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

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Message-ID: <20181213085605.j2e76y67srg5qfbc@pengutronix.de>
Date:   Thu, 13 Dec 2018 09:56:05 +0100
From:   Uwe Kleine-König 
        <u.kleine-koenig@...gutronix.de>
To:     Vokáč Michal <Michal.Vokac@...ft.com>
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

Hello,

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.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |