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

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Message-ID: <CAKwvOd=-u3grX3O4CtBayJYhv=mmsxMrRTF=AMcKMbphN5Xkgg@mail.gmail.com>
Date:   Mon, 13 Apr 2020 12:23:38 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Russell King - ARM Linux admin <linux@...linux.org.uk>
Cc:     Ard Biesheuvel <ardb@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Jian Cai <caij2003@...il.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Peter Smith <Peter.Smith@....com>,
        Stefan Agner <stefan@...er.ch>,
        David Howells <dhowells@...hat.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Manoj Gupta <manojgupta@...gle.com>,
        Benjamin Gaignard <benjamin.gaignard@...aro.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Ilie Halip <ilie.halip@...il.com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Krzysztof Kozlowski <krzk@...nel.org>,
        Bartosz Golaszewski <bgolaszewski@...libre.com>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        "Steven Rostedt (VMware)" <rostedt@...dmis.org>,
        Jian Cai <jiancai@...gle.com>,
        Doug Anderson <armlinux@...isordat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Patrick Bellasi <patrick.bellasi@....com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Tejun Heo <tj@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] ARM: do not assemble iwmmxt.S with LLVM toolchain

On Fri, Apr 10, 2020 at 5:33 AM Russell King - ARM Linux admin
<linux@...linux.org.uk> wrote:
>
> For older CPUs, it doesn't matter what the latest ARM ARM says, the
> appropriate version of the ARM ARM is the one relevant for the CPU
> architecture.  This is a mistake frequently made, and it's been pointed
> out by Arm Ltd in the past (before ARMv6 even came on the scene) that
> keeping older revisions is necessary if you want to be interested in
> the older architectures.

As if it never existed *waves hands*.  Interesting.  Does ARM still
distribute these older reference manuals? Do you keep copies of the
older revisions?

>
> However, there's an additional complication here: DEC's license from
> Arm Ltd back in the days of StrongARM allowed them to make changes to
> the architecture - that was passed over to Intel when they bought that
> part of DEC.  Consequently, these "non-Arm vendor" cores contain
> extensions that are not part of the ARM ARM.  iWMMXT is one such
> example, which first appeared in the Intel PXA270 SoC (an ARMv5
> derived CPU).
>
> In fact, several of the features found in later versions of the ARM
> architecture came from DEC and Intel enhancements.
>
> If your compiler/assembler only implements what is in the latest ARM
> ARM, then it is not going to be suitable for these older CPUs and
> alternate vendor "ARM compatible" CPUs.

This is a neat piece of history, thanks for sharing.
-- 
Thanks,
~Nick Desaulniers