linux-kernel - Re: [PATCH v2] Makefile: Fix detection of clang when cross-compiling

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Date:	Mon, 13 Jul 2015 20:59:33 +1000
From:	Anton Blanchard <anton@...ba.org>
To:	Behan Webster <behanw@...verseincode.com>
Cc:	Michal Marek <mmarek@...e.cz>,
	Paul Cercueil <paul@...pouillou.net>,
	linux-kbuild@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH v2] Makefile: Fix detection of clang when
 cross-compiling

Hi,

>      > When the host's C compiler is clang, and when attempting to
>      > cross-compile Linux e.g. to MIPS with mipsel-linux-gcc, the
>      > Makefile would incorrectly detect the use of clang, which
>      > resulted in clang-specific flags being passed to
>      > mipsel-linux-gcc.
>      >
>      > This can be verified under Debian by installing the "clang"
>      > package, and then using it as the default compiler with:
>      > sudo update-alternatives --config cc
>      >
>      > This patch moves the detection of clang after the $(CC)
>      > variable is initialized to the name of the cross-compiler, so
>      > that the check applies
>      > to the cross-compiler and not the host's C compiler.
>      >
>      > v2: Move the detection of clang after the inclusion of the
>      > arch/*/Makefile (as they might set $(CROSS_COMPILE))
>      >
>      > Signed-off-by: Paul Cercueil <paul@...pouillou.net
>     <mailto:paul@...pouillou.net>>
> 
>     Applied to kbuild.git#kbuild. I will push it after v4.1-rc1
>     becomes available, though.
> 
> Drat. I wish I saw this earlier.
> 
> This breaks patches which check for the value of COMPILER in 
> arch/*/Makefile. This detection must be performed before the
> inclusion of the arch Makefile.
> 
> Can I move this to after the initialization of CC but before the
> include?
> 
> I'm not sure that being able to define the default compiler per arch
> is necessary. But I know I need to be able to add arch specific flags
> for clang.

I can confirm the patch breaks ppc64le clang builds.

Anton
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