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

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Date:	Fri, 28 Sep 2012 07:45:48 +0200
From:	Thierry Reding <thierry.reding@...onic-design.de>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Philip, Avinash" <avinashphilip@...com>,
	grant.likely@...retlab.ca, rob.herring@...xeda.com,
	rob@...dley.net, rpurdie@...ys.net,
	broonie@...nsource.wolfsonmicro.com, shawn.guo@...aro.org,
	devicetree-discuss@...ts.ozlabs.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org, nsekhar@...com,
	gururaja.hebbar@...com
Subject: Re: [PATCH v3] pwm_backlight: Add device tree support for Low
 Threshold Brightness

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.

Thierry

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