linux-kernel - Re: [PATCH, RFC] byteorder: sanity check toolchain vs kernel endianess

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Date:   Mon, 13 May 2019 09:39:45 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Nick Kossifidis <mick@....forth.gr>,
        Christoph Hellwig <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH, RFC] byteorder: sanity check toolchain vs kernel endianess

From: Arnd Bergmann <arnd@...db.de>
Date: Sat, May 11, 2019 at 2:51 AM
To: Dmitry Vyukov
Cc: Nick Kossifidis, Christoph Hellwig, Linus Torvalds, Andrew Morton,
linux-arch, Linux Kernel Mailing List, linuxppc-dev

> On Fri, May 10, 2019 at 6:53 AM Dmitry Vyukov <dvyukov@...gle.com> wrote:
> > >
> > > I think it's good to have a sanity check in-place for consistency.
> >
> >
> > Hi,
> >
> > This broke our cross-builds from x86. I am using:
> >
> > $ powerpc64le-linux-gnu-gcc --version
> > powerpc64le-linux-gnu-gcc (Debian 7.2.0-7) 7.2.0
> >
> > and it says that it's little-endian somehow:
> >
> > $ powerpc64le-linux-gnu-gcc -dM -E - < /dev/null | grep BYTE_ORDER
> > #define __BYTE_ORDER__ __ORDER_LITTLE_ENDIAN__
> >
> > Is it broke compiler? Or I always hold it wrong? Is there some
> > additional flag I need to add?
>
> It looks like a bug in the kernel Makefiles to me. powerpc32 is always
> big-endian,
> powerpc64 used to be big-endian but is now usually little-endian. There are
> often three separate toolchains that default to the respective user
> space targets
> (ppc32be, ppc64be, ppc64le), but generally you should be able to build
> any of the
> three kernel configurations with any of those compilers, and have the Makefile
> pass the correct -m32/-m64/-mbig-endian/-mlittle-endian command line options
> depending on the kernel configuration. It seems that this is not happening
> here. I have not checked why, but if this is the problem, it should be
> easy enough
> to figure out.


Thanks! This clears a lot.
This may be a bug in our magic as we try to build kernel files outside
of make with own flags (required to extract parts of kernel
interfaces).
So don't spend time looking for the Makefile bugs yet.