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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Fri, 10 Apr 2020 11:56:16 +0200
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Subject: Re: [PATCH] ARM: do not assemble iwmmxt.S with LLVM toolchain
To:     Jian Cai <caij2003@gmail.com>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Manoj Gupta <manojgupta@google.com>, Peter.Smith@arm.com,
        Stefan Agner <stefan@agner.ch>,
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        Russell King <linux@armlinux.org.uk>,
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        Andrew Morton <akpm@linux-foundation.org>,
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        Masahiro Yamada <masahiroy@kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
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On Fri, Apr 10, 2020 at 1:28 AM Jian Cai <caij2003@gmail.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@gmail.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