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

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Date:   Tue, 10 Nov 2020 22:35:27 +0100
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Nick Desaulniers <ndesaulniers@...gle.com>,
        Jian Cai <jiancai@...gle.com>,
        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, 10 Nov 2020 at 21:38, Arnd Bergmann <arnd@...nel.org> wrote:
>
> 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!
>

Thanks for the perspective. I wasn't aware of all these details, but I
just wanted to confirm that .inst works as expected in all
configurations.