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    SubjectRe: [RFC PATCH 1/2] dt-bindings: pwm: imx: Allow switching PWM output between PWM and GPIO
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    On 22.8.2018 08:14, Lothar Waßmann wrote:
    > Michal Vokáč <michal.vokac@ysoft.com> 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 Vokáč <michal.vokac@ysoft.com>
    >> ---
    >> 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.

    >> +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
    >> + >;
    >> +};
    >
    >
    >

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