The goodix i2c-hid bindings currently support two models of
touchscreen: GT7375P and GT7986U. The datasheets of both touchscreens
show the following things:
* The mainboard that the touchscreen is connected to is only expected
  to supply one voltage to the touchscreen: 3.3V.
* The touchscreen, depending on stuffing options, can accept IO to the
  touchscreen as either 3.3V or 1.8V. Presumably this means that the
  touchscreen has its own way internally to make or deal with 1.8V
  signals when it's configured for 1.8V IO.

NOTE: you've got to look very carefully at the datasheet for the
touchscreen to see that the above bullets are true. Specifically, the
datasheet shows a signal called VDDIO and one might think that this is
where a mainboard would provide VDDIO to the touchscreen. Upon closer
inspection, however, a footnote can be found that says "When VDDIO is
left floating, the logic level is 1.8V [...]; when VDDIO is connected
to AVDD, the logic level is AVDD.". Thus the VDDIO pin on the
touchscreen IC is actually a selector and not a pin whre the mainboard
would pass a reference voltage.

The fact that the touchscreen isn't supplied 1.8V by the mainboard
means that when I originally submitted bindings for these touchscreens
I only listed the 3.3V rail in the bindings. It can be noted that the
original bindings and driver were added for sc7180-trogdor boards and
these boards all use 3.3V IO via a level shifter on the mainboard.

It turns out that with sc7280-herobrine-evoker, we've got a bit of a
strange monkey on our hands. Due to some very interesting but
(unfortunately) set-in-stone hardware design, we are doing 1.8V IO to
the touchscreen but we _also_ have some extra buffers on the mainboard
that need to be powered up to make the IO lines work. After much
pondering about this, it seems like the best way to handle this is to
add an optional "mainboard-vddio" rail to the bindings that is used to
power up the buffers. Specifically, the fact that the touchscreen
datasheet documents that its IOs can be at a different voltage level
than its main power rail means that there truly are two voltage rails
associated with the touchscreen, even if we don't actually provide the
IO rail to it. Thus it doesn't feel absurd for the DT node on the host
to have a 1.8V rail to power up anything related to its 1.8V logic.
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230206184744.5.Ia77a96c6c5564f9cc25e6220b5a9171d5c2639e8@changeidSigned-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>