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

[Michael Ellerman]

Dmitry Vyukov <dvyukov at google.com> writes:
> From: Arnd Bergmann <arnd at arndb.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 at google.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.

OK :)

We did have some bugs in the past (~1-2 y/ago) but AFAIK they are all
fixed now. These days I build most of my kernels with a bi-endian 64-bit
toolchain, and switching endian without running `make clean` also works.

cheers