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

From: Laurent Pinchart <laurent.pinchart-ryLnwIuWjnjg/C1BVhZhaw@public.gmane.org>
To: Thierry Reding <thierry.reding-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Cc: Stephen Warren <swarren-3lzwWm7+Weoh9ZMKESR00Q@public.gmane.org>,
	Jean-Christophe PLAGNIOL-VILLARD
	<plagnioj-sclMFOaUSTBWk0Htik3J/w@public.gmane.org>,
	Denis Carikli <denis-fO0SIAKYzcbQT0dZR+AlfA@public.gmane.org>,
	Mark Rutland <mark.rutland-5wv7dgnIgG8@public.gmane.org>,
	devicetree-u79uwXL29TY76Z2rM5mHXA@public.gmane.org,
	Ian Campbell
	<ijc+devicetree-KcIKpvwj1kUDXYZnReoRVg@public.gmane.org>,
	Eric B??nard <eric-fO0SIAKYzcbQT0dZR+AlfA@public.gmane.org>,
	Pawel Moll <pawel.moll-5wv7dgnIgG8@public.gmane.org>,
	Jingoo Han <jg1.han-Sze3O3UU22JBDgjK7y7TUQ@public.gmane.org>,
	Rob Herring <rob.herring-bsGFqQB8/DxBDgjK7y7TUQ@public.gmane.org>,
	Richard Purdie <rpurdie-Fm38FmjxZ/leoWH0uzbU5w@public.gmane.org>,
	Sascha Hauer <kernel-bIcnvbaLZ9MEGnE8C9+IrQ@public.gmane.org>,
	linux-arm-kernel-IAPFreCvJWM7uuMidbF8XUB+6BGkLq7r@public.gmane.org,
	Lothar Wa??mann
	<LW-AvR2QvxeiV7DiMYJYoSAnRvVK+yQ3ZXh@public.gmane.org>
Subject: Re: [PATCHv4] video: backlight: gpio-backlight: Add DT support.
Date: Thu, 24 Oct 2013 00:38:59 +0200	[thread overview]
Message-ID: <2730370.0UKsY9Ox0H@avalon> (raw)
In-Reply-To: <20131023202011.GD8828@mithrandir>

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Hi Thierry,

On Wednesday 23 October 2013 22:20:12 Thierry Reding wrote:
> On Wed, Oct 23, 2013 at 05:51:13PM +0100, Stephen Warren wrote:
> > On 10/22/2013 09:01 PM, Thierry Reding wrote:
> > > On Tue, Oct 22, 2013 at 05:34:45PM +0200, Jean-Christophe
> > 
> > > PLAGNIOL-VILLARD wrote:
> > ...
> > 
> > >> I'm sorry but the blacklight descibe in DT have nothing to do
> > >> with the common pratice that the current driver have today
> > > 
> > > That's not at all what I said. What I said was that the majority
> > > of backlight drivers currently default to turning the backlight on
> > > when probed. Therefore I think it would be consistent if this
> > > driver did the same.
> > > 
> > > I also said that I don't think it's a very good default, but at the
> > > same time we can't just go and change the default behaviour at will
> > > because people may rely on it.
> > 
> > It may well be reasonable to change the default behaviour for devices
> > instantiated from DT. If it's not possible to instantiate the device
> > from DT yet, then it's not possible for anyone to be relying on the
> > default behaviour yet, since there is none. So, perhaps the default
> > could be:
> > 
> > * If device instantiated from a board file, default to on, for
> > backwards-compatibility.
> > 
> > * If device instantiated from DT, there is no backwards compatibility
> > to be concerned with, since this is a new feature, hence default to
> > off, since we think that's the correct thing to do.
> 
> I actually had a patch to do precisely that. However I then realized
> that people have actually been using pwm-backlight in DT for a while
> already and therefore may be relying on that behaviour as well.
> 
> It also isn't really an issue of DT vs. non-DT. The simple fact is that
> besides the backlight driver there's usually no other code that enables
> a backlight on boot. The only way to do so that I know of is using the
> DRM panel patches that I've been working on.

I would very much welcome a refactoring of the backlight code that would 
remove the fbdev dependency and hook backlights to panel drivers. That's 
something I wanted to work on myself, but that I pushed back after CDF :-)

> That said, it is true that the number of DT users of the pwm-backlight
> driver is smaller than the number of board file users, and it is much
> more likely that people are still actively using them, so if we can get
> everyone to agree on changing the default behaviour that might still be
> possible.

-- 
Regards,

Laurent Pinchart

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