linux-kernel - Re: using IS_ENABLED(CONFIG

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Message-ID: <C2D7FE5348E1B147BCA15975FBA23075F44D1969@IN01WEMBXA.internal.synopsys.com>
Date:	Mon, 16 Nov 2015 08:35:05 +0000
From:	Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
CC:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Paul Gortmaker <paul.gortmaker@...driver.com>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Michal Marek <mmarek@...e.cz>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	arcml <linux-snps-arc@...ts.infradead.org>
Subject: Re: using IS_ENABLED(CONFIG_xyz) effectively

Hi Geert,

On Monday 16 November 2015 01:58 PM, Geert Uytterhoeven wrote:
> Hi Vineet,
>
> On Mon, Nov 16, 2015 at 9:00 AM, Vineet Gupta
> <Vineet.Gupta1@...opsys.com> wrote:
>> I've been using IS_ENABLED for some time and once in a while run into an issue
>> which prevents seamless use. Hence posing this question to experts in the area.
>>
>> C macro processor evaluates the ensuing control block even if IS_ENABLED evaluates
>> to false. This requires dummy #defines or worse still removing usage of IS_ENABLED
>> altogether.
>>
>> e.g. In example below even for ARCOMPACT builds, we need the ARCV2 specific define
>> ARCV2_IRQ_DEF_PRIO.
>>
>> void arch_cpu_idle(void)
>> {
>>         if (is_isa_arcompact()) {    <---- IS_ENABLED(CONFIG_ISA_ARCOMPACT)
>>                 __asm__("sleep 0x3");
>>         } else {
>>                 const int arg = 0x10 | ARCV2_IRQ_DEF_PRIO;
>>                __asm__("sleep 0x10");
>>         }
>> }
>>
>> One could argue that the interface needs to be cleanly defined to not have such
>> specific #defines in common code in first place. However sometime that becomes
>> just too tedious.
>>
>> Is there a way to get around by this ?
> Use #ifdef CONFIG_...?
>
> The advantage of IS_ENABLED() over #ifdef is that it allows compile-testing of
> the disabled code path. Of course it should only be compiled if it makes
> sense. And that's exactly what you're running into.

And I thought it was to de-uglify the code with same semantics - which doesn't
seem to be the case !
Oh well !

Thx,
-Vineet

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