linux-kernel - Re: [PATCH] pwm-backlight: add support for device tree gpio control

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Date:	Mon, 11 Nov 2013 10:39:57 +0100
From:	Thierry Reding <thierry.reding@...il.com>
To:	Mike Dunn <mikedunn@...sguy.com>
Cc:	Tomi Valkeinen <tomi.valkeinen@...com>,
	Richard Purdie <rpurdie@...ys.net>,
	Jingoo Han <jg1.han@...sung.com>,
	Jean-Christophe Plagniol-Villard <plagnioj@...osoft.com>,
	Grant Likely <grant.likely@...aro.org>,
	Rob Herring <rob.herring@...xeda.com>,
	linux-pwm@...r.kernel.org, linux-fbdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org,
	Robert Jarzmik <robert.jarzmik@...e.fr>,
	Marek Vasut <marex@...x.de>
Subject: Re: [PATCH] pwm-backlight: add support for device tree gpio control

On Fri, Sep 27, 2013 at 06:35:31AM -0700, Mike Dunn wrote:
> On 09/26/2013 05:50 AM, Thierry Reding wrote:
> > On Thu, Sep 26, 2013 at 03:26:13PM +0300, Tomi Valkeinen wrote:
> >> On 26/09/13 15:02, Thierry Reding wrote:
> >>> On Thu, Sep 26, 2013 at 01:13:18PM +0300, Tomi Valkeinen wrote:
> >>>> On 11/09/13 14:40, Mike Dunn wrote:
> >>>>> On 09/10/2013 10:21 AM, Thierry Reding wrote:
> >>>>
> >>>>>> Do you have a real setup that actually needs multiple GPIOs? Usually
> >>>>>> such a setup requires some kind of timing or other additional constraint
> >>>>>> which can't be represented by this simple binding.
> >>>>>>
> >>>>>> Looking at the Palm Treo code it seems like the reason why multiple
> >>>>>> GPIOs are needed is because one is to enable the backlight, while the
> >>>>>> other is in fact used to enable the LCD panel. 
> >>>>>
> >>>>>
> >>>>> There are actually four GPIOs involved!  (There is an embarrasingly horrible
> >>>>> hack in arch/arm/mach-pxa/palmtreo.c that handles the others.)  One is almost
> >>>>> certainly simply backlight power.  The other three are probably LCD related.
> >>>>
> >>>> When you say "power", do you mean the gpio enables a regulator to feed
> >>>> power to the backlight? If so, wouldn't that be a regulator, not gpio,
> >>>> from the bl driver's point of view?
> >>>>
> >>>> Generally speaking, this same problem appears in many places, but for
> >>>> some reason especially in display. I'm a bit hesitant in adding "free
> >>>> form" gpio/regulator support for drivers, as, as Thierry pointed out,
> >>>> there are often timing requirements, or sometimes the gpios are
> >>>> inverted, or sometimes the gpio is, in fact, a reset gpio, where you'll
> >>>> assert the gpio for a short moment only.
> >>>
> >>> I sent out another series a few days ago that somewhat obsoletes this
> >>> patch. What it does is basically add a single enable GPIO that can be
> >>> used to turn the backlight on and off. In a separate patch, support is
> >>> added for a power regulator. The combination of both should be able to
> >>> cover the majority of use-cases.
> >>
> >> But Mike's case required 4 GPIOs? Or can that be reduced to one gpio and
> >> one regulator?
> > 
> > Well, at least for the backlight it only seemed to involve a single
> > GPIO. The other three were probably related to LCD and therefore not
> > really suitable for a backlight driver. Traditionally it has been that
> > the backlight driver handled these things as well (via the callbacks
> > installed by board setup code). While really they should be handled by a
> > separate driver (for the LCD).
> 
> 
> Yes, this is currently my best guess.  This is reverse-engineered and
> unfortunately I'm not yet able to accurately describe my particular use-case.
> Probably as wacky as anything you can imagine, Thierry :)
> 
> The gpio and regulator patches will probably suffice.  Thierry, can you please
> point me to those patches?  I don't see them in your gitorious tree.  If they
> were posted to linux-pwm, I missed them; sorry.

I've stumbled across this email and it's not marked as answered, so here
goes: these patches will be part of my pull request for 3.13. They
should now be in my tree, although that's now moved to kernel.org. You
can find it here:

	https://git.kernel.org/cgit/linux/kernel/git/thierry.reding/linux-pwm.git

Thierry

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