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

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Message-ID: <20090206155512.GF13758@n2100.arm.linux.org.uk>
Date:	Fri, 6 Feb 2009 15:55:12 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Jaswinder Singh Rajput <jaswinder@...nel.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	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

On Fri, Feb 06, 2009 at 09:18:48PM +0530, Jaswinder Singh Rajput wrote:
> On Fri, 2009-02-06 at 15:33 +0000, Russell King - ARM Linux wrote:
> > On Fri, Feb 06, 2009 at 08:59:01PM +0530, Jaswinder Singh Rajput wrote:
> > > Jaswinder Singh Rajput (2):
> > >       Neither asm/types.h nor linux/types.h is required for arch/ia64/include/asm/fpu.h
> > >       make linux/types.h as assembly safe
> > 
> > I continue to disagree with the need for the second patch.
> 
> Like Ingo suggested:
> 
> On Fri, 2009-02-06 at 15:58 +0100, Ingo Molnar wrote:
> >  Well types.h easily gets included in other files though, which might be 
> > partially suited for assembly - and have !__ASSEMBLY__ portions that rely on 
> > a types.h include.
> > 
> > So making this file an invariant in .S files does not sound like a bad idea 
> > to me. Is there any downside?
> > 
> 
> 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.

2. if it doesn't need C types from linux/types.h, then that header has
   no business including linux/types.h, and the include should be
   eliminated to save the already dirbolically slow compiler from
   having to read and parse that file, and more importantly allowing
   it to eliminate linux/types.h from the build dependencies.

Yes, you can wrap linux/types.h with that ifndef, and yes it will fix
any problems, but I view it as a hack rather than fixing the real problem
which is lazyness by code writers to get their include dependencies right.
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