From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752321AbcFJHoQ (ORCPT ); Fri, 10 Jun 2016 03:44:16 -0400 Received: from mail-wm0-f48.google.com ([74.125.82.48]:35392 "EHLO mail-wm0-f48.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751645AbcFJHoO (ORCPT ); Fri, 10 Jun 2016 03:44:14 -0400 Date: Fri, 10 Jun 2016 08:44:49 +0100 From: Lee Jones To: Lothar =?iso-8859-1?Q?Wa=DFmann?= Cc: Jean-Christophe Plagniol-Villard , Jingoo Han , Thierry Reding , Tomi Valkeinen , linux-fbdev@vger.kernel.org, linux-kernel@vger.kernel.org, linux-pwm@vger.kernel.org, Marcel Ziswiler , Ian Campbell , Kumar Gala , Mark Rutland , Pawel Moll , Rob Herring , devicetree@vger.kernel.org Subject: Re: [PATCHv2] backlight: pwm_bl: disable PWM when 'duty_cycle' is zero Message-ID: <20160610074449.GB1537@dell> References: <1465294429-8570-1-git-send-email-LW@KARO-electronics.de> <20160609135125.GA2385@dell> <20160610072346.51b0a5d5@ipc1.ka-ro> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20160610072346.51b0a5d5@ipc1.ka-ro> User-Agent: Mutt/1.5.24 (2015-08-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 10 Jun 2016, Lothar Waßmann wrote: > Hi, > > On Thu, 9 Jun 2016 14:51:25 +0100 Lee Jones wrote: > > On Tue, 07 Jun 2016, Lothar Waßmann wrote: > > > > > 'brightness' is usually an index into a table of duty_cycle values, > > > where the value at index 0 may well be non-zero > > > (tegra30-apalis-eval.dts and tegra30-colibri-eval-v3.dts are real-life > > > examples). > > > Thus brightness == 0 does not necessarily mean that the PWM output > > > will be inactive. > > > Check for 'duty_cycle == 0' rather than 'brightness == 0' to decide > > > whether to disable the PWM. > > > > > > Signed-off-by: Lothar Waßmann > > > --- > > > Changes wrt. v1: > > > - update binding docs to reflect the change > > > > > > .../devicetree/bindings/leds/backlight/pwm-backlight.txt | 9 ++++++--- > > > drivers/video/backlight/pwm_bl.c | 4 ++-- > > > 2 files changed, 8 insertions(+), 5 deletions(-) > > > > > > diff --git a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt > > > index 764db86..95fa8a9 100644 > > > --- a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt > > > +++ b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt > > > @@ -4,10 +4,13 @@ Required properties: > > > - compatible: "pwm-backlight" > > > - pwms: OF device-tree PWM specification (see PWM binding[0]) > > > - brightness-levels: Array of distinct brightness levels. Typically these > > > - are in the range from 0 to 255, but any range starting at 0 will do. > > > + are in the range from 0 to 255, but any range will do. > > > The actual brightness level (PWM duty cycle) will be interpolated > > > - from these values. 0 means a 0% duty cycle (darkest/off), while the > > > - last value in the array represents a 100% duty cycle (brightest). > > > + from these values. 0 means a 0% duty cycle, while the highest value in > > > + the array represents a 100% duty cycle. > > > + The range may be in reverse order (starting with the maximum duty cycle > > > + value) to create a PWM signal with the 100% duty cycle representing > > > + minimum and 0% duty cycle maximum brigthness. > > > - default-brightness-level: the default brightness level (index into the > > > array defined by the "brightness-levels" property) > > > - power-supply: regulator for supply voltage > > > diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c > > > index b2b366b..80b2b52 100644 > > > --- a/drivers/video/backlight/pwm_bl.c > > > +++ b/drivers/video/backlight/pwm_bl.c > > > @@ -103,8 +103,8 @@ static int pwm_backlight_update_status(struct backlight_device *bl) > > > if (pb->notify) > > > brightness = pb->notify(pb->dev, brightness); > > > > > > - if (brightness > 0) { > > > - duty_cycle = compute_duty_cycle(pb, brightness); > > > + duty_cycle = compute_duty_cycle(pb, brightness); > > > + if (duty_cycle > 0) { > > > > How does this work in the aforementioned: > > > > "The range may be in reverse order" > > > > ... case? Surely when duty_cycle is when the screen should be at it's > > brightest? Wouldn't it confuse the user if they turn their brightness > > *up* and the screen goes *off*? > > > Assuming that the PWM output is inactive (LOW) when the duty_cycle is > set to zero, there will be no difference between operating the PWM at > duty_cycle 0 or disabling it. > > Currently, the screen will go bright when it should be off in this > case. It sounds like we need something that lets the framework know if duty_cycle = MAX is the brightest or if duty_cycle = 0 is. Either way someone is going to get screwed by this logic. -- Lee Jones Linaro STMicroelectronics Landing Team Lead Linaro.org │ Open source software for ARM SoCs Follow Linaro: Facebook | Twitter | Blog