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: <CAMo8BfJDRStMDGThMQhmZBQs2LLwxFOthuqQaC1qyws14gwhRA@mail.gmail.com>
Date:   Thu, 25 May 2017 15:43:41 -0700
From:   Max Filippov <jcmvbkbc@...il.com>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Babu Moger <babu.moger@...cle.com>,
        "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 Thu, May 25, 2017 at 3:27 PM, Max Filippov <jcmvbkbc@...il.com> wrote:
> On Wed, May 24, 2017 at 3:18 AM, Arnd Bergmann <arnd@...db.de> wrote:
>> On Wed, May 24, 2017 at 11:59 AM, Geert Uytterhoeven
>> <geert@...ux-m68k.org> wrote:
>>> 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
>>
>> This way, we only need to add 'select CPU_BIG_ENDIAN' to the
>> architectures that are always big-endian, and we don't need to
>> change anything for the ones that have a single 'CPU_BIG_ENDIAN'
>> option.
>>
>> The three architectures that have a 'choice' statement (mips, ppc and
>> sh) will have to convert, and m32r will have to replace the
>> option with the opposite one, which could break 'make oldconfig',
>> but nobody really cares about m32r any more.
>
> Xtensa may have either endianness and for xtensa we define
> CONFIG_CPU_BIG_ENDIAN or CONFIG_CPU_LITTLE_ENDIAN
> in the arch/xtensa/Makefile based on the value of the compiler builtin
> macro.

Also, in outside the Kconfig files there's much more instances of
__{BIG,LITTLE}_ENDIAN than CONFIG_CPU_{BIG,LITTLE}_ENDIAN:

$ git grep '__\(BIG\|LITTLE\)_ENDIAN' | wc -l
4537
$ git grep 'CONFIG_CPU_\(BIG\|LITTLE\)_ENDIAN' | wc -l
247

My understanding is that CONFIG_CPU_{BIG,LITTLE}_ENDIAN was
intended to be used only in Kconfig files, and perhaps all of its uses
outside should be replaced with __{BIG,LITTLE}_ENDIAN.

-- 
Thanks.
-- Max