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

From: Geert Uytterhoeven <geert@linux-m68k.org>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: Babu Moger <babu.moger@oracle.com>,
	"David S. Miller" <davem@davemloft.net>,
	Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Arnd Bergmann <arnd@arndb.de>,
	sparclinux <sparclinux@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Linux-Arch <linux-arch@vger.kernel.org>,
	"devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
	"linux-serial@vger.kernel.org" <linux-serial@vger.kernel.org>
Subject: Re: CPU_BIG_ENDIAN in generic code (was: Re: [PATCH v3 3/7] arch/sparc: Define config parameter CPU_BIG_ENDIAN)
Date: Mon, 29 May 2017 14:15:35 +0200	[thread overview]
Message-ID: <CAMuHMdU1kne4tKRfyx0hr1LXh_c75bYMs0b=vncHqrhB69jqYg@mail.gmail.com> (raw)
In-Reply-To: <87a85v7to2.fsf@concordia.ellerman.id.au>

Hi Michael,

On Mon, May 29, 2017 at 2:07 PM, Michael Ellerman <mpe@ellerman.id.au> wrote:
> Geert Uytterhoeven <geert@linux-m68k.org> writes:
>> On Tue, May 23, 2017 at 11:45 PM, Babu Moger <babu.moger@oracle.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).

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds