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SubjectRe: [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@ti.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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