linux-kernel - Re: CPU_BIG_ENDIAN in generic code (was: Re: [PATCH v3 3/7] arch/sparc: Define config parameter CPU_BIG

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Message-ID: <CAK8P3a2gk07SW2nGZdNXSzRwx5+Ap5hDf4wDksunLtYkpC5WOg@mail.gmail.com>
Date:   Wed, 24 May 2017 17:09:01 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Babu Moger <babu.moger@...cle.com>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        "David S. Miller" <davem@...emloft.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        sparclinux <sparclinux@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-serial@...r.kernel.org" <linux-serial@...r.kernel.org>
Subject: Re: CPU_BIG_ENDIAN in generic code (was: Re: [PATCH v3 3/7]
 arch/sparc: Define config parameter CPU_BIG_ENDIAN)

On Wed, May 24, 2017 at 4:45 PM, Babu Moger <babu.moger@...cle.com> wrote:
> On 5/24/2017 5:18 AM, Arnd Bergmann wrote:
>> On Wed, May 24, 2017 at 11:59 AM, Geert Uytterhoeven
>> <geert@...ux-m68k.org> wrote:
>>> On Tue, May 23, 2017 at 11:45 PM, Babu Moger <babu.moger@...cle.com>
>>> wrote:
>>> include/asm-generic/qrwlock.h is also generic, but depends on the
>>> architecture to select ARCH_USE_QUEUED_RWLOCKS, which only very few do
>>> (x86, and now sparc).
>>>
>>> I guess the time is ripe for adding (both) symbols to all architectures?
>>
>> Good idea. I think we can do most of this by adding a few lines to
>> arch/Kconfig:
>>
>> config CPU_BIG_ENDIAN
>>          bool
>>
>> config CPU_LITTLE_ENDIAN
>>         def_bool !CPU_BIG_ENDIAN
>
> I noticed that even x86 does not define CPU_LITTLE_ENDIAN.  Strange.

There is no architecture-independent code that tests for
CONFIG_CPU_LITTLE_ENDIAN, unlike CONFIG_CPU_BIG_ENDIAN,
so that's not very suprising.

> With this code all the architecture will default to
> CONFIG_CPU_LITTLE_ENDIAN.

What I meant is that we have to 'select CPU_BIG_ENDIAN' on all architectures
that actually are big-endian:

These are all configurable:
$ git grep -l linux/byteorder/big_endian.h | xargs grep -l
linux/byteorder/little_endian.h
arch/arc/include/uapi/asm/byteorder.h
arch/arm/include/uapi/asm/byteorder.h
arch/arm64/include/uapi/asm/byteorder.h
arch/c6x/include/uapi/asm/byteorder.h
arch/m32r/include/uapi/asm/byteorder.h
arch/microblaze/include/uapi/asm/byteorder.h
arch/mips/include/uapi/asm/byteorder.h
arch/powerpc/include/uapi/asm/byteorder.h
arch/sh/include/uapi/asm/byteorder.h
arch/tile/include/uapi/asm/byteorder.h

These are always big-endian:
$ git grep -l linux/byteorder/big_endian.h | xargs grep -L
linux/byteorder/little_endian.h
arch/avr32/include/uapi/asm/byteorder.h
arch/frv/include/uapi/asm/byteorder.h
arch/m68k/include/uapi/asm/byteorder.h
arch/openrisc/include/uapi/asm/byteorder.h
arch/parisc/include/uapi/asm/byteorder.h
arch/s390/include/uapi/asm/byteorder.h
arch/sparc/include/uapi/asm/byteorder.h

And these are always little-endian:
arch/alpha/include/uapi/asm/byteorder.h
arch/blackfin/include/uapi/asm/byteorder.h
arch/cris/include/uapi/asm/byteorder.h
arch/hexagon/include/uapi/asm/byteorder.h
arch/ia64/include/uapi/asm/byteorder.h
arch/metag/include/uapi/asm/byteorder.h
arch/mn10300/include/uapi/asm/byteorder.h
arch/score/include/uapi/asm/byteorder.h
arch/unicore32/include/uapi/asm/byteorder.h
arch/x86/include/uapi/asm/byteorder.h

So if we 'select CPU_BIG_ENDIAN' from avr32, frv, m68k, openrisc, parisc,
s390 and sparc, this covers all the fixed-endian architectures, and the
other ones are those that already have either CPU_BIG_ENDIAN
as a 'bool' option, or both as a 'choice'.

> I can make it as a separate patch. But I can only test SPARC and little bit
> of x86. Is that ok?

I think that's ok.

        Arnd