
 $(build-dirs): prepare
 	$(Q)$(MAKE) $(build)=$@ \
 	single-build=$(if $(filter-out $@/, $(filter $@/%, $(KBUILD_SINGLE_TARGETS))),1) \
-	need-builtin=1 need-modorder=1
+	$(if $(KBUILD_MIXED_TREE),,need-builtin=1) need-modorder=1
 
 clean-dirs := $(addprefix _clean_, $(clean-dirs))
 PHONY += $(clean-dirs) clean
@@ -2002,7 +2022,7 @@ quiet_cmd_rmfiles = $(if $(wildcard $(rm-files)),CLEAN   $(wildcard $(rm-files))
 # Run depmod only if we have System.map and depmod is executable
 quiet_cmd_depmod = DEPMOD  $(KERNELRELEASE)
       cmd_depmod = $(CONFIG_SHELL) $(srctree)/scripts/depmod.sh $(DEPMOD) \
-                   $(KERNELRELEASE)
+                   $(KERNELRELEASE) $(mixed-build-prefix)
 
 # read saved command lines for existing targets
 existing-targets := $(wildcard $(sort $(targets)))

scripts/Makefile.modpost

@@ -46,6 +46,8 @@ include scripts/Kbuild.include
 # for ld_flags
 include scripts/Makefile.lib
 
+mixed-build-prefix = $(if $(KBUILD_MIXED_TREE),$(KBUILD_MIXED_TREE)/)
+
 MODPOST = scripts/mod/modpost								\
 	$(if $(CONFIG_MODVERSIONS),-m)							\
 	$(if $(CONFIG_MODULE_SRCVERSION_ALL),-a)					\
@@ -67,7 +69,7 @@ else
 
 ifeq ($(KBUILD_EXTMOD),)
 
-input-symdump := vmlinux.symvers
+input-symdump := $(mixed-build-prefix)vmlinux.symvers
 output-symdump := Module.symvers
 module_srcpath := $(srctree)
 

scripts/depmod.sh

@@ -3,14 +3,15 @@
 #
 # A depmod wrapper used by the toplevel Makefile
 
-if test $# -ne 2; then
+if test $# -ne 2 -a $# -ne 3; then
-	echo "Usage: $0 /sbin/depmod <kernelrelease>" >&2
+	echo "Usage: $0 /sbin/depmod <kernelrelease> [System.map folder]" >&2
 	exit 1
 fi
 DEPMOD=$1
 KERNELRELEASE=$2
+KBUILD_MIXED_TREE=$3
 
-if ! test -r System.map ; then
+if ! test -r ${KBUILD_MIXED_TREE}System.map ; then
 	echo "Warning: modules_install: missing 'System.map' file. Skipping depmod." >&2
 	exit 0
 fi
@@ -41,7 +42,7 @@ if $depmod_hack_needed; then
 	KERNELRELEASE=99.98.$KERNELRELEASE
 fi
 
-set -- -ae -F System.map
+set -- -ae -F ${KBUILD_MIXED_TREE}System.map
 if test -n "$INSTALL_MOD_PATH"; then
 	set -- "$@" -b "$INSTALL_MOD_PATH"
 fi