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

Hi,

On Wed, 8 Nov 2017 09:12:12 +0000 Lee Jones wrote:

On Wed, 08 Nov 2017, Lothar Waßmann wrote:

quoted

Hi,

On Tue, 7 Nov 2017 18:46:18 -0600 Rob Herring wrote:

quoted

On Mon, Oct 30, 2017 at 5:33 AM, Lee Jones [off-list ref] wrote:

quoted

On Fri, 27 Oct 2017, Rob Herring wrote:

quoted

On Thu, Oct 26, 2017 at 02:49:46PM +0200, Lothar Waßmann wrote:

quoted

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 <redacted>
---
 .../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).

quoted

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