linux-kernel - Re: [PATCH] ARM: do not assemble iwmmxt.S with LLVM toolchain

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Date:   Fri, 10 Apr 2020 11:56:16 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Jian Cai <caij2003@...il.com>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Manoj Gupta <manojgupta@...gle.com>, Peter.Smith@....com,
        Stefan Agner <stefan@...er.ch>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Ilie Halip <ilie.halip@...il.com>, jiancai@...gle.com,
        Russell King <linux@...linux.org.uk>,
        Linus Walleij <linus.walleij@...aro.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Doug Anderson <armlinux@...isordat.com>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        David Howells <dhowells@...hat.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH] ARM: do not assemble iwmmxt.S with LLVM toolchain

On Fri, Apr 10, 2020 at 1:28 AM Jian Cai <caij2003@...il.com> wrote:
>
> iwmmxt.S contains XScale instructions LLVM ARM backend does not support.
> Skip this file if LLVM integrated assemmbler or LLD is used to build ARM
> kernel.
>
> Signed-off-by: Jian Cai <caij2003@...il.com>

It clearly makes sense to limit the Kconfig option to compilers that
can actually build it.
A few questions though:

- Given that Armada XP with its PJ4B was still marketed until fairly
recently[1],
  wouldn't it make sense to still add support for it? Is it a lot of work?

- Why does the linker have to understand it, rather than just the assembler?

> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 66a04f6f4775..39de8fc64a73 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -804,7 +804,7 @@ source "arch/arm/mm/Kconfig"
>
>  config IWMMXT
>         bool "Enable iWMMXt support"
> -       depends on CPU_XSCALE || CPU_XSC3 || CPU_MOHAWK || CPU_PJ4 || CPU_PJ4B
> +       depends on !AS_IS_CLANG && !LD_IS_LLD && (CPU_XSCALE || CPU_XSC3 || CPU_MOHAWK || CPU_PJ4 || CPU_PJ4B)

I would suggest splitting it into two lines for readability:

       depends on  CPU_XSCALE || CPU_XSC3 || CPU_MOHAWK || CPU_PJ4 || CPU_PJ4B
       depends on !AS_IS_CLANG && !LD_IS_LLD

    Arnd

[1] http://web.archive.org/web/20191015165247/https://www.marvell.com/embedded-processors/armada/index.jsp