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:   Mon, 29 May 2017 22:07:41 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>,
        Babu Moger <babu.moger@...cle.com>
Cc:     "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:

> 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__ ?

cheers