[RFC 1/2] dt-bindings: pwm-backlight: add brightness-levels-scale property

[Enric Balletbo i Serra <enric.balletbo@collabora.com>]

Brightness is not perceived linearly; rather, it typically follows some
kind of parabolic curve. We can support this by skipping values in the
brightness-levels array in a pseudo-quadratic curve. Typically we used
less than 256 levels, which yields no more than 1KiB of memory in our
device tree. But, we've noticed that on some devices the backlight
performs much smoother at lower ranges if we have more than 256-levels of
granularity. On kevin device, for example, if we support all 64K, that
will waste us at least 256KiB in our device tree.

Let's avoid to waste memory and have a huge table of numbers in our device
tree of numbers by adding a brightness-levels-scale property to let the
driver compute the brightness levels based on one algorithm and their
property parameters.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
---
 .../bindings/leds/backlight/pwm-backlight.txt       | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
index 764db86..11c5583 100644
--- a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
+++ b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
@@ -17,6 +17,16 @@ Optional properties:
                "pwms" property (see PWM binding[0])
   - enable-gpios: contains a single GPIO specifier for the GPIO which enables
                   and disables the backlight (see GPIO binding[1])
+  - brightness-levels-scale: this can be used instead of 'brightness-levels',
+                             to represent the perceived luminance. So rather
+                             than specifying 'brightness-levels = <0
+                             1 2 ... 65535>', one can simply say
+                             'brightness-levels-scale = <255 65535>', where the
+                             first number is the max number of levels and the
+                             second number is the max PWM value that represent a
+                             100% duty cycle (brightest). The result is a
+                             correction table for PWM values to create linear
+                             brightness based on the CIE1931 algorithm.
 
 [0]: Documentation/devicetree/bindings/pwm/pwm.txt
 [1]: Documentation/devicetree/bindings/gpio/gpio.txt
@@ -33,3 +43,14 @@ Example:
 		power-supply = <&vdd_bl_reg>;
 		enable-gpios = <&gpio 58 0>;
 	};
+
+	backlight {
+		compatible = "pwm-backlight";
+		pwms = <&pwm 0 5000000>;
+
+		brightness-levels-scale = <255 65535>;
+		default-brightness-level = <128>;
+
+		power-supply = <&vdd_bl_reg>;
+		enable-gpios = <&gpio 58 0>;
+	};
-- 
2.9.3