[PATCH 0/2] backlight: pwm_bl: prevent backlight flicker when switching PWM on

[PATCH 0/2] backlight: pwm_bl: prevent backlight flicker when switching PWM on

[Lothar Waßmann]

These patches implement some measures to prevent backlight flicker
when the backlight is being switched on for a display with an active
low brightness control pin.

[PATCH 0/2] backlight: pwm_bl: prevent backlight flicker when switching PWM on

[Lothar Waßmann]

These patches implement some measures to prevent backlight flicker
when the backlight is being switched on for a display with an active
low brightness control pin.
GIT: [PATCH 1/2] backlight: pwm_bl: Enable PWM before switching regulator
GIT: [PATCH 2/2] backlight: pwm_bl: add configurable delay between

Re: [PATCH 0/2] backlight: pwm_bl: prevent backlight flicker when switching PWM on

[Daniel Thompson]

On 26/10/17 13:49, Lothar Waßmann wrote:
> These patches implement some measures to prevent backlight flicker
> when the backlight is being switched on for a display with an active
> low brightness control pin.
> GIT: [PATCH 1/2] backlight: pwm_bl: Enable PWM before switching regulator
> GIT: [PATCH 2/2] backlight: pwm_bl: add configurable delay between

Other than hoisting the pwm_enable() even earlier in the setup sequence 
this patchset seems to have a significant overlap with Enric's recent 
posting:

   https://lkml.org/lkml/2017/7/21/211

Any chance of a shared view on this, especially on the DT bindings?



Daniel.

Re: [PATCH 0/2] backlight: pwm_bl: prevent backlight flicker when switching PWM on

[Enric Balletbo i Serra]

Hi all,

On 08/11/17 11:48, Daniel Thompson wrote:
> On 26/10/17 13:49, Lothar Waßmann wrote:
>> These patches implement some measures to prevent backlight flicker
>> when the backlight is being switched on for a display with an active
>> low brightness control pin.
>> GIT: [PATCH 1/2] backlight: pwm_bl: Enable PWM before switching regulator
>> GIT: [PATCH 2/2] backlight: pwm_bl: add configurable delay between
> 
> Other than hoisting the pwm_enable() even earlier in the setup sequence this
> patchset seems to have a significant overlap with Enric's recent posting:
> 
>   https://lkml.org/lkml/2017/7/21/211
> 
> Any chance of a shared view on this, especially on the DT bindings?
> 

The DT binding were discussed some time ago for the patch series I sent, though
there isn't a final ack from DT maintainer.

Lothar, the series I sent have been reviewed and acked, can you test if the
series fixes the problem for you too?

Thanks,
 Enric

> 
> 
> Daniel.

Re: [PATCH 0/2] backlight: pwm_bl: prevent backlight flicker when switching PWM on

[Lothar Waßmann]

Hi,

On Fri, 10 Nov 2017 12:22:15 +0100 Enric Balletbo i Serra wrote:
> Hi all,
> 
> On 08/11/17 11:48, Daniel Thompson wrote:
> > On 26/10/17 13:49, Lothar Waßmann wrote:
> >> These patches implement some measures to prevent backlight flicker
> >> when the backlight is being switched on for a display with an active
> >> low brightness control pin.
> >> GIT: [PATCH 1/2] backlight: pwm_bl: Enable PWM before switching regulator
> >> GIT: [PATCH 2/2] backlight: pwm_bl: add configurable delay between
> > 
> > Other than hoisting the pwm_enable() even earlier in the setup sequence this
> > patchset seems to have a significant overlap with Enric's recent posting:
> > 
> >   https://lkml.org/lkml/2017/7/21/211
> > 
> > Any chance of a shared view on this, especially on the DT bindings?
> > 
> 
> The DT binding were discussed some time ago for the patch series I sent, though
> there isn't a final ack from DT maintainer.
> 
> Lothar, the series I sent have been reviewed and acked, can you test if the
> series fixes the problem for you too?
> 
I'll try to test it within the next couple of days and will report back.


Lothar Waßmann

[PATCH 1/2] backlight: pwm_bl: Enable PWM before switching regulator on

[Lothar Waßmann]

When the PWM is re-enabled after being switched off, the backlight may
flicker due to the backlight power being switched on while the PWM is
still unconfigured. This is especially true if the PWM output is
active low (assuming that the PWM output is low when the PWM is not
enabled).
Configure the PWM before switching backlight power on to prevent this.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
---
 drivers/video/backlight/pwm_bl.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 002f1ce..921f322 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -50,6 +50,8 @@ static void pwm_backlight_power_on(struct pwm_bl_data *pb, int brightness)
 	if (pb->enabled)
 		return;
 
+	pwm_enable(pb->pwm);
+
 	err = regulator_enable(pb->power_supply);
 	if (err < 0)
 		dev_err(pb->dev, "failed to enable power supply\n");
@@ -57,7 +59,6 @@ static void pwm_backlight_power_on(struct pwm_bl_data *pb, int brightness)
 	if (pb->enable_gpio)
 		gpiod_set_value_cansleep(pb->enable_gpio, 1);
 
-	pwm_enable(pb->pwm);
 	pb->enabled = true;
 }
 
-- 
2.1.4

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

[Lothar Waßmann]

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@KARO-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.
 
 [0]: Documentation/devicetree/bindings/pwm/pwm.txt
 [1]: Documentation/devicetree/bindings/gpio/gpio.txt
diff --git a/drivers/video/backlight/pwm_bl.c b/drivers/video/backlight/pwm_bl.c
index 921f322..9578f65 100644
--- a/drivers/video/backlight/pwm_bl.c
+++ b/drivers/video/backlight/pwm_bl.c
@@ -23,6 +23,7 @@
 #include <linux/pwm_backlight.h>
 #include <linux/regulator/consumer.h>
 #include <linux/slab.h>
+#include <linux/delay.h>
 
 struct pwm_bl_data {
 	struct pwm_device	*pwm;
@@ -35,6 +36,7 @@ struct pwm_bl_data {
 	struct gpio_desc	*enable_gpio;
 	unsigned int		scale;
 	bool			legacy;
+	int			turn_on_delay_ms;
 	int			(*notify)(struct device *,
 					  int brightness);
 	void			(*notify_after)(struct device *,
@@ -52,6 +54,17 @@ static void pwm_backlight_power_on(struct pwm_bl_data *pb, int brightness)
 
 	pwm_enable(pb->pwm);
 
+	if (pb->turn_on_delay_ms > 0) {
+		dev_dbg(pb->dev, "Sleeping %u..%uµs\n",
+			pb->turn_on_delay_ms * 1000,
+			pb->turn_on_delay_ms * 1100);
+		if (pb->turn_on_delay_ms > 20)
+			msleep(pb->turn_on_delay_ms);
+		else
+			usleep_range(pb->turn_on_delay_ms * 1000,
+			     pb->turn_on_delay_ms * 1100);
+	}
+
 	err = regulator_enable(pb->power_supply);
 	if (err < 0)
 		dev_err(pb->dev, "failed to enable power supply\n");
@@ -108,8 +121,9 @@ static int pwm_backlight_update_status(struct backlight_device *bl)
 		duty_cycle = compute_duty_cycle(pb, brightness);
 		pwm_config(pb->pwm, duty_cycle, pb->period);
 		pwm_backlight_power_on(pb, brightness);
-	} else
+	} else {
 		pwm_backlight_power_off(pb);
+	}
 
 	if (pb->notify_after)
 		pb->notify_after(pb->dev, brightness);
@@ -264,9 +278,9 @@ static int pwm_backlight_probe(struct platform_device *pdev)
 				pb->scale = data->levels[i];
 
 		pb->levels = data->levels;
-	} else
+	} else {
 		pb->scale = data->max_brightness;
-
+	}
 	pb->notify = data->notify;
 	pb->notify_after = data->notify_after;
 	pb->check_fb = data->check_fb;
@@ -329,6 +343,8 @@ static int pwm_backlight_probe(struct platform_device *pdev)
 		goto err_alloc;
 	}
 
+	of_property_read_u32(node, "turn-on-delay-ms", &pb->turn_on_delay_ms);
+
 	dev_dbg(&pdev->dev, "got pwm for backlight\n");
 
 	/*
-- 
2.1.4

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

[Rob Herring]

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@KARO-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.

>  
>  [0]: Documentation/devicetree/bindings/pwm/pwm.txt
>  [1]: Documentation/devicetree/bindings/gpio/gpio.txt

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

[Lee Jones]

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@KARO-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?

> >  [0]: Documentation/devicetree/bindings/pwm/pwm.txt
> >  [1]: Documentation/devicetree/bindings/gpio/gpio.txt

-- 
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
Follow Linaro: Facebook | Twitter | Blog

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

[Rob Herring]

On Mon, Oct 30, 2017 at 5:33 AM, Lee Jones <lee.jones@linaro.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@KARO-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.

Rob

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

[Lothar Waßmann]

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@linaro.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@KARO-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

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

[Lee Jones]

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@linaro.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@KARO-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
Follow Linaro: Facebook | Twitter | Blog

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

[Lothar Waßmann]

Hi,

On Wed, 8 Nov 2017 09:12:12 +0000 Lee Jones wrote:
> 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@linaro.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@KARO-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?
> 
It actually is a Power Control input of the LCD panel, that is
modeled as a fiexd-regulator in Linux.


Lothar Waßmann