Re: [RFC PATCH 1/2] dt-bindings: pwm: imx: Allow switching PWM output between PWM and GPIO

From: Michal VokÃÄ
Date: Fri Aug 31 2018 - 09:20:23 EST

On 31.8.2018 14:45, Lothar WaÃmann wrote:
Rob Herring <robh@xxxxxxxxxx> wrote:

On Tue, Aug 21, 2018 at 04:38:52PM +0200, Michal VokÃÄ wrote:
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 VokÃÄ <michal.vokac@xxxxxxxxx>
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,
- 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.
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.
+&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.

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

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.

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.

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.