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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Date:   Wed, 8 Nov 2017 09:12:12 +0000
From:   Lee Jones <lee.jones@...aro.org>
To:     Lothar Waßmann <LW@...O-electronics.de>
Cc:     Rob Herring <robh@...nel.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

On Wed, 08 Nov 2017, Lothar Waßmann wrote:

> 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.

I read it as the delay being between "configuring the PWM" (still not
on) and "switching <the> PWM on" (enabling the regulator).

> 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.

How is the regulator described in the datasheet?

Is it for the Backlight or the PWM?

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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