On 22.8.2018 08:14, Lothar Waßmann wrote:

Michal Vokáč [off-list ref] wrote:

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.

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

The "default" function of a PWM is to deliver a PWM signal. So it is
more sensible to me to have the PWM function as "default" and a "gpio"
function as alternative state.

Yes, I totally agree that using "default" for PWM and "gpio" as the
alternative function seems more sensible. That is actually how I started.
Then I realized that that way you end up with the PWM pad set to zero
until the first call of imx_pwm_apply_v2 where you can select the GPIO
function. On my system that first call is made by pwm-backlight more than
3s after pinctrl init.

I suggested to use the "default" state as a GPIO function as the only way
how to get a truly inverted PWM output all the time from power-up to
power-down.

In my opinion it is up to the DT author what pad configuration he uses for
each pinctrl function as he knows what the HW really needs. I see that this
approach is kind of controversial but I hope that with good documentation
this would not be a problem. And as I wrote in the intro, it is absolutely
optional. If you do not need it, you do not use it.

quoted

+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
+	>;
+};