This is a fairly broad question, so I will try to keep it as focused as I can.
I currently own a Lenovo laptop with Ubuntu installed and touchscreen functionality and own a pressure-sensitive Bluetooth pen, and been trying to make the two work together as a cheap Cintiq-like tablet.
The pen has, unfortunately, support for only specific apps for iOS phones and tablets.
So after lots of research, I've managed to interface with the pen and create a
uinput device for it, so I can register button clicks and pressure changes on the pen and even see them routed to GIMP when configuring the device through the Input Controllers menu.
The code I have so far for that interface is available here.
The trouble starts when trying to test it out with GIMP.
From what I gather, this is because GIMP assumes Wacom devices report their own position, treats touchscreen touches as mouse movements and only allows input from a single device at a time.
My question is, how can I work around this?
More specifically, how can I create a
uinput device that would behave as a Wacom tablet and supersede/block the behavior I described?
Or if there's a different solution, such as patching GIMP or writing a plugin for it.
The code mentioned above now works.
I have written a blog post on the process of getting this to work: http://gerev.github.io/laptop-cintiq
As you said, Gimp expects you to provide ABS_X and ABS_Y along with ABS_PRESSURE in your driver - which is not strange, because you are using you virtual device as input, so it wouldn't make much sense to pick ABS_X and ABS_Y coordinates from one device and ABS_PRESSURE from another (although they will always be the same in this case). Maybe you can just read the current coordinates of the mouse and copy them as your own device coordinates.
As an example, the project GfxTablet does something similar to what you are trying, they have an Android application for tablets with pen and use uinput to create virtual device that works like pressure-sensitive pen on Linux. I have used it and it worked like a charm in Gimp and mypaint on my laptop, and I had no problem with having a mouse (or the touchpad) active at the same time as the uinput device (I think that Krita added support for generic pressure-sensitive devices recently). You can take a look at the source code of the driver here (surprinsingly simple, to be fair).
Note that this is not a faulty behavior of Gimp, because this is what is expected from a tablet-like device. Take a look at the event codes kernel documentation page, in the last section (Guidelines), it is said that tablets must report ABS_X and ABS_Y. Moreover, they should use BTN_STYLUS and BTN_STYLUS2 to report the tool buttons and some BTN_TOOL_* (e.g. BTN_TOOL_PEN) to report activity (you can find all the available codes in input.h); however, these last does not seem that important, as GfxTablet does not implement them and worked without problem.