linux-kernel - Re: [PATCH] drivers: depend on instead of select BACKLIGHT_CLASS_DEVICE and ACPI

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Date:	Wed, 29 Oct 2014 19:27:42 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Jani Nikula <jani.nikula@...el.com>
Cc:	Randy Dunlap <rdunlap@...radead.org>,
	Tomi Valkeinen <tomi.valkeinen@...com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	linux-fbdev@...r.kernel.org, Jingoo Han <jg1.han@...sung.com>,
	Daniel Drake <dsd@...top.org>,
	Jens Frederich <jfrederich@...il.com>,
	David Airlie <airlied@...ux.ie>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jon Nettleton <jon.nettleton@...il.com>,
	linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
	dri-devel@...ts.freedesktop.org,
	platform-driver-x86@...r.kernel.org,
	Lee Jones <lee.jones@...aro.org>,
	Laurent Pinchart <laurent.pinchart@...asonboard.com>,
	Darren Hart <dvhart@...radead.org>,
	Daniel Vetter <daniel.vetter@...el.com>,
	Bryan Wu <cooloney@...il.com>, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH] drivers: depend on instead of select
 BACKLIGHT_CLASS_DEVICE and ACPI_VIDEO

On Wed, 2014-10-29 at 09:54 +0200, Jani Nikula wrote:
> On Wed, 29 Oct 2014, Michael Ellerman <mpe@...erman.id.au> wrote:
> > On Tue, 2014-10-28 at 13:29 -0700, Randy Dunlap wrote:
> >> On 10/27/14 06:13, Tomi Valkeinen wrote:
> >> > I also think the 'depends on BACKLIGHT_CLASS_DEVICE ||
> >> > BACKLIGHT_CLASS_DEVICE=n' pattern is quite... interesting (i.e. sounds
> >> > like a hack to me =).
> >> 
> >> It does exactly what is needed and it is used in many places in kernel
> >> Kconfig files.
> >
> > Is there any reason you can't do:
> >
> >   depends on BACKLIGHT_CLASS_DEVICE != m
> 
> That's not the same thing. The FOO || FOO=n allows for all options, but
> forbids it being a module when the option depending on it is
> built-in.

OK right. Because "BAR depends on FOO" is short for "depends on FOO=y || FOO=m",
but also adds the implicit condition that if FOO=m then BAR must also be m.

Thanks for clueing me in.

cheers


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