Re: [libgpiod v2][PATCH v2 5/5] bindings: python: add the implementation for v2 API

[Bartosz Golaszewski <brgl@xxxxxxxx>]

On Fri, Jul 8, 2022 at 1:28 PM Bartosz Golaszewski <brgl@xxxxxxxx> wrote:
>
> On Fri, Jul 8, 2022 at 12:56 PM Kent Gibson <warthog618@xxxxxxxxx> wrote:
> >
>
> [snip]
>
> > >
> > > The limitation of the uAPI is what keeps us from making it true in
> > > user-space (that each line can have its own config). As it is, only up
> > > to 9-10 lines can have distinct configs and making the API look and
> > > behave as if it wasn't the case is more confusing (E2BIG errors) than
> > > simply admitting we have the concept of defaults and overrides (to
> > > which the interface is greatly simplified in the high-level
> > > libraries). The idea about making the most common config attributes
> > > become the defaults is simply bad. It would require the user to
> > > anticipate how the library will behave for every attribute and lead to
> >
> > It requires nothing from the user.  They are not even aware of the
> > concept of "defaults" or "overrides".  They just set config on lines.
> > If that is too complicated, which is quite unlikely, then they get
> > E2BIG and they need to repartition their lines into multiple requests.
> >
> > Anyway, that horse is dead.
> >
>
> For a python user, this:
>
> lc = gpiod.LineConfig()
> lc.set_props(offsets=[2, 3], direction=Direction.OUTPUT)
> req = gpiod.request_lines("/dev/gpiochip0", line_cfg=lc)
>
> is pretty much as simple as it gets. They still don't need to be aware
> of the underlying split into defaults and overrides. I believe it's
> GoodEnough™.
>
> I imagine in Rust bindings we'll be able to chain set_props() as is
> customary and we'll get a one-liner out of that.
>

The code I posted here is wrong as it's missing the request config but
it made me think: how about in case of req_cfg=None or not passed at
all, we derive the lines to request from overridden offsets in the
line config? In that case if the user does:

  lc = gpiod.LineConfig()
  lc.set_props(offsets=[0, 1], direction=Direction.OUTPUT,
output_value=Value.ACTIVE)
  lc.set_props(offset=4, direction=Direction.INPUT)
  req = gpiod.request_lines("/dev/gpiochip0", line_cfg=lc)

Then it will be interpreted as lines=[0, 1, 4]?

I'm also thinking that we could allow the output values to be mapped
as <line name> -> <value> within gpiod.LineConfig like that:

  lc.set_props(lines=["foo", 4], direction=Direction.OUTPUT)
  lc.set_output_values({"foo": Value.Active, 4: Value.INACTIVE})

It would require us to retrieve the names of all lines from the chip
at the time of the request and store them in the request structure
(for reconfigure to work) but it would make the entire thing even more
"pythonic".

Bart