From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751440AbdIELIB (ORCPT ); Tue, 5 Sep 2017 07:08:01 -0400 Received: from mail-wm0-f50.google.com ([74.125.82.50]:37607 "EHLO mail-wm0-f50.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751119AbdIELH7 (ORCPT ); Tue, 5 Sep 2017 07:07:59 -0400 X-Google-Smtp-Source: ADKCNb4iDX9HIWHms4fbIcTz1M9S5X3eNnOQ2Q0/ttgWJ5HU4xjkqv2t02qi55NaGhdIdzOBPzmS1A== Subject: Re: [RFC 1/2] dt-bindings: pwm-backlight: add brightness-levels-scale property To: Enric Balletbo i Serra , Lee Jones , Jingoo Han , Richard Purdie , Jacek Anaszewski , Pavel Machek , Rob Herring , Mark Rutland , Doug Anderson , Brian Norris , Guenter Roeck Cc: linux-leds@vger.kernel.org, devicetree@vger.kernel.org, linux-kernel@vger.kernel.org References: <20170904153504.27963-1-enric.balletbo@collabora.com> <20170904153504.27963-2-enric.balletbo@collabora.com> From: Daniel Thompson Message-ID: Date: Tue, 5 Sep 2017 12:07:55 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.3.0 MIME-Version: 1.0 In-Reply-To: <20170904153504.27963-2-enric.balletbo@collabora.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/09/17 16:35, Enric Balletbo i Serra wrote: > 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 > --- > .../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. Even if we do keep this property (see first e-mail) this second value seems pointless to me; can't the driver just use the actual PWM max counter value for this? Daniel. > > [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>; > + }; >