Re: [PATCH v3] pwm_backlight: Add device tree support for Low Threshold Brightness
From: Andrew Morton <akpm@linux-foundation.org>
Date: 2012-09-27 21:33:13
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On Wed, 26 Sep 2012 20:17:07 +0530 "Philip, Avinash" [off-list ref] wrote:
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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?