Support for RZW T101P136CQ in ili9881c

[RomanHargrave]

Note: this is in draft because I need to test it, and possibly deal with some other driver shenanigans of a similar nature to do so.

This adds support for the above-mentioned model of panel from RZW. This panel is found in certain industrial HMIs manufactured by EDATEC (e.g. the ED3010-*). Unfortunately, these enclosures are (currently) only usable with a modified build of Linux 6.6 that EDATEC have not released any source code for. Among other changes, that kernel image includes additional ili9881c device profiles for the display found in the enclosures I wish to use. Notably, where ili9881c would turn on the reset pin to reset the device, the modified ili9881c found in that image turns off the reset pin. I have added functionality to invert reset polarity as such.

[6by9]

Releasing a kernel image without offering the source code is in breach of the GPLv2 licence the Linux kernel is released under. You could point them to term 3 of https://www.gnu.org/licenses/old-licenses/gpl-2.0.html and ask them which of the 3 options they comply.
We do work with EDATEC, so I can give them a nudge if you get no joy.

The extra init sequences, display modes, and compatible lookups I have no issue with.

Inverting the reset line should be done via the overlay. If it were our V2 panels that use ILI9881 with the vc4-kms-dsi-ili9881-7inch overlay, changing line 56 from:

reset-gpio = <&display_mcu 0 GPIO_ACTIVE_LOW>;

to

reset-gpio = <&display_mcu 0 GPIO_ACTIVE_HIGH>;

would do the same as your code change.
The state requested in the driver is meant to be the logical state for the signal, hence gpiod_set_value_cansleep(ctx->reset, 0); to deassert reset and become active. Physical state is conveyed via the ACTIVE_[HIGH|LOW] option.

[RomanHargrave]

That's nice to know WRT the overlay. I'll make that change. I did send them the usual GPL email and have not heard back, nor have I received a response to an earlier contact of similar nature. It would save some time reverse engineering their more involved (and more inscrutable) changes to the attiny regulator driver for the panel backlight if they could be compelled via some other more effective channel.