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Date: Mon, 17 Sep 2018 20:11:26 -0700
From: Stefan Agner <stefan@agner.ch>
Subject: Re: [PATCH] kbuild: allow to use GCC toolchain not in Clang search
 path
In-Reply-To: <CAK7LNASz4zabEdjO2AVscWbqGyjpEBE6Li7busC_sbYkuuJ7tQ@mail.gmail.com>
References: <20180918023157.23951-1-stefan@agner.ch>
 <CAK7LNASz4zabEdjO2AVscWbqGyjpEBE6Li7busC_sbYkuuJ7tQ@mail.gmail.com>
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To: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Michal Marek <michal.lkml@markovi.net>, Nick Desaulniers <ndesaulniers@google.com>, Arnd Bergmann <arnd@arndb.de>, Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>

On 17.09.2018 20:06, Masahiro Yamada wrote:
> Hi Stefan,
> 
> 2018-09-18 11:31 GMT+09:00 Stefan Agner <stefan@agner.ch>:
>> When using a GCC cross toolchain which is not in a compiled in
>> Clang search path, Clang reverts to the system assembler and
>> linker. This leads to assembler or linker errors, depending on
>> which tool is first used for a given architecture.
>>
>> It seems that Clang is not searching $PATH for a matching
>> assembler or linker.
> 
> 
> The current code passes the absolute path of toolchains
> to --gcc-toolchain option.
> In my understanding, this is enough for Clang
> to find the proper linker and the assembler.
> 
> 
> Why is the --prefix option also needed ?
> 

It seems that Clang makes a difference between "toolchain" and search path for toolchain binaries.

Without this patch building fails for the described use case, see the github link for more context.

--
Stefan


> 
> Am I missing something?
> 
> 
> 
>> Make sure that Clang picks up the correct assembler or linker by
>> passing the cross compilers bin directory as search path.
>>
>> This allows to use Clang provided by distributions with GCC
>> toolchains not in /usr/bin.
>>
>> Link: https://github.com/ClangBuiltLinux/linux/issues/78
>> Signed-off-by: Stefan Agner <stefan@agner.ch>
>> ---
>> Nick, I removed your Reviewed-by since I had to change variable
>> assignment slightly...
>>
>>  Makefile | 8 +++++---
>>  1 file changed, 5 insertions(+), 3 deletions(-)
>>
>> diff --git a/Makefile b/Makefile
>> index 4d5c883a98e5..d5de2db4b549 100644
>> --- a/Makefile
>> +++ b/Makefile
>> @@ -495,13 +495,15 @@ endif
>>  ifeq ($(cc-name),clang)
>>  ifneq ($(CROSS_COMPILE),)
>>  CLANG_TARGET   := --target=$(notdir $(CROSS_COMPILE:%-=%))
>> -GCC_TOOLCHAIN  := $(realpath $(dir $(shell which $(LD)))/..)
>> +GCC_TOOLCHAIN_DIR := $(dir $(shell which $(LD)))
>> +CLANG_PREFIX   := --prefix=$(GCC_TOOLCHAIN_DIR)
>> +GCC_TOOLCHAIN  := $(realpath $(GCC_TOOLCHAIN_DIR)/..)
>>  endif
>>  ifneq ($(GCC_TOOLCHAIN),)
>>  CLANG_GCC_TC   := --gcc-toolchain=$(GCC_TOOLCHAIN)
>>  endif
>> -KBUILD_CFLAGS += $(CLANG_TARGET) $(CLANG_GCC_TC)
>> -KBUILD_AFLAGS += $(CLANG_TARGET) $(CLANG_GCC_TC)
>> +KBUILD_CFLAGS += $(CLANG_TARGET) $(CLANG_GCC_TC) $(CLANG_PREFIX)
>> +KBUILD_AFLAGS += $(CLANG_TARGET) $(CLANG_GCC_TC) $(CLANG_PREFIX)
>>  KBUILD_CFLAGS += $(call cc-option, -no-integrated-as)
>>  KBUILD_AFLAGS += $(call cc-option, -no-integrated-as)
>>  endif
>> --
>> 2.19.0
>>