[PATCH 08/20] build: add make variables for kernel source and object directories

[Elliott Mitchell]

Precursor to splitting the two apart.  Linux kernel sources can be
reused for multiple kernel builds with different configurations.
The object directory cannot be reused.

Signed-off-by: Elliott Mitchell <ehem+openwrt at m5p.com>
---
 include/kernel.mk | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/kernel.mk b/include/kernel.mk
index 82d87e8491..ff34f74bed 100644
--- a/include/kernel.mk
+++ b/include/kernel.mk
@@ -45,6 +45,10 @@ else
   endif
   KERNEL_BUILD_DIR ?= $(BUILD_DIR)/linux-$(BOARD)$(if $(SUBTARGET),_$(SUBTARGET))
   LINUX_DIR ?= $(KERNEL_BUILD_DIR)/linux-$(LINUX_VERSION)
+  # Location of kernel source files
+  LINUX_SRC_DIR ?= $(LINUX_DIR)
+  # Location of kernel object files
+  LINUX_OBJ_DIR ?= $(LINUX_DIR)
   LINUX_UAPI_DIR=uapi/
   LINUX_VERMAGIC:=$(strip $(shell cat $(LINUX_DIR)/.vermagic 2>/dev/null))
   LINUX_VERMAGIC:=$(if $(LINUX_VERMAGIC),$(LINUX_VERMAGIC),unknown)
@@ -107,6 +111,7 @@ KERNEL_MAKE_FLAGS = \
 	HOSTCFLAGS="$(HOST_CFLAGS) -Wall -Wmissing-prototypes -Wstrict-prototypes" \
 	CROSS_COMPILE="$(KERNEL_CROSS)" \
 	ARCH="$(LINUX_KARCH)" \
+	O="$(LINUX_OBJ_DIR)" \
 	KBUILD_HAVE_NLS=no \
 	KBUILD_BUILD_USER="$(call qstrip,$(CONFIG_KERNEL_BUILD_USER))" \
 	KBUILD_BUILD_HOST="$(call qstrip,$(CONFIG_KERNEL_BUILD_DOMAIN))" \
@@ -136,7 +141,7 @@ ifneq ($(HOST_OS),Linux)
   export SKIP_STACK_VALIDATION:=1
 endif
 
-KERNEL_MAKEOPTS = -C $(LINUX_DIR) $(KERNEL_MAKE_FLAGS)
+KERNEL_MAKEOPTS = -C $(LINUX_SRC_DIR) $(KERNEL_MAKE_FLAGS)
 
 ifdef CONFIG_USE_SPARSE
   KERNEL_MAKEOPTS += C=1 CHECK=$(STAGING_DIR_HOST)/bin/sparse
-- 
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