linux-kernel - Re: [PATCH 1/4] ARM: tegra: Unify tegra{20,30,114}_init

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Date:	Sat, 9 Feb 2013 12:18:41 +0000
From:	Arnd Bergmann <arnd@...db.de>
To:	Hiroshi Doyu <hdoyu@...dia.com>
Cc:	"balbi@...com" <balbi@...com>,
	"linux-tegra@...r.kernel.org" <linux-tegra@...r.kernel.org>,
	"swarren@...dotorg.org" <swarren@...dotorg.org>,
	"linux@....linux.org.uk" <linux@....linux.org.uk>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/4] ARM: tegra: Unify tegra{20,30,114}_init_early()

On Friday 08 February 2013, Hiroshi Doyu wrote:
> > > +#if defined(CONFIG_ARCH_TEGRA_3x_SOC)
> >
> > how about using:
> >
> > #if IS_BUILTIN(CONFIG_ARCH_TEGRA_3x_SOC)
> >
> > instead ?
> 
> Why is IS_BUILTIN() prefered?
> 

Inside of a function, if(IS_ENABLED(CONFIG_FOO)) or the respective IS_BUILTIN is
preferred over #ifdef because it provides better compile coverage and better
readability. Also, IS_ENABLED() is nice when you want to check if something
is builtin or module, without having to write a complex expression.

If you just replace the #ifdef with #if IS_BUILTIN as Felipe suggested, I see
no real benefit, but it would be nice to write this as


void __init tegra_hotplug_init(void)
{
	if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
		return;
	
	if (IS_ENABLED(CONFIG_ARCH_TEGRA_2x_SOC)) && tegra_chip_id == TEGRA20)
		tegra_hotplug_shutdown = tegra20_hotplug_shutdown;
	if (IS_ENABLED(CONFIG_ARCH_TEGRA_3x_SOC)) && tegra_chip_id == TEGRA30)
		tegra_hotplug_shutdown = tegra30_hotplug_shutdown;
}

which completely avoids all preprocessor conditionals and replaces them
with compile-time choices based on constant expressions to eliminate the
code paths for disabled platforms.

	Arnd
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