linux-kernel - Re: [PATCH] backlight: add CONFIG_PM

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Date:	Wed, 12 Jun 2013 14:08:21 +0200
From:	Jean-Christophe PLAGNIOL-VILLARD <plagnioj@...osoft.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Arnd Bergmann <arnd@...db.de>, Jingoo Han <jg1.han@...sung.com>,
	'Shuah Khan' <shuah.kh@...sung.com>,
	linux-fbdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	shuahkhan@...il.com, rpurdie@...ys.net, FlorianSchandinat@....de,
	tomi.valkeinen@...com, rafael.j.wysocki@...el.com
Subject: Re: [PATCH] backlight: add CONFIG_PM_SLEEP to suspend/resume
 functions

On 16:31 Mon 10 Jun     , Andrew Morton wrote:
> On Fri, 07 Jun 2013 12:02:31 +0200 Arnd Bergmann <arnd@...db.de> wrote:
> 
> > On Friday 07 June 2013 10:39:20 Jingoo Han wrote:
> > > Add CONFIG_PM_SLEEP to suspend/resume functions to fix the following
> > > build warning when CONFIG_PM_SLEEP is not selected. This is because
> > > sleep PM callbacks defined by SIMPLE_DEV_PM_OPS are only used when
> > > the CONFIG_PM_SLEEP is enabled.
> > > 
> > > drivers/video/backlight/backlight.c:211:12: warning: 'backlight_suspend' defined but not used [-Wunused-function]
> > > drivers/video/backlight/backlight.c:225:12: warning: 'backlight_resume' defined but not used [-Wunused-function]
> > > 
> > > Signed-off-by: Jingoo Han <jg1.han@...sung.com>
> > > ---
> > >  drivers/video/backlight/backlight.c |    2 ++
> > >  1 file changed, 2 insertions(+)
> > 
> > Your patch looks ok, but I find it extremely annoying to have new warnings
> > like this one come up every single day in linux-next. It really shouldn't
> > be this hard to use a macro called SIMPLE_DEV_PM_OPS() correctly.
> > 
> > Below is an implementation of SIMPLE_DEV_PM_OPS and UNIVERSAL_DEV_PM_OPS
> > that avoids this issue by introducing an unused reference to the suspend
> > and resume functions. gcc is smart enough to leave out that unused code
> > by itself, and it would actually improve compile-time coverage to have
> > something like this, besides being harder to misuse.
> > 
> > This would be a better approach if we didn't already have all the "#ifdef
> > CONFIG_PM_SLEEP" in place that hide the functions now. Unfortunately we
> > already have over 300 uses of SIMPLE_DEV_PM_OPS/UNIVERSAL_DEV_PM_OPS
> > in the kernel today, so removing all the #ifdef atomically without
> > creating more build errors is rather hard to do.
> > 
> > Maybe someone has an idea how to extend my approach so it works with
> > and without the #ifdef, to let us transition to a situation that no
> > longer needs them.
> 
> You could create new macros, and add a checkpatch rule to remind people
> to not use the old ones.  Then people can migrate over from the old
> macros at a leisurely pace.
> 
> The problem will be in thinking up decent names for the new macros.

Agreed

Best Regards,
J.
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