Tablet pad not recognized as input device
Summary
I added support for the XP-Pen Artist 15.6 Pro that works fine under the Wacom X driver but whose pad isn't recognized by libinput at all unless I declare the buttons as keys.
Steps to reproduce
Here is my branch with the finished support. In particular, here is the report descriptor of the pad. I based it off a known working Huion tablet I have available. If I change the pad buttons to keys as seen in this older commit the pad is recognized and works. But this should be showing up as a tablet pad and sending pad button events and the current latest report descriptor / input device node is recognized and configured properly by xf86-input-wacom.
In particular I can find nothing wrong with the recording on its event node. But then libinput just skips event5 in its devices list and has nothing for it.
There is probably something I am doing wrong with the report descriptor that makes libinput not pick it up as a tablet pad, but trying to dig through the documentation for a few hours hasn't turned anything up. I've tried changing the usage groups around throughout the report but, for example, having it be a Digitizer Pen device with Application Tablet Function Keys and physical Buttons breaks it under Wacom (probably because it then shares device class with the pen) and doesn't fix it under libinput. But at this point I'm not sure where the problem is, and nobody around Digimend I talked to in IRC about it has any idea either since most of them just use the Wacom X driver.
libinput version you encountered the bug on
1.14.1
Hardware information:
XP-Pen 15.6 Pro