Hi Michal,

On 23.8.2018 12:40, Lukasz Majewski wrote:

Hi Lukasz, thanks for the reply!

quoted

Hi Michal,
  

quoted

Output of the PWM block of i.MX SoCs is always zero volts when the
block is disabled. This can caue issues when inverted PWM polarity
is needed. With inverted polarity a duty cycle = 0% corresponds to
solid high level on the output. If the PWM is dissabled its output
instantly goes to solid zero which corresponds to duty cycle =
100%.

To have a trully inverted PWM output configure the PWM pad as a
GPIO with pull-up. Then switch the pad to PWM output whenever
non-zero duty cycle is needed.  

Just to ask - Is your display equipped with power supply
enable/disable pin?  

No it is not. The backlight on my display is just a bunch of serial-
parallel connected LEDs with separate GND and VCC pins on a separate
flex cable. And the display itself also does not have a reset or
enable signal. It is a PITA to use it I must say..

Yes, it seems so. I must have had more luck than you with the HW....

quoted

As fair as I remember the trick to avoid flickering the display
was to disable the display (enable-gpio property) and set the PWM
PIN as GPIO to high in u-boot.  

Yes, I know about that. I can not use this as the PWM output is the
only signal I have to control the backlight. I mentioned that
somewhere in the previous discussion with Lothar.

Yes, I've read it. I also find the PWM pinctrl as "default" state more
natural.

One more idea - though.

In iMX6Q it was possible to specify the pinctrl PIN setup as 0x80000000
- this means that it goes untouched to the IP block (configured by
  bootloader).

Maybe it would work to:

1. Setup the PWM output as GPIO in u-boot (high)

2. In PWM IMX probe configure PWM to be 100% duty cycle. And switch
iomux to PWM function of the pin

3. Then latter in the code PWM gets configured and we can control it in
"normal" way ?

Or am I missing some important point?

I also think this could be useful not only for backlight. Any circuit
that requires truly inverted PWM signal can use it. I see it as an
enhancement to what you with Lothar have already done ;)
  

quoted

quoted

Signed-off-by: Michal Vokáč <redacted>
---
  Documentation/devicetree/bindings/pwm/imx-pwm.txt | 44
+++++++++++++++++++++++ 1 file changed, 44 insertions(+)

diff --git a/Documentation/devicetree/bindings/pwm/imx-pwm.txt

b/Documentation/devicetree/bindings/pwm/imx-pwm.txt index
c61bdf8..3b1bc4c 100644 ---
a/Documentation/devicetree/bindings/pwm/imx-pwm.txt +++
b/Documentation/devicetree/bindings/pwm/imx-pwm.txt @@ -14,6 +14,12

@@ See the clock consumer binding,

Documentation/devicetree/bindings/clock/clock-bindings.txt
  - interrupts: The interrupt for the pwm controller
  
+Optional properties:
+- pinctrl: For i.MX27 and newer SoCs. Add extra pinctrl to
configure the PWM
+  pin to gpio function.  It allows control over the pin output
level when the
+  PWM block is disabled. This is meant to be used if inverted
polarity of the
+  PWM signal is required. See "Inverted PWM output" section
bellow. +
  Example:
  
  pwm1: pwm@53fb4000 {

@@ -25,3 +31,41 @@ pwm1: pwm@53fb4000 {
  	clock-names = "ipg", "per";
  	interrupts = <61>;
  };
+
+Inverted PWM output
+-------------------
+
+The i.MX SoC has such limitation that whenever a pad is configured

as a PWM +output, the output level is always zero volts when the
PWM block is disabled. +The zero output level is actively driven
by the output stage of the PWM block +and can not be overridden by
pull-up. It also does not matter what PWM polarity +a PWM client
(e.g. backlight) requested. +
+To gain control of the PWM output level in disabled state two
pinctrl states +can be used. The "default" state and the "pwm"
state. In the default state the +PWM output is configured as a
GPIO with pull-up. In the "pwm" state the output +is configured as
a PWM output. This setup assures that the PWM output is at +the
required level that corresponds to duty cycle = 0 when PWM is
disabled. +E.g. at boot. +
+Example:
+
+&pwm1 {
+	pinctrl-names = "default", "pwm";
+	pinctrl-0 = <&pinctrl_backlight_gpio>;
+	pinctrl-1 = <&pinctrl_backlight_pwm>;
+}
+
+pinctrl_backlight_gpio: pwm1grp-gpio {
+	fsl,pins = <
+		/* GPIO with 22kOhm pull-up */
+		MX6QDL_PAD_GPIO_9__GPIO1_IO09	0xF008
+	>;
+};
+
+pinctrl_backlight_pwm: pwm1grp-pwm {
+	fsl,pins = <
+		/* PWM output */
+		MX6QDL_PAD_GPIO_9__PWM1_OUT	0x8
+	>;
+};  

Best regards,

Lukasz Majewski

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