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

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Date:   Tue, 9 Oct 2018 11:03:59 +0000
From:   Vokáč Michal <Michal.Vokac@...ft.com>
To:     Lukasz Majewski <lukma@...x.de>
CC:     Thierry Reding <thierry.reding@...il.com>,
        Rob Herring <robh+dt@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-pwm@...r.kernel.org" <linux-pwm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Fabio Estevam <fabio.estevam@....com>,
        Lothar Waßmann <LW@...o-electronics.de>
Subject: Re: [RFC PATCH 1/2] dt-bindings: pwm: imx: Allow switching PWM output
 between PWM and GPIO

On 23.8.2018 14:36, Lukasz Majewski wrote:
> Hi Michal,
> 
>> On 23.8.2018 12:40, Lukasz Majewski wrote:
>>
>> Hi Lukasz, thanks for the reply!
>>> Hi Michal,
>>>    
>>>> 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.
>>>
>>> Just to ask - Is your display equipped with power supply
>>> enable/disable pin?
>>
>> No it is not. The backlight on my display is just a bunch of serial-
>> parallel connected LEDs with separate GND and VCC pins on a separate
>> flex cable. And the display itself also does not have a reset or
>> enable signal. It is a PITA to use it I must say..
> 
> Yes, it seems so. I must have had more luck than you with the HW....
> 
>>    
>>> As fair as I remember the trick to avoid flickering the display
>>> was to disable the display (enable-gpio property) and set the PWM
>>> PIN as GPIO to high in u-boot.
>>
>> Yes, I know about that. I can not use this as the PWM output is the
>> only signal I have to control the backlight. I mentioned that
>> somewhere in the previous discussion with Lothar.
> 
> Yes, I've read it. I also find the PWM pinctrl as "default" state more
> natural.
> 
> One more idea - though.
> 
> In iMX6Q it was possible to specify the pinctrl PIN setup as 0x80000000
> - this means that it goes untouched to the IP block (configured by
>    bootloader).

Hi Lukasz,
Thank you for the ideas and sorry for the long delay.

 From reading a lot of replies from (not only) DT maintainers I got the
impression that using the 0x80000000 configuration value is generally
discouraged. And I support that idea.

> Maybe it would work to:
> 
> 1. Setup the PWM output as GPIO in u-boot (high)
> 
> 2. In PWM IMX probe configure PWM to be 100% duty cycle. And switch
> iomux to PWM function of the pin

The probe function should only prepare structures, not configure the HW.
Also you somehow need to know whether or not to enable the PWM/GPIO.
Normally it is the client (backlight, sysfs..) who calls the pwm_apply to
configure and enable/disable the PWM.

> 3. Then latter in the code PWM gets configured and we can control it in
> "normal" way ?
> 
> Or am I missing some important point?

Ideally, the final solution should allow all possible use-cases:

- inverted PWM, enabled from bootloader to userspace
- inverted PWM, disabled from bootloader to userspace

The same for non-inverted PWM.

What you are suggesting does not seem like it supports that idea.

I will send v2 with some proposed changes soon.

>>
>> I also think this could be useful not only for backlight. Any circuit
>> that requires truly inverted PWM signal can use it. I see it as an
>> enhancement to what you with Lothar have already done ;)
>>    
>>>>
>>>> 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
>>>> +	>;
>>>> +};
>>>> +
>>>> +pinctrl_backlight_pwm: pwm1grp-pwm {
>>>> +	fsl,pins = <
>>>> +		/* PWM output */
>>>> +		MX6QDL_PAD_GPIO_9__PWM1_OUT	0x8
>>>> +	>;
>>>> +};