linux-kernel - Re: [linux-next][PATCH] revert headers

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Message-ID: <20090206163314.GA11216@elte.hu>
Date:	Fri, 6 Feb 2009 17:33:14 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
Cc:	Jaswinder Singh Rajput <jaswinder@...nel.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Jaswinder Singh Rajput <jaswinderlinux@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Sam Ravnborg <sam@...nborg.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	hskinnemoen@...el.com, cooloney@...nel.org, tony.luck@...el.com,
	ralf@...ux-mips.org, dhowells@...hat.com, matthew@....cx,
	chris@...kel.net, LKML <linux-kernel@...r.kernel.org>,
	linux-next <linux-next@...r.kernel.org>,
	linux-ia64 <linux-ia64@...r.kernel.org>
Subject: Re: [linux-next][PATCH] revert headers_check fix: ia64, fpu.h


* Russell King - ARM Linux <linux@....linux.org.uk> wrote:

> On Fri, Feb 06, 2009 at 05:12:29PM +0100, Ingo Molnar wrote:
> > 
> > * Russell King - ARM Linux <linux@....linux.org.uk> wrote:
> > 
> > > > We cannot see any downside of this patch.
> > > > 
> > > > But we can see upside of this patch is:
> > > > 1. No need to protect linux/types.h with #ifndef __ASSEMBLY__ in many
> > > > files
> > > > 2. So we trying to replace multiple #ifndef __ASSEMBLY__ with one.
> > > 
> > > The point is:
> > > 
> > > 1. If the parent include needs to include linux/types.h to get at C
> > >    types _and_ the include file needs to also be included by assembly
> > >    code, it itself needs to have #ifndef __ASSEMBLY__ to protect those
> > >    uses from the assembly code.
> > > 
> > >    In that case, the linux/types.h include should be contained within
> > >    the #ifndef __ASSEMBLY__ .. #endif block along with all C only
> > >    parts of the header file.
> > 
> > That makes the code much less clean: putting #include's in the middle of a 
> > header is poor style and leads to people failing to consider dependencies. 
> > We generally put them to the header portion.
> > 
> > Putting an #include line in the middle of a header file is a receipe for a 
> > dependency hell (it can easily fall inside #ifdefs, can be overlooked, 
> > etc.), so it's _strongly_ discouraged (at least on arch/x86).
> 
> Put them at the top then with an additional ifndef.

So you advocate 40 stupid pairs of #ifdefs spread out, instead of a 
_single_, obvious #ifdef in a commonly used header?

Case closed.

	Ingo
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