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SubjectRe: [PATCH v2] kbuild: Add better clang cross build support
Hi Matthias,


2017-04-11 6:09 GMT+09:00 Matthias Kaehlcke <mka@chromium.org>:


> COMPILER is previously set to "clang" if CC=clang was set from the
> make command line. So -target and -gcc-toolchain can be added to CC,
> since we already know that it is set.

Can you reword or delete this paragraph?
I think this is confusing because COMPILER was replaced with cc-name
by commit 5631d9c4.




> Clang will fallback to using things like ld, as, and libgcc if
> (respectively) one of the llvm linkers isn't available, the integrated
> assembler is turned off, or an appropriately cross-compiled version of
> compiler-rt isn't available. To this end, you can specify the path to
> this fallback gcc toolchain with GCC_TOOLCHAIN.
>
> From: Behan Webster <behanw@converseincode.com>
> Signed-off-by: Behan Webster <behanw@converseincode.com>
> Reviewed-by: Jan-Simon Möller <dl9pf@gmx.de>
> Reviewed-by: Mark Charlebois <charlebm@gmail.com>
> Signed-off-by: Greg Hackmann <ghackmann@google.com>
> Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> ---
> Changes in v2:
> - Fix GCC_TOOLCHAIN definition. $(LD) is needed to link the kernel, use
> its path to determine the installation prefix of the toolchain.
> (From: Greg Hackmann <ghackmann@google.com>)
>
> Makefile | 18 ++++++++++++++----
> 1 file changed, 14 insertions(+), 4 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index e11989d36c87..d3a02786bb53 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -375,6 +375,16 @@ LDFLAGS_vmlinux =
> CFLAGS_GCOV = -fprofile-arcs -ftest-coverage -fno-tree-loop-im -Wno-maybe-uninitialized
> CFLAGS_KCOV := $(call cc-option,-fsanitize-coverage=trace-pc,)
>
> +ifeq ($(cc-name),clang)
> +ifneq ($(CROSS_COMPILE),)
> +CLANG_TARGET := -target $(notdir $(CROSS_COMPILE:%-=%))
> +GCC_TOOLCHAIN := $(realpath $(dir $(shell which $(LD)))/..)
> +endif
> +ifneq ($(GCC_TOOLCHAIN),)
> +CLANG_GCC_TC := -gcc-toolchain $(GCC_TOOLCHAIN)
> +endif
> +CLANG_FLAGS := $(CLANG_TARGET) $(CLANG_GCC_TC)
> +endif
>
> # Use USERINCLUDE when you must reference the UAPI directories only.
> USERINCLUDE := \
> @@ -395,18 +405,18 @@ LINUXINCLUDE := \
>
> LINUXINCLUDE += $(filter-out $(LINUXINCLUDE),$(USERINCLUDE))
>
> -KBUILD_CPPFLAGS := -D__KERNEL__
> +KBUILD_CPPFLAGS := -D__KERNEL__ $(CLANG_FLAGS)
>
> KBUILD_CFLAGS := -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs \
> -fno-strict-aliasing -fno-common \
> -Werror-implicit-function-declaration \
> -Wno-format-security \
> - -std=gnu89 $(call cc-option,-fno-PIE)
> -
> + -std=gnu89 $(call cc-option,-fno-PIE) \
> + $(CLANG_FLAGS)
>
> KBUILD_AFLAGS_KERNEL :=
> KBUILD_CFLAGS_KERNEL :=
> -KBUILD_AFLAGS := -D__ASSEMBLY__ $(call cc-option,-fno-PIE)
> +KBUILD_AFLAGS := -D__ASSEMBLY__ $(call cc-option,-fno-PIE) $(CLANG_FLAGS)


This adds multiple -gcc-toolchain because both KBUILD_CPPFLAGS and KBUILD_CFLAGS
are added to c_flags. See scripts/Makefile.lib

orig_c_flags = $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) $(KBUILD_SUBDIR_CCFLAGS) \
$(ccflags-y) $(CFLAGS_$(basetarget).o)


Pre-processing is related to the front-end part,
so -target and -gcc-toolchain are not necessary for KBUILD_CPPFLAGS?

Maybe, adding it to KBUILD_CFLAGS and KBUILD_AFLAGS could be enough?




Is it possible to move these to around line 700?

ifeq ($(cc-name),clang)
< add here ? >
KBUILD_CPPFLAGS += $(call cc-option,-Qunused-arguments,)
KBUILD_CPPFLAGS += $(call cc-option,-Wno-unknown-warning-option,)



I do not want to sprinkle ugly ifeq ($(cc-name,clang).





--
Best Regards
Masahiro Yamada

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