linux-kernel - Re: [PATCH] [RFC] #define __BYTE

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Message-ID: <4BAA5F99.3050904@caviumnetworks.com>
Date:	Wed, 24 Mar 2010 11:53:13 -0700
From:	David Daney <ddaney@...iumnetworks.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Joakim Tjernlund <Joakim.Tjernlund@...nsmode.se>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] [RFC] #define __BYTE_ORDER

On 03/24/2010 11:37 AM, Geert Uytterhoeven wrote:
> On Wed, Mar 24, 2010 at 19:21, Andrew Morton<akpm@...ux-foundation.org>  wrote:
>> On Wed, 17 Mar 2010 19:10:55 +0100
>> Joakim Tjernlund<Joakim.Tjernlund@...nsmode.se>  wrote:
>>
>>> Linux does not define __BYTE_ORDER in its endian header files
>>> which makes some header files bend backwards to get at the
>>> current endian. Lets #define __BYTE_ORDER in big_endian.h/litte_endian.h
>>> to make it easier for header files that are used in user space too.
>>
>> I don't get it.  Why not nuke __BYTE_ORDER altogether and do `#ifdef
>> __LITTLE_ENDIAN' and `#ifdef __BIG_ENDIAN' everywhere?
>
> Because in userspace the convention is that
>    1. _both_ __LITTLE_ENDIAN and __BIG_ENDIAN are defined,
>    2. you have to test for e.g. __BYTE_ORDER == __BIG_ENDIAN.
>

I have stumbled on this issue as well.

However, consider this:

If you make such a change, then you will start to see:

#if __BYTE_ORDER == __BIG_ENDIAN

appearing in kernel source code.  Do we want two different endian 
checking idioms in the kernel?  Or would it be just a single idiom, but 
one that is different than the status quo?

The only time I can see that it makes a difference is if you want to 
share things like driver source code files between in-kernel drivers and 
userspace.  A discussion of which, would probably provoke much discussion.

David Daney


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