Messages in this thread |  | | Subject | Re: [RFC PATCH 1/2] dt-bindings: pwm: imx: Allow switching PWM output between PWM and GPIO | From | Michal Vokáč <> | Date | Fri, 31 Aug 2018 15:17:10 +0200 |
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On 31.8.2018 14:45, Lothar Waßmann wrote: > Rob Herring <robh@kernel.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@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 >>> +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.
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.
Michal
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