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 | Wed, 22 Aug 2018 09:01:50 +0200 |
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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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