linux-kernel - Re: [PATCH 2/2] backlight: pwm_bl: add configurable delay between re-enabling PWM and switching backlight power on

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Message-ID: <20171108083407.1ae55f91@karo-electronics.de>
Date:   Wed, 8 Nov 2017 08:34:07 +0100
From:   Lothar Waßmann <LW@...O-electronics.de>
To:     Rob Herring <robh@...nel.org>
Cc:     Lee Jones <lee.jones@...aro.org>,
        Bartlomiej Zolnierkiewicz <b.zolnierkie@...sung.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        Jacek Anaszewski <jacek.anaszewski@...il.com>,
        Jingoo Han <jingoohan1@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        Pavel Machek <pavel@....cz>,
        Richard Purdie <rpurdie@...ys.net>,
        Thierry Reding <thierry.reding@...il.com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-fbdev@...r.kernel.org" <linux-fbdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linux LED Subsystem <linux-leds@...r.kernel.org>,
        Linux PWM List <linux-pwm@...r.kernel.org>
Subject: Re: [PATCH 2/2] backlight: pwm_bl: add configurable delay between
 re-enabling PWM and switching backlight power on

Hi,

On Tue, 7 Nov 2017 18:46:18 -0600 Rob Herring wrote:
> On Mon, Oct 30, 2017 at 5:33 AM, Lee Jones <lee.jones@...aro.org> wrote:
> > On Fri, 27 Oct 2017, Rob Herring wrote:
> >
> >> On Thu, Oct 26, 2017 at 02:49:46PM +0200, Lothar Waßmann wrote:
> >> > When switching the backlight on, the LCD may need some time to adjust
> >> > to the configured PWM duty cycle. Add a configurable delay between
> >> > configuring the PWM and enabling the backlight regulator to account
> >> > for this.
> >> >
> >> > Signed-off-by: Lothar Waßmann <LW@...O-electronics.de>
> >> > ---
> >> >  .../bindings/leds/backlight/pwm-backlight.txt      |  4 ++++
> >> >  drivers/video/backlight/pwm_bl.c                   | 22 +++++++++++++++++++---
> >> >  2 files changed, 23 insertions(+), 3 deletions(-)
> >> >
> >> > diff --git a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
> >> > index 764db86..95594c3 100644
> >> > --- a/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
> >> > +++ b/Documentation/devicetree/bindings/leds/backlight/pwm-backlight.txt
> >> > @@ -17,6 +17,10 @@ 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])
> >> > +  - turn-on-delay-ms: delay in milliseconds between configuring the PWM
> >> > +             and switching PWM on. This may be required to eliminate
> >> > +             flicker when switching the PWM on after it has been
> >> > +             disabled.
> >>
> >> The subject talks about turning on the backlight regulator, but this
> >> only talks about the PWM.
> >
> > From my understanding "switching PWM on" == enabling the regulator.
> >
> > I don't see any disparity here.  Or am I missing something?
> 
> The delay is from (re-)enabling the PWM (turning it on) to switching
> backlight power on. That sounds like 2 different things to me.
> 
The text should better read:
+  - turn-on-delay-ms: delay in milliseconds between configuring the PWM
+             and switching the backlight regulator on. This may be
+             required to eliminate flicker when switching the backlight
+             on after it had been disabled.



Lothar Waßmann