Re: libgpiod gpiod_line_request output and keep existing value

[Mark Deneen <mdeneen@xxxxxxxxx>]

Hi Bartosz,

Thanks for the response, it is greatly appreciated. I've added a few
comments below.

On Fri, Mar 6, 2020 at 6:50 AM Bartosz Golaszewski <brgl@xxxxxxxx> wrote:
>
> czw., 5 mar 2020 o 15:57 Mark Deneen <mdeneen@xxxxxxxxx> napisał(a):
> >
> > Is it possible to use gpiod_line_request to set a given line as an
> > output but retain the current state of the line?  I can not use
> > gpiod_line_get_value to query the state since the line in question has
> > not been requested yet.
>
> No, because it doesn't make sense. If you're driving the line, you
> define its state.

I can test this, but does the line state reset when the chip is closed?

> >
> > Let's say that I have a userspace process which sets a gpio to high
> > and I restart the process.  I would like for the output to remain high
> > when I request the line again in the new process.  This was possible
> > with gpiolib+sysfs, is it possible with libgpiod?
> >
>
> This was rather a side effect of sysfs keeping the line exported
> without any user-space process controlling it. This is not the case
> with a character device where if the process exits, all resources are
> freed ie. GPIOs are tied to processes rather than to a sysfs
> interface.

I understand.  For some purposes this is a step backward although it
is definitely far more deterministic and that's probably a good thing.

Let's say, for example, that a GPIO output is connected to a circuit
which controls a relay.  My process initialzes this gpio and engages
the relay.  At some point this process is restarted, possibly due to a
new version of the software being installed, and I wish for the relay
to remain engaged during this operation.  I can do this with the sysfs
interface by initializing the state to a predetermined value if the
direction had not been set to output yet.  With libgpiod I would have
to record the last state of the gpio somewhere and use this value when
requesting the line.

Should I be writing a kernel module instead of using libgpiod if I
need this sort of control?