linux-kernel - Re: [PATCH v6 3/3] Documentation/llvm: update CROSS

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Date:   Mon, 2 Aug 2021 12:16:21 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Masahiro Yamada <masahiroy@...nel.org>
Cc:     Miguel Ojeda <ojeda@...nel.org>, Fangrui Song <maskray@...gle.com>,
        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 v6 3/3] Documentation/llvm: update CROSS_COMPILE
 inferencing

On 8/2/2021 11:39 AM, 'Nick Desaulniers' via Clang Built Linux 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.
> 
> Change use of env vars to command line parameters.
> 
> Suggested-by: Masahiro Yamada <masahiroy@...nel.org>
> Reviewed-by: Fangrui Song <maskray@...gle.com>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>

Thanks for the update, I think this is much easier for non-CBL folks to 
understand.

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
> Changes v5 -> v6:
> * Pick up Fangrui's RB tag.
> * Change use of env vars to command line parameters for consistency.
> 
>   Documentation/kbuild/llvm.rst | 19 ++++++++++++++++++-
>   1 file changed, 18 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/kbuild/llvm.rst b/Documentation/kbuild/llvm.rst
> index b18401d2ba82..f8a360958f4c 100644
> --- a/Documentation/kbuild/llvm.rst
> +++ b/Documentation/kbuild/llvm.rst
> @@ -38,7 +38,7 @@ Cross Compiling
>   A single Clang compiler binary will typically contain all supported backends,
>   which can help simplify cross compiling. ::
>   
> -	ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- make CC=clang
> +	make ARCH=arm64 CC=clang CROSS_COMPILE=aarch64-linux-gnu-
>   
>   ``CROSS_COMPILE`` is not used to prefix the Clang compiler binary, instead
>   ``CROSS_COMPILE`` is used to set a command line flag: ``--target=<triple>``. For
> @@ -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 linker.
> +
> +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::
> +
> +	make ARCH=arm64 LLVM=1 LLVM_IAS=1
> +
>   Supported Architectures
>   -----------------------
>   
>