linux-kernel - Re: kernel coding style for if ... else which cross #ifdef

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Date:	Sat, 24 May 2008 15:35:58 +0100
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Sam Ravnborg <sam@...nborg.org>
CC:	"H. Peter Anvin" <hpa@...or.com>,
	Steve French <smfrench@...il.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: kernel coding style for if ... else which cross #ifdef

Sam Ravnborg wrote:
> I assume you wanted to say:
>   
>>    #undef  CONFIG_FOO
>>    #define CFG_FOO   1
>>    #define CONFIG_FOO_MODULE
>>    #define CFG_FOO_MODULE 1
>>     
> Because then the CONFIG_* is not changed
> and we do not want to change that.
>   

Yeah, I didn't intend to change the meaning of CONFIG_FOO.

> I'm not fully convinced about:
>   
>>    #define CFG_FOO   1
>>     
> But on the other hand it is only in odd
> cases we distingush between built-in and module.
> So it makes most sense.
>   

I think CONFIG_ and CFG_ should be exact parallels, so if CONFIG_FOO is 
undefined, CFG_FOO should be 0.

>> Not sure what CFG_* should be for string/numeric options.  Probably "1" 
>> if the value is defined, "0" if not, with CONFIG_* being the actual 
>> value (so a CONFIG_ value of 0 is distinguishable from not defined).
>>     
> For non-boolean/tristate values we simply skip CFG_ values - thats
> the most simple approach.

I suppose, but it might be useful to know whether a constant is present:

	if (CFG_THINGY_LIMIT && x > CONFIG_THINGY_LIMIT) {...}

(which fails if CONFIG_THINGY_LIMIT is undefined, so I guess it still 
doesn't work very well).

    J
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