linux-kernel - Re: [RFC] x86: merge nmi

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Message-ID: <alpine.LFD.1.10.0805180023100.18798@apollo.tec.linutronix.de>
Date:	Sun, 18 May 2008 00:34:15 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Mikael Pettersson <mikpe@...uu.se>
cc:	"Maciej W. Rozycki" <macro@...ux-mips.org>,
	Tom Spink <tspink@...il.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Jiri Slaby <jirislaby@...il.com>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [RFC] x86: merge nmi_32-64 to nmi.c

On Sat, 17 May 2008, Mikael Pettersson wrote:
> Maciej W. Rozycki writes:
>  > On Sat, 17 May 2008, Tom Spink wrote:
>  > 
>  > > static inline unsigned int get_nmi_count(int cpu)
>  > > {
>  > > #ifdef CONFIG_X86_64
>  > >         return cpu_pda(cpu)->__nmi_count;
>  > > #else
>  > >         return nmi_count(cpu);
>  > > #endif
>  > > }
>  > > 
>  > > I know it introduces a lot of these conditionals, but at least there
>  > > is one place to look for the get_nmi_count function, instead of
>  > > searching for all variants of the function.
>  > 
>  >  Well, I suppose some header should provide a definition like:
>  > 
>  > #ifdef CONFIG_X86_64
>  > #define cpu_x86_64 1
>  > #else
>  > #define cpu_x86_64 0
>  > #endif
>  > 
>  > and the you can remove the horrible #ifdef clutter and make the quoted 
>  > function look like:
>  > 
>  > static inline unsigned int get_nmi_count(int cpu)
>  > {
>  > 	return cpu_x86_64 ? cpu_pda(cpu)->__nmi_count : nmi_count(cpu);
>  > }
>  > 
>  > Much better -- isn't it?
> 
> IMO, no, the #ifdef is preferable.
> 
> Why? Because the #ifdef is a very visible signal to the platform
> people that there are (in this case) subarch differences that force
> "clients" to behave differently on different subarchs. By removing
> the #ifdef you're IMO making it less likely for the platform people
> to take notice and work towards eliminating those differences.

The #ifdef is a poor choice. Maciej is damned right, that the single
function with a clear distinction of the return value is better in
terms of readability and maintenance.

As I said before, We can make this more visible with an uppercase
CONFIG_WHATEVER instaed of the innocent cpu_x86_64 one, but both
solutions are better than #ifdefs and provide simple grepable
patterns.

The awareness of those differences does not depend at all on an
#ifdef. Developers who are aware of the platform differences prefer a
readable not ifdef poluted code base. People who need to be poked into
the difference via an #ifdef are probably not those who can actually
clean it up.

Thanks,

	tglx
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