Michal Vokáč [off-list ref] wrote:

On 31.8.2018 14:45, Lothar Waßmann wrote:

quoted

Rob Herring [off-list ref] wrote:
  

quoted

On Tue, Aug 21, 2018 at 04:38:52PM +0200, Michal Vokáč 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
+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  

There's a slight problem here if I remember the i.MX pin muxing. In GPIO
mode, doesn't the GPIO block control the direction and level if an
output. I guess as long as unused GPIOs are all initialized to inputs it
will be okay.  

I am not sure if I understand you correctly. Did you mean: "..doesn't the
GPIO block control the PULL-UP/DOWN and level if an output."? Yes, that is
true. And as you said, all GPIOs are configured as inputs after reset.

quoted

One could set the pad_ctl DSE value to 0, so that the pin cannot be
driven even if configured as output:
		MX6QDL_PAD_GPIO_9__GPIO1_IO09	0xF000  

Yes, it will make no harm to set the pin to high-Z if configured as
output. Though I am not sure that this makes sense.

If you want to rely on the function of the Pull resistors this is
exactly what you need.

In case we choose the pull-up to keep the level high the pin needs to stay
configured as input. And as the GPIO is reserved for us there is actually
no one else who could re-configure it.

U-Boot may have configured the PWM pin as output to enable the
backlight without brightness control.

In case we choose to actively drive the pin instead of relying on the
internal pull-up we need to use gpiod lib and configure the pin as output.
In that case DSE must be set non-zero.

That is my personal preference too.


Lothar Waßmann
-- 
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