linux-kernel - Re: [PATCH] mtd: dc21285.c: remove double check of CONFIG_ARCH

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Date:	Tue, 29 May 2012 11:19:20 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Christian Dietrich <christian.dietrich@...ormatik.uni-erlangen.de>
Cc:	Artem Bityutskiy <artem.bityutskiy@...ux.intel.com>,
	Dmitry Eremin-Solenikov <dbaryshkov@...il.com>,
	linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org,
	vamos-dev@...ts.cs.fau.de
Subject: Re: [PATCH] mtd: dc21285.c: remove double check of
 CONFIG_ARCH_NETWINDER

On Tue, 2012-05-29 at 12:06 +0200, Christian Dietrich wrote:
> When CONFIG_ARCH_NETWINDER is unset nw_en_write is a NOP. But
> machine_is_netwinder() also checks for
> CONFIG_ARCH_NETWINDER. Therefore in the !netwinder case the
> preprocessed code is:
> 
> if (0)
>    do {} while(0); 

It's not a double check. It's a compile time check for "do we even need
to build this code at all?", and a separate run time check for "do we
need to run this code now?".

Think about the case where CONFIG_ARCH_NETWINDER is set, but you aren't
actually running this kernel binary on a netwinder *today*.

Yes, we might not support multi-platform kernels on ARM yet, but we are
slowly getting there. This kind of change just makes that harder.

And even if the machine_is_netwinder() "function" is *currently* a macro
which is hard-coded to return one or zero, that just means that your
change achieves nothing in the compiler output. It'll be silently
optimised away, or not, as appropriate.

-- 
dwmw2

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