Re: Reading current output value

[Yegor Yefremov <yegorslists@xxxxxxxxxxxxxx>]

Hi Kent,

On Wed, Jun 22, 2022 at 12:26 PM Kent Gibson <warthog618@xxxxxxxxx> wrote:
>
> On Wed, Jun 22, 2022 at 09:54:41AM +0200, Yegor Yefremov wrote:
> > On a am335x based board I have a GPIO pin that enables/disables power
> > of an external device (the bootloader sets this pin to output and 1,
> > and the kernel is instructed to not change it). Using kernel
> > 5.19.0-rc2 and sysfs interface, I can read the current status as
> > follows:
> >
> > echo 68 > /sys/class/gpio/export
> > cat /sys/class/gpio/gpio68/value
> >
> > As a result, I read 1.
> >
> > Using gpioget (libgpiod) v1.6.3, the line will be configured to
> > "input" and the value is set to 0:
> >
> > # gpioget 2 4
> > 0
> >
> > So, how can I read the state without changing it? I am mostly
> > interested in using the kernel userspace API directly.
> >
>
> The API itself supports it, but it isn't exposed in gpioget v1.6.3.
> The gpioget in libgpiod master has a --dir-as-is option for exactly
> this case, but that hasn't made its way into a libgpiod release yet.
> (commit 3a912fbc1e2 tools: gpioget: add new --dir-as-is option for GPO read-back)
> Can you try master?

# gpioget -v
gpioget (libgpiod) v2.0-devel
Copyright (C) 2017-2018 Bartosz Golaszewski
License: LGPLv2.1
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Now, I get my "1", but as soon as gpioget exits, the pin goes at "0".

> > By the way, setting pin to 0 works but not to 1:
> > gpioset 2 4=0 - OK
> > gpioset 2 4=1 - no level change
> >
>
> gpioset has to remain running to guarantee the output level.
> The pin is probably reverting when gpioset exits.
> Try the --mode=wait option.

Yes, this works, as long as gpioset is running.

Yegor