linux-kernel - RE: [PATCH v3] pwm_backlight: Add device tree support for Low Threshold Brightness

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Date:	Thu, 11 Oct 2012 08:32:21 +0000
From:	"Philip, Avinash" <avinashphilip@...com>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Thierry Reding <thierry.reding@...onic-design.de>
CC:	"grant.likely@...retlab.ca" <grant.likely@...retlab.ca>,
	"rob.herring@...xeda.com" <rob.herring@...xeda.com>,
	"rob@...dley.net" <rob@...dley.net>,
	"rpurdie@...ys.net" <rpurdie@...ys.net>,
	"broonie@...nsource.wolfsonmicro.com" 
	<broonie@...nsource.wolfsonmicro.com>,
	"shawn.guo@...aro.org" <shawn.guo@...aro.org>,
	"devicetree-discuss@...ts.ozlabs.org" 
	<devicetree-discuss@...ts.ozlabs.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Nori, Sekhar" <nsekhar@...com>,
	"Hebbar, Gururaja" <gururaja.hebbar@...com>
Subject: RE: [PATCH v3] pwm_backlight: Add device tree support for Low
 Threshold Brightness

On Fri, Sep 28, 2012 at 11:15:48, Thierry Reding wrote:
> On Thu, Sep 27, 2012 at 02:33:09PM -0700, Andrew Morton wrote:
> > On Wed, 26 Sep 2012 20:17:07 +0530
> > "Philip, Avinash" <avinashphilip@...com> wrote:
> > 
> > > Some back lights perform poorly when driven by a PWM with a short
> > > duty-cycle. For such devices, the low threshold can be used to specify a
> > > lower bound for the duty-cycle and should be chosen to exclude the
> > > problematic range.
> > > 
> > > Add device tree probing support for lth_brightness putting
> > > low-threshold-brightness as optional property.
> > > 
> > > ...
> > >
> > > --- a/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt
> > > +++ b/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt
> > > @@ -14,6 +14,15 @@ Required properties:
> > >  Optional properties:
> > >    - pwm-names: a list of names for the PWM devices specified in the
> > >                 "pwms" property (see PWM binding[0])
> > > +  - low-threshold-brightness: brightness threshold low level. Low threshold
> > > +    brightness set to value so that backlight present on low end of
> > > +    brightness.
> > > +    Some panels, backlight would absent if duty percentage of PWM wave is less
> > > +    than certain level (say 20%). By setting low-threshold-brightness to a
> > > +    value above (percentage of brightness-levels max) 50 (20% of 255, if 255
> > > +    is max). On setting low-threshold-brightness, range of brightness-levels
> > > +    is calculated in a region of low-threshold-brightness to brightness-levels
> > > +    max.
> > 
> > hoo boy, that's hard to follow.  How does this look?
> > 
> > --- a/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt~pwm_backlight-add-device-tree-support-for-low-threshold-brightness-fix
> > +++ a/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt
> > @@ -14,15 +14,15 @@ Required properties:
> >  Optional properties:
> >    - pwm-names: a list of names for the PWM devices specified in the
> >                 "pwms" property (see PWM binding[0])
> > -  - low-threshold-brightness: brightness threshold low level. Low threshold
> > -    brightness set to value so that backlight present on low end of
> > -    brightness.
> > -    Some panels, backlight would absent if duty percentage of PWM wave is less
> > -    than certain level (say 20%). By setting low-threshold-brightness to a
> > -    value above (percentage of brightness-levels max) 50 (20% of 255, if 255
> > -    is max). On setting low-threshold-brightness, range of brightness-levels
> > -    is calculated in a region of low-threshold-brightness to brightness-levels
> > -    max.
> > +  - low-threshold-brightness: brightness threshold low level. Sets the lowest
> > +    brightness value.
> > +    On some panels the backlight misbehaves if the duty cycle percentage of the
> > +    PWM wave is less than a certain level (say 20%).  In this example the user
> > +    can set low-threshold-brightness to a value above 50 (ie, 20% of 255), thus
> > +    preventing the PWM duty cycle from going too low.
> > +    On setting low-threshold-brightness the range of brightness levels is
> > +    calculated in the range low-threshold-brightness to the maximum value in
> > +    brightness-levels, described above.
> >  
> >  [0]: Documentation/devicetree/bindings/pwm/pwm.txt
> >  
> > 
> > 
> > Also, I'm wondering if we really needed a new property - couldn't one
> > do this simply by setting brightness-levels to 50..255?
> 
> Yes. This was discussed in the thread that followed the posting of this
> patch's v2. We've decided to drop it and go with brightness-levels only
> for device tree data. Eventually all existing users should convert to
> that as well so we can remove some of the cruft from the platform data
> up.

Andrew/Thierry,

It was decided that patch as dropped and still see it in linux-next.
http://git.kernel.org/?p=linux/kernel/git/next/linux-next.git;a=commit;h=1801ff7f87131190b0a469c26bd7c829601e198c

Thanks
Avinash

> 
> Thierry
> 

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