From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932360Ab2IURnp (ORCPT ); Fri, 21 Sep 2012 13:43:45 -0400 Received: from avon.wwwdotorg.org ([70.85.31.133]:48566 "EHLO avon.wwwdotorg.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932122Ab2IURnn (ORCPT ); Fri, 21 Sep 2012 13:43:43 -0400 Message-ID: <505CA74B.2090405@wwwdotorg.org> Date: Fri, 21 Sep 2012 11:43:39 -0600 From: Stephen Warren User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:15.0) Gecko/20120827 Thunderbird/15.0 MIME-Version: 1.0 To: "Philip, Avinash" CC: "grant.likely@secretlab.ca" , "rob.herring@calxeda.com" , "rob@landley.net" , "rpurdie@rpsys.net" , "thierry.reding@avionic-design.de" , "broonie@opensource.wolfsonmicro.com" , "shawn.guo@linaro.org" , "devicetree-discuss@lists.ozlabs.org" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "Nori, Sekhar" , "Hebbar, Gururaja" Subject: Re: [PATCH v2] pwm_backlight: Add device tree support for Low Threshold Brightness References: <1348203080-16348-1-git-send-email-avinashphilip@ti.com> <505BF83D.8040201@wwwdotorg.org> <518397C60809E147AF5323E0420B992E3E9A685B@DBDE01.ent.ti.com> In-Reply-To: <518397C60809E147AF5323E0420B992E3E9A685B@DBDE01.ent.ti.com> X-Enigmail-Version: 1.4.4 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 09/21/2012 12:03 AM, Philip, Avinash wrote: > Hi Stephen, > > On Fri, Sep 21, 2012 at 10:46:45, Stephen Warren wrote: >> On 09/20/2012 10:51 PM, Philip, Avinash wrote: >>> Some backlights 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. >>> >>> This patch adds support for an optional low-threshold-brightness >>> property. >> >>> diff --git a/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/video/backlight/pwm-backlight.txt >> >>> 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. >> >> For my education, why not just specify values above this value in the >> brightness-levels array; how do those two interact? > > Please find details from > https://lkml.org/lkml/2012/7/18/284 Hmm. That still doesn't really explain what this property does. I'm going to guess that if this property is present, and values in the brightness-levels property get scaled between the low-threshold-brightness and 255 instead of being used directly. But then, in the email you linked to, what does "But brightness-levels won't be uniformly divided" mean? Why would doing the calculation at run-time be any better than simply putting the correct values into brightness-levels in the first place? Either way, the DT binding should explain exactly what this value is used for, and how it affects the interpretation of values in brightness-levels.