[PATCH v3] pwm_backlight: Add device tree support for Low Threshold Brightness

["Philip, Avinash" <avinashphilip@ti.com>]

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.

Signed-off-by: Philip, Avinash <avinashphilip@ti.com>
---
Changes since v2:
        - Added documentation part for configuring low-threshold-brightness

Changes since v1:
        - Updated commit message.
	- Changes to low-threshold-brightness.
	- Merged example section to original.

:100644 100644 1e4fc72... 4e29912... M	Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt
:100644 100644 995f016... 29e6fe1... M	drivers/video/backlight/pwm_bl.c
 .../bindings/video/backlight/pwm-backlight.txt     |   10 ++++++++++
 drivers/video/backlight/pwm_bl.c                   |    5 +++++
 2 files changed, 15 insertions(+), 0 deletions(-)

diff --git a/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt
index 1e4fc72..4e29912 100644
--- 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.
 
 [0]: Documentation/devicetree/bindings/pwm/pwm.txt
 
@@ -25,4 +34,5 @@ Example:
 
 		brightness-levels = <0 4 8 16 32 64 128 255>;
 		default-brightness-level = <6>;
+		low-threshold-brightness = <50>;
 	};
diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 995f016..29e6fe1 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -143,6 +143,11 @@ static int pwm_backlight_parse_dt(struct device *dev,
 
 		data->dft_brightness = value;
 		data->max_brightness--;
+
+		ret = of_property_read_u32(node, "low-threshold-brightness",
+					   &value);
+		if (!ret)
+			data->lth_brightness = value;
 	}
 
 	/*
-- 
1.7.0.4