[OE-core][PATCH 3/3] kernel-arch: add KERNEL_TOOLCHAIN for building the kernel with clang

[mark.yang@lge.com]

From: "mark.yang" <mark.yang@lge.com>

kernel-arch pins TOOLCHAIN to gcc, so a clang kernel needs TOOLCHAIN
and the KERNEL_* variables overridden per recipe. Modules and
make-mod-scripts then keep the gcc defaults and run make against the
shared kernel build tree with a different compiler: kbuild detects the
CC_VERSION_TEXT mismatch, re-runs syncconfig and silently drops
compiler-dependent options such as CONFIG_CFI_CLANG.

Add toolchain-clang variants of KERNEL_CC, KERNEL_LD and KERNEL_AR and
a KERNEL_TOOLCHAIN switch covering all kernel-arch recipes, so the
kernel, modules and make-mod-scripts change toolchain together. The
default stays gcc.

-fuse-ld is left out of KERNEL_CC because the kernel links with
KERNEL_LD directly and kbuild's -Werror=unused-command-line-argument
rejects the unused option. KERNEL_LD uses ld.lld when the distro has
ld-is-lld (clang-cross only installs the symlink then), GNU ld.bfd
otherwise.

Signed-off-by: mark.yang <mark.yang@lge.com>
---
 meta/classes-recipe/kernel-arch.bbclass | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/meta/classes-recipe/kernel-arch.bbclass b/meta/classes-recipe/kernel-arch.bbclass
index 7aea9cd3e8..7e83a09b1c 100644
--- a/meta/classes-recipe/kernel-arch.bbclass
+++ b/meta/classes-recipe/kernel-arch.bbclass
@@ -76,9 +76,20 @@ KERNEL_CC = "${CCACHE}${HOST_PREFIX}gcc ${HOST_CC_KERNEL_ARCH} \
  -ffile-prefix-map=${STAGING_KERNEL_DIR}=${KERNEL_SRC_PATH} \
  -ffile-prefix-map=${STAGING_KERNEL_BUILDDIR}=${KERNEL_SRC_PATH} \
 "
+KERNEL_CC:toolchain-clang = "${CCACHE}${HOST_PREFIX}clang ${HOST_CC_KERNEL_ARCH} \
+ ${DEBUG_PREFIX_MAP} \
+ -ffile-prefix-map=${STAGING_KERNEL_DIR}=${KERNEL_SRC_PATH} \
+ -ffile-prefix-map=${STAGING_KERNEL_BUILDDIR}=${KERNEL_SRC_PATH} \
+"
 KERNEL_LD = "${HOST_PREFIX}ld.bfd ${HOST_LD_KERNEL_ARCH}"
+KERNEL_LD:toolchain-clang = "${@bb.utils.contains('DISTRO_FEATURES', 'ld-is-lld', '${HOST_PREFIX}ld.lld', '${HOST_PREFIX}ld.bfd', d)} ${HOST_LD_KERNEL_ARCH}"
 KERNEL_AR = "${HOST_PREFIX}ar ${HOST_AR_KERNEL_ARCH}"
+KERNEL_AR:toolchain-clang = "${HOST_PREFIX}llvm-ar ${HOST_AR_KERNEL_ARCH}"
 KERNEL_OBJCOPY = "${HOST_PREFIX}objcopy ${HOST_OBJCOPY_KERNEL_ARCH}"
 # Code in package.py can't handle options on KERNEL_STRIP
 KERNEL_STRIP = "${HOST_PREFIX}strip"
-TOOLCHAIN = "gcc"
+# The kernel, out-of-tree modules and make-mod-scripts share one kernel build
+# tree and must agree on the toolchain, so switch it with KERNEL_TOOLCHAIN
+# rather than setting TOOLCHAIN on individual recipes.
+KERNEL_TOOLCHAIN ?= "gcc"
+TOOLCHAIN = "${KERNEL_TOOLCHAIN}"