linux-kernel - Re: Kconfig, Makefile and ifdef: mod as yes vs. no?

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Date:	Sun, 12 Jul 2009 12:07:26 +0300
From:	Pekka Paalanen <pq@....fi>
To:	Kyle McMartin <kyle@...artin.ca>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Kconfig, Makefile and ifdef: mod as yes vs. no?

On Sat, 11 Jul 2009 17:53:43 -0400
Kyle McMartin <kyle@...artin.ca> wrote:

> On Sun, Jul 12, 2009 at 12:06:25AM +0300, Pekka Paalanen wrote:
> > +#ifdef CONFIG_BACKLIGHT_CLASS_DEVICE
> >
> 
> You need to test _MODULE for the symbol too... since it's necessary to
> be able to distinguish between symbols which exist in the vmlinux, and
> symbols which require a module to be loaded.
> 
> #if defined(CONFIG_BACKLIGHT_CLASS_DEVICE) &&
> 	defined(CONFIG_BACKLIGHT_CLASS_DEVICE_MODULE)
> 
> should work.

Just || instead of &&, and yes, this is a solution for half of the problem,
thanks.

But what about the Makefile?

nouveau-y := nouveau_drv.o ...
nouveau-$(CONFIG_BACKLIGHT_CLASS_DEVICE) += nouveau_backlight.o
obj-$(CONFIG_DRM_NOUVEAU)+= nouveau.o

With this, nouveau_backlight.o is not built if
CONFIG_BACKLIGHT_CLASS_DEVICE=m. Should I write:

nouveau-$(if $(findstring m,$(CONFIG_BACKLIGHT_CLASS_DEVICE)),y,n) += \
	nouveau_backlight.o

which to me looks ugly, or add a new Kconfig boolean option that is
selected 'if CONFIG_BACKLIGHT_CLASS_DEVICE' and use that in both Makefile
and code, or is there a canonical way to do this?

Other code seems to deal with a similar situation by #ifdef'ing the
whole c-file contents. That cannot be the preferred way, can it?

Thanks.

-- 
Pekka Paalanen
http://www.iki.fi/pq/
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