Does Line.request()'s default arg matter for Input lines?

[tal-zvon]

Does it matter what we set this arg for an input line? If not, can we get a short note in the docs that it doesn't matter. Maybe have it be an Option<u8> that can be set to None?

[JonathonReinhart]

Does it matter what we set this arg for an input line?

Nope. I was in the neighborhood and tracked this down.

In the uAPI, default_values is documented:

 * @default_values: if the %GPIOHANDLE_REQUEST_OUTPUT is set for a requested
 * line, this specifies the default output value, should be 0 (low) or
 * 1 (high), anything else than 0 or 1 will be interpreted as 1 (high)

So it only applies for REQUEST_OUTPUT.

This is confirmed by looking in the kernel:

		if (lflags & GPIOHANDLE_REQUEST_OUTPUT) {
			int val = !!handlereq.default_values[i];

			ret = gpiod_direction_output(desc, val);
			if (ret)
				goto out_free_lh;
		} else if (lflags & GPIOHANDLE_REQUEST_INPUT) {
			ret = gpiod_direction_input(desc);
			if (ret)
				goto out_free_lh;
		}

You can see that for REQUEST_OUTPUT, the value from default_values[i] (which is the default arg in Rust code) is passed to gpiod_direction_output() to actually set the mode to output and the default state.

For REQUEST_INPUT however, default_values[i] is not used. (It's not used anywhere else in that function.)

I think a short note in the doc to say it only applies for outputs would make sense. Changing it to Option<u8> would just add unnecessary complexity, IMO.