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

Message ID 1509022186-19636-4-git-send-email-LW@KARO-electronics.de
State Superseded
Headers show
Series
  • backlight: pwm_bl: prevent backlight flicker when switching PWM on
Related show

Commit Message

Lothar Waßmann Oct. 26, 2017, 12:49 p.m.
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(-)

Comments

Rob Herring Oct. 27, 2017, 2:38 p.m. | #1
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
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Lee Jones Oct. 30, 2017, 10:33 a.m. | #2
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
Rob Herring Nov. 8, 2017, 12:46 a.m. | #3
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
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Lothar Waßmann Nov. 8, 2017, 7:34 a.m. | #4
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
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Lee Jones Nov. 8, 2017, 9:12 a.m. | #5
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?
Lothar Waßmann Nov. 8, 2017, 9:44 a.m. | #6
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
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Patch

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");
 
 	/*