[PATCHv4] video: backlight: gpio-backlight: Add DT support.

[Jean-Christophe PLAGNIOL-VILLARD]

On 09:23 Tue 22 Oct     , Thierry Reding wrote:
> On Tue, Oct 22, 2013 at 06:58:33AM +0200, Jean-Christophe PLAGNIOL-VILLARD wrote:
> > On 11:13 Mon 21 Oct     , Denis Carikli wrote:
> > > Cc: Richard Purdie <rpurdie at rpsys.net>
> > > Cc: Jingoo Han <jg1.han at samsung.com>
> > > Cc: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > > Cc: Rob Herring <rob.herring at calxeda.com>
> > > Cc: Pawel Moll <pawel.moll at arm.com>
> > > Cc: Mark Rutland <mark.rutland at arm.com>
> > > Cc: Stephen Warren <swarren at wwwdotorg.org>
> > > Cc: Ian Campbell <ijc+devicetree at hellion.org.uk>
> > > Cc: devicetree at vger.kernel.org
> > > Cc: Sascha Hauer <kernel at pengutronix.de>
> > > Cc: linux-arm-kernel at lists.infradead.org
> > > Cc: Lothar Waßmann <LW at KARO-electronics.de>
> > > Cc: Jean-Christophe Plagniol-Villard <plagnioj at jcrosoft.com>
> > > Cc: Eric Bénard <eric at eukrea.com>
> > > Signed-off-by: Denis Carikli <denis at eukrea.com>
> > > ---
> > > ChangeLog v3->v4:
> > > - The default-brightness property is now optional, it defaults to 1 if not set.
> > by default we set OFF not ON
> > 
> > do not actiate driver or properti by default you can not known to consequence
> > on the hw
> 
> Turning on a backlight by default is what pretty much every backlight
> driver does. I personally think that's the wrong default, I even tried
> to get some discussion started recently about how we could change this.
> However, given that this has been the case for possibly as long as the
> subsystem has existed, suddenly changing it might cause quite a few of
> our users to boot the new kernel and not see their display come up. As
> with any other ABI, this isn't something we can just change without a
> very good migration path.

I'm sorry but the blacklight descibe in DT have nothing to do with the common
pratice that the current driver have today

put on by default if wrong specially without the property define. Even put it
on by default it wrong as the bootloader may have set it already for splash
screen and to avoid glitch the drivers need to detect this.

For me this should not even be a property but handled by the driver them
selves in C.

Best Regards,
J.
> 
> In my opinion every backlight should be hooked up to a display panel,
> and the display panel driver should be the one responsible for turning
> the backlight on or off. That's the only way we can guarantee that the
> backlight is turned on at the right moment so that glitches can be
> avoided.
> 
> One possible migration path would be to update all display panel drivers
> to cope with an associated backlight device, but even if somebody would
> even find the time to write the code to do that, I can imagine that we'd
> have a hard time getting this tested since a lot of the boards that rely
> on these backlight drivers are legacy and probably no longer very
> actively used.
> 
> Thierry