Re: [libgpiod] feature request: output state read and sustain

[Kent Gibson <warthog618@xxxxxxxxx>]

On Tue, Jun 28, 2022 at 03:08:20PM +0200, Jiří Prchal wrote:
> Hi,
> using new libgpiod / chardev driver, is there any way to get state of
> output? I mean one process sets it for example to 1 and another process
> reads this output state for example to show that on web page.
> I have to say that old sysfs interface was more user friendly...
> 

"new" being anything since Linux 4.8 ;-)?
And strictly speaking it isn't a driver - libgpiod and the GPIO subsystem
provide an interface to the chip driver.  More on that later.

Only the process holding the line has access to the current value.
If you need that value elsewhere then it has to be published by that
process - it is not available through the GPIO API itself.
There is nothing preventing that process publishing the value
in whatever way is appropriate to your application.
e.g. write it to a file that can be read by your webapp, just as it
would from sysfs.

Less restrictive access models are frequently "more user friendly", but
have other issues. e.g. some misbehaving process just reset your
modem for you.

And sysfs has other great features like being slow and being complete
rubbish for events on input lines.

> And at second: it would be better to NOT "the state of a GPIO line
> controlled over the character device reverts to default when the last
> process referencing the file descriptor representing the device file exits."
> "Set and forget" behavior is more natural to what some gpios are used. For
> example resetting external modems, need set 1 for short time, then to 0 and
> leave it for long long time until next reset is needed. It's non sense to
> keep running some process only to keep output at 0.

Agreed, that might be more natural, but that behaviour is not by choice,
it is a consequence of the kernel internals.  In short, if the GPIO
subsystem does not hold the chip then the driver is free to do what it
likes to it.
So when you release a line all bets are off.
It may stay where you left it, but it may not - it may even switch to an
input - it depends on the driver.
If it works for you that's great, but without major kernel changes
libgpiod has no better option than to provide the caveat as the "set and
forget" behaviour is something that it cannot guarantee.

Cheers,
Kent.