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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Date:   Tue, 30 May 2017 12:56:39 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     Babu Moger <babu.moger@...cle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        sparclinux <sparclinux@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        "devicetree\@vger.kernel.org" <devicetree@...r.kernel.org>,
        "linux-serial\@vger.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)

Geert Uytterhoeven <geert@...ux-m68k.org> writes:

> Hi Michael,
>
> On Mon, May 29, 2017 at 2:07 PM, Michael Ellerman <mpe@...erman.id.au> wrote:
>> Geert Uytterhoeven <geert@...ux-m68k.org> writes:
>>> On Tue, May 23, 2017 at 11:45 PM, Babu Moger <babu.moger@...cle.com> wrote:
>>>> Found this problem while enabling queued rwlock on SPARC.
>>>> The parameter CONFIG_CPU_BIG_ENDIAN is used to clear the
>>>> specific byte in qrwlock structure. Without this parameter,
>>>> we clear the wrong byte. Here is the code.
>>>>
>>>> static inline u8 *__qrwlock_write_byte(struct qrwlock *lock)
>>>>  {
>>>>         return (u8 *)lock + 3 * IS_BUILTIN(CONFIG_CPU_BIG_ENDIAN);
>>>>  }
>>>>
>>>> Define CPU_BIG_ENDIAN for SPARC to fix it.
>>>
>>>> --- a/arch/sparc/Kconfig
>>>> +++ b/arch/sparc/Kconfig
>>>> @@ -92,6 +92,10 @@ config ARCH_DEFCONFIG
>>>>  config ARCH_PROC_KCORE_TEXT
>>>>         def_bool y
>>>>
>>>> +config CPU_BIG_ENDIAN
>>>> +       bool
>>>> +       default y if SPARC
>>>
>>> Nice catch!
>>>
>>> Traditionally, CPU_BIG_ENDIAN and CPU_LITTLE_ENDIAN were defined only on
>>> architectures that may support both.  And it was checked in platform code
>>> and drivers only.
>>> Hence the symbol is lacking from most architectures. Heck, even
>>> architectures that support both may default to one endiannes, and declare
>>> only the symbol for the other endianness:
>>
>> I guess there's a reason we can't use __BIG_ENDIAN__ / __LITTLE_ENDIAN__ ?
>
> I (C/asm) code we can, in Kconfig we cannot.
>
> So far we tried always doing that, but a few checks for the semi-existing
> Kconfig symbol crept in in generic code. Those could be replaced by the __*__
> variants, but consistently having the Kconfig symbols would be useful anyway
> (e.g. to avoid building the broken-on-big-endian ISDN drivers).

Ah OK, the original mail was citing C code, but yeah I guess it would be
handy in Makefiles etc.

cheers