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

["Michal Vokáč" <michal.vokac@ysoft.com>]

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