dt-bindings: pwm-backlight: Add a num-interpolation-steps property

The num-interpolated-steps property specifies the number of
interpolated steps between each value of brightness-level table. This is
useful for high resolution PWMs to not have to list out every possible
value in the brightness-level array.
Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt

@@ -21,6 +21,11 @@ Optional properties:
                           and enabling the backlight using GPIO.
   - pwm-off-delay-ms: Delay in ms between disabling the backlight using GPIO
                       and setting PWM value to 0.
+  - num-interpolated-steps: Number of interpolated steps between each value
+                            of brightness-levels table. This way a high
+                            resolution pwm duty cycle can be used without
+                            having to list out every possible value in the
+                            brightness-level array.
 
 [0]: Documentation/devicetree/bindings/pwm/pwm.txt
 [1]: Documentation/devicetree/bindings/gpio/gpio.txt
@@ -39,3 +44,17 @@ Example:
 		post-pwm-on-delay-ms = <10>;
 		pwm-off-delay-ms = <10>;
 	};
+
+Example using num-interpolation-steps:
+
+	backlight {
+		compatible = "pwm-backlight";
+		pwms = <&pwm 0 5000000>;
+
+		brightness-levels = <0 2048 4096 8192 16384 65535>;
+		num-interpolated-steps = <2048>;
+		default-brightness-level = <4096>;
+
+		power-supply = <&vdd_bl_reg>;
+		enable-gpios = <&gpio 58 0>;
+	};