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

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Date:   Fri, 31 Aug 2018 14:45:48 +0200
From:   Lothar Waßmann <LW@...O-electronics.de>
To:     Rob Herring <robh@...nel.org>
Cc:     Michal Vokáč <michal.vokac@...ft.com>,
        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

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.
> 
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


Lothar Waßmann
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