linux-kernel - Re: [PATCH v2] Make iwmmxt.S support Clang's integrated assembler

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Date:   Tue, 10 Nov 2020 21:38:01 +0100
From:   Arnd Bergmann <arnd@...nel.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Jian Cai <jiancai@...gle.com>, Ard Biesheuvel <ardb@...nel.org>,
        Manoj Gupta <manojgupta@...gle.com>,
        Luis Lozano <llozano@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Russell King <linux@...linux.org.uk>,
        Nathan Chancellor <natechancellor@...il.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] Make iwmmxt.S support Clang's integrated assembler

On Tue, Nov 10, 2020 at 9:11 PM 'Nick Desaulniers' via Clang Built
Linux <clang-built-linux@...glegroups.com> wrote:
>
> On Tue, Nov 10, 2020 at 12:10 PM Jian Cai <jiancai@...gle.com> wrote:
> >
> > I tried to verify with ixp4xx_defconfig, and I noticed it also used CONFIG_CPU_BIG_ENDIAN=y to enable big endianness as follows,
> >
> > linux$ grep ENDIAN arch/arm/configs/ixp4xx_defconfig
> > CONFIG_CPU_BIG_ENDIAN=y
> >
> > Also it appeared arch/arm/kernel/iwmmxt.o was not built with ixp4xx_defconfig. The commands I used
> >
> > linux$ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make CC=clang ixp4xx_defconfig
> > linux$ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make -j32 CC=clang
> > linux$ ls arch/arm/kernel/iwmmxt.o
> > ls: cannot access 'arch/arm/kernel/iwmmxt.o': No such file or directory
> >
> > Did I miss any steps?
>
> Yes, you need to manually enable CONFIG_IWMMXT in menuconfig or via
> `scripts/configs -e`.

To clarify: ixp4xx and pxa3xx were two platforms based on the XScale core.
ixp4xx was commonly used in big-endian mode but lacked iWMMXt. pxa3xx
had iWMMXt but doesn't allow enabling big-endian mode because of a Kconfig
dependency, meaning that nobody has ever tried it, and it's likely broken.

Later 'mvebu' parts (Armada 510) do have iWMMXt and allow big-endian
mode, but those are BE8, with non-reversed byteorder for the instructions.

So none of this matters in practice, but it's very satifiying to know
it is finally
all working with the integrated assembler in all those combinations, at
least in theory!

      Arnd