linux-kernel - Re: [PATCH] backlight: use of_find_backlight_by

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Message-ID: <20141104171532.GS17577@x1>
Date:	Tue, 4 Nov 2014 17:15:32 +0000
From:	Lee Jones <lee.jones@...aro.org>
To:	Heiko Stübner <heiko@...ech.de>
Cc:	Jingoo Han <jg1.han@...sung.com>, 'Bryan Wu' <cooloney@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] backlight: use of_find_backlight_by_node stub when
 backlight class disabled

On Tue, 04 Nov 2014, Heiko Stübner wrote:

> Am Dienstag, 4. November 2014, 14:42:20 schrieb Lee Jones:
> > On Tue, 04 Nov 2014, Jingoo Han wrote:
> > > On Tuesday, November 04, 2014 6:08 PM, Heiko Stübner wrote:
> > > > On Mon, 03 Nov 2014, Lee Jones wrote:
> > > > > On Wed, 29 Oct 2014, Heiko Stübner wrote:
> > > > > > Drivers may want to search for an optional backlight even when the
> > > > > > backlight class is disabled. In this case the linker would miss the
> > > > > > function referenced in the backlight header.
> > > > > > 
> > > > > > Therefore use the stub function also when the backlight class is
> > > > > > disabled.
> > > > > > 
> > > > > > Signed-off-by: Heiko Stuebner <heiko@...ech.de>
> > > > > > ---
> > > > > > 
> > > > > >  include/linux/backlight.h | 2 +-
> > > > > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > > > 
> > > > > Applied to Backlight -next with Jingoo's Ack.
> > > > 
> > > > I've removed this patch, as it causes unexpected:
> > > >   Redefinition of of_find_backlight_by_node()
> > > 
> > > I reproduced the same build error.
> > > 
> > > Then, how about folding the following two patches into
> > > one single patch? These two patches were already sent by Heiko Stübner.
> > > 
> > >   [PATCH] backlight: use of_find_backlight_by_node stub when backlight
> > >   class disabled [PATCH] backlight: extend of_find_backlight_by_node
> > >   stub-check to modules> 
> > > Then, the one single patch will do as follows.
> > > 
> > > -#ifdef CONFIG_OF
> > > +#if defined(CONFIG_OF) && (defined(CONFIG_BACKLIGHT_CLASS_DEVICE) || \
> > > +			   defined(CONFIG_BACKLIGHT_CLASS_DEVICE_MODULE))
> > > 
> > > In this case, I cannot find any build errors.
> > 
> > That's a neat trick.  I didn't know you could do that.
> > 
> > However, it's bit messy consider different formatting, or a nested
> > #ifdef instead please.
> 
> I guess it is a matter of me "not seeing the forrest for the trees", but how
> would a nested ifdef look like, as this would result in 3 possible results
> when for CONFIG_OF first and then for one of the BACKLIGHT_CLASS defines?
> 
> Formatting wise, when applied both defined(CONFIG_BACKLIGHT_foo) parts
> are exactly below each other, making it (hopefully) clear where the "or" is
> part of. What would look better?

Actually there is a better way still:

#ifdef CONFIG_OF && IS_ENABLED(CONFIG_BACKLIGHT_CLASS_DEVICE)
struct backlight_device *of_find_backlight_by_node(struct device_node *node);
#else
static inline struct backlight_device *
of_find_backlight_by_node(struct device_node *node)
{
      return NULL;
}
#endif


-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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