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Date:   Fri, 31 Aug 2018 15:30:43 +0200
From:   Lothar Waßmann <LW@...O-electronics.de>
To:     Michal Vokáč <michal.vokac@...ft.com>
Cc:     Rob Herring <robh@...nel.org>,
        Thierry Reding <thierry.reding@...il.com>,
        Mark Rutland <mark.rutland@....com>,
        devicetree@...r.kernel.org, linux-pwm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Lukasz Majewski <l.majewski@...ess.pl>,
        Fabio Estevam <fabio.estevam@....com>
Subject: Re: [RFC PATCH 1/2] dt-bindings: pwm: imx: Allow switching PWM
 output between PWM and GPIO

Michal Vokáč <michal.vokac@...ft.com> wrote:

> On 31.8.2018 14:45, Lothar Waßmann wrote:
> > Rob Herring <robh@...nel.org> wrote:
> >   
> >> On Tue, Aug 21, 2018 at 04:38:52PM +0200, Michal Vokáč 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@...ft.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@...b4000 {
> >>> @@ -25,3 +31,41 @@ pwm1: pwm@...b4000 {
> >>>   	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.
> 
> > 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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