linux-kernel - Re: [PATCH v5 3/3] Documentation/llvm: update CROSS

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Message-ID: <20210730231252.r6fi6c3i6obbcgfi@google.com>
Date:   Fri, 30 Jul 2021 16:12:52 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Masahiro Yamada <masahiroy@...nel.org>,
        Miguel Ojeda <ojeda@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Arnd Bergmann <arnd@...nel.org>, linux-kernel@...r.kernel.org,
        linux-kbuild@...r.kernel.org, clang-built-linux@...glegroups.com,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Christoph Hellwig <hch@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH v5 3/3] Documentation/llvm: update CROSS_COMPILE
 inferencing

On 2021-07-30, Nick Desaulniers wrote:
>As noted by Masahiro, document how we can generally infer CROSS_COMPILE
>(and the more specific details about --target and --prefix) based on
>ARCH.
>
>Suggested-by: Masahiro Yamada <masahiroy@...nel.org>
>Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
>---
> Documentation/kbuild/llvm.rst | 17 +++++++++++++++++
> 1 file changed, 17 insertions(+)
>
>diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
>index b18401d2ba82..4292f0686316 100644
>--- a/Documentation/kbuild/llvm.rst
>+++ b/Documentation/kbuild/llvm.rst
>@@ -63,6 +63,23 @@ They can be enabled individually. The full list of the parameters: ::
> Currently, the integrated assembler is disabled by default. You can pass
> ``LLVM_IAS=1`` to enable it.
>
>+Omitting CROSS_COMPILE
>+----------------------
>+
>+As explained above, ``CROSS_COMPILE`` is used to set ``--target=<triple>``.
>+
>+Unless ``LLVM_IAS=1`` is specified, ``CROSS_COMPILE`` is also used to derive
>+``--prefix=<path>`` to search for the GNU assembler.

and the GNU linker.

Reviewed-by: Fangrui Song <maskray@...gle.com>

>+If ``CROSS_COMPILE`` is not specified, the ``--target=<triple>`` is inferred
>+from ``ARCH``.
>+
>+That means if you use only LLVM tools, ``CROSS_COMPILE`` becomes unnecessary.
>+
>+For example, to cross-compile the arm64 kernel::
>+
>+	ARCH=arm64 make LLVM=1 LLVM_IAS=1

Is  ARCH=arm64 make LLVM=1 LLVM_IAS=1

preferred over

   make ARCH=arm64 LLVM=1 LLVM_IAS=1

?

> Supported Architectures
> -----------------------
>
>-- 
>2.32.0.554.ge1b32706d8-goog
>