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

From: Michal VokÃÄ
Date: Thu Jan 24 2019 - 03:59:35 EST

On 13.12.2018 09:56, Uwe Kleine-KÃnig wrote:
On Wed, Dec 12, 2018 at 12:04:51PM +0000, VokÃÄ 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..