Re: using IS_ENABLED(CONFIG_xyz) effectively

[Geert Uytterhoeven]

Hi Vineet,

On Mon, Nov 16, 2015 at 9:00 AM, Vineet Gupta
<Vineet.Gupta1@xxxxxxxxxxxx> 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.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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