linux-kernel - Re: [PATCH] kbuild: initialize CLANG_FLAGS correctly in the top Makefile

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Message-ID: <CAK7LNASyjUbM1ij8E1tO3uD3Rpi_qC6-X4=LKR+ZKh5g6vvBnQ@mail.gmail.com>
Date:   Wed, 31 Jul 2019 01:45:56 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>
Cc:     Stephen Boyd <swboyd@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        stable <stable@...r.kernel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] kbuild: initialize CLANG_FLAGS correctly in the top Makefile

On Mon, Jul 29, 2019 at 6:16 PM Masahiro Yamada
<yamada.masahiro@...ionext.com> wrote:
>
> CLANG_FLAGS is initialized by the following line:
>
>   CLANG_FLAGS     := --target=$(notdir $(CROSS_COMPILE:%-=%))
>
> ..., which is run only when CROSS_COMPILE is set.
>
> Some build targets (bindeb-pkg etc.) recurse to the top Makefile.
>
> When you build the kernel with Clang but without CROSS_COMPILE,
> the same compiler flags such as -no-integrated-as are accumulated
> into CLANG_FLAGS.
>
> If you run 'make CC=clang' and then 'make CC=clang bindeb-pkg',
> Kbuild will recompile everything needlessly due to the build command
> change.
>
> Fix this by correctly initializing CLANG_FLAGS.
>
> Fixes: 238bcbc4e07f ("kbuild: consolidate Clang compiler flags")
> Cc: <stable@...r.kernel.org> # v4.20+
> Signed-off-by: Masahiro Yamada <yamada.masahiro@...ionext.com>
> ---

Applied to linux-kbuild.



>
>  Makefile | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/Makefile b/Makefile
> index fa0fbe7851ea..5ee6f6889869 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -472,6 +472,7 @@ KBUILD_CFLAGS_MODULE  := -DMODULE
>  KBUILD_LDFLAGS_MODULE := -T $(srctree)/scripts/module-common.lds
>  KBUILD_LDFLAGS :=
>  GCC_PLUGINS_CFLAGS :=
> +CLANG_FLAGS :=
>
>  export ARCH SRCARCH CONFIG_SHELL HOSTCC KBUILD_HOSTCFLAGS CROSS_COMPILE AS LD CC
>  export CPP AR NM STRIP OBJCOPY OBJDUMP PAHOLE KBUILD_HOSTLDFLAGS KBUILD_HOSTLDLIBS
> @@ -519,7 +520,7 @@ endif
>
>  ifneq ($(shell $(CC) --version 2>&1 | head -n 1 | grep clang),)
>  ifneq ($(CROSS_COMPILE),)
> -CLANG_FLAGS    := --target=$(notdir $(CROSS_COMPILE:%-=%))
> +CLANG_FLAGS    += --target=$(notdir $(CROSS_COMPILE:%-=%))
>  GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit))
>  CLANG_FLAGS    += --prefix=$(GCC_TOOLCHAIN_DIR)
>  GCC_TOOLCHAIN  := $(realpath $(GCC_TOOLCHAIN_DIR)/..)
> --
> 2.17.1
>


-- 
Best Regards
Masahiro Yamada