linux-kernel - Re: [RFC PATCH v4 2/2] pwm: imx: Configure output to GPIO in disabled state

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Date:   Thu, 24 Jan 2019 09:54:16 +0100
From:   Michal Vokáč <michal.vokac@...ft.com>
To:     Uwe Kleine-König <u.kleine-koenig@...gutronix.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>,
        Lukasz Majewski <l.majewski@...ess.pl>,
        Fabio Estevam <fabio.estevam@....com>,
        Lothar Waßmann <LW@...o-electronics.de>,
        Linus Walleij <linus.walleij@...aro.org>
Subject: Re: [RFC PATCH v4 2/2] pwm: imx: Configure output to GPIO in disabled
 state

On 13.12.2018 09:56, Uwe Kleine-König wrote:
> On Wed, Dec 12, 2018 at 12:04:51PM +0000, Vokáč Michal wrote:
>> Normally the PWM output is held LOW when PWM is disabled. This can cause
>> problems when inverted PWM signal polarity is needed. With this behavior
>> the connected circuit is fed by 100% duty cycle instead of being shut-off.
>>
>> Allow users to define a "pwm" and a "gpio" pinctrl states. The pwm pinctrl
>> state is selected when PWM is enabled and the gpio pinctrl state is
>> selected when PWM is disabled. In the gpio state the new pwm-gpios GPIO is
>> configured as input and the internal pull-up resistor is used to pull the
>> output level high.
>>
>> If all the pinctrl states and the pwm-gpios GPIO are not correctly
>> specified in DT the PWM work as usual.
>>
>> As an example, with this patch a PWM controlled backlight with inversed
>> signal polarity can be used in full brightness range. Without this patch
>> the backlight can not be turned off as brightness = 0 disables the PWM
>> and that in turn set PWM output LOW, that is full brightness.
>>
>> Inverted output of the PWM with "default" and with "pwm"+"gpio" pinctrl:
>>
>> +--------------+------------+---------------+----------- +-------------+
>> | After reset  | Bootloader |   PWM probe   |     PWM    |     PWM     |
>> | 100k pull-up |            |               | enable 30% |   disable   |
>> +--------------+------------+---------------+------------+-------------+
>> | pinctrl      |    none    |    default    |   default  |   default   |
>> | out H   __________________                   __   __                 |
>> | out L                     \_________________/  \_/  \_/\____________ |
>> |                                           ^    ^    ^                |
>> +--------------+------------+---------------+------------+-------------+
>> | pinctrl      |    none    |      gpio     |     pwm    |     gpio    |
>> | out H   __________________________________   __   __   _____________ |
>> | out L                                     \_/  \_/  \_/              |
>> |                                           ^    ^    ^                |
>> +----------------------------------------------------------------------+
> 
> Just for the record: My last concern against this patch set (that I sent
> for v3) and v4 of the series criss-crossed. So the problem with the
> peaks that could happen is still unaddressed.

Hi Uwe et al.

Sorry for the huge delay. I hope we will be able to refresh our
memories and continue on the discussion. I will react to your
comments in the appropriate v3 thread. And sorry for this v4.
I was too fast on the trigger back then..

Michal