libwacom and Huion/Gaomon devices

TLDR: Thanks to José Exposito, libwacom 2.12 will support all [1] Huion and Gaomon devices when running on a 6.10 kernel.

libwacom, now almost 13 years old, is a C library that provides a bunch of static information about graphics tablets that is not otherwise available by looking at the kernel device. Basically, it's a set of APIs in the form of libwacom_get_num_buttons and so on. This is used by various components to be more precise about initializing devices, even though libwacom itself has no effect on whether the device works. It's only a library for historical reasons [2], if I were to rewrite it today, I'd probably ship libwacom as a set of static json or XML files with a specific schema.

Here are a few examples on how this information is used: libinput uses libwacom to query information about tablet tools.The kernel event node always supports tilt but the individual tool that is currently in proximity may not. libinput can get the tool ID from the kernel, query libwacom and then initialize the tool struct correctly so the compositor and Wayland clients will get the right information. GNOME Settings uses libwacom's information to e.g. detect if a tablet is built-in or an external display (to show you the "Map to Monitor" button or not, if builtin), GNOME's mutter uses the SVGs provided by libwacom to show you an OSD where you can assign keystrokes to the buttons. All these features require that the tablet is supported by libwacom.

Huion and Gamon devices [3] were not well supported by libwacom because they re-use USB ids, i.e. different tablets from seemingly different manufacturers have the same vendor and product ID. This is understandable, the 16-bit product id only allows for 65535 different devices and if you're a company that thinks about more than just the current quarterly earnings you realise that if you release a few devices every year (let's say 5-7), you may run out of product IDs in about 10000 years. Need to think ahead! So between the 140 Huion and Gaomon devices we now have in libwacom I only counted 4 different USB ids. Nine years ago we added name matching too to work around this (i.e. the vid/pid/name combo must match) but, lo and behold, we may run out of unique strings before the heat death of the universe so device names are re-used too! [4] Since we had no other information available to userspace this meant that if you plugged in e.g. a Gaomon M106 and it was detected as S620 and given wrong button numbers, a wrong SVG, etc.

A while ago José got himself a tablet and started contributing to DIGIMEND (and upstreaming a bunch of things). At some point we realised that the kernel actually had the information we needed: the firmware version string from the tablet which conveniently gave us the tablet model too. With this kernel patch scheduled for 6.10 this is now exported as the uniq property (HID_UNIQ in the uevent) and that means it's available to userspace. After a bit of rework in libwacom we can now match on the trifecta of vid/pid/uniq or the quadrella of vid/pid/name/uniq. So hooray, for the first time we can actually detect Huion and Gaomon devices correctly.

The second thing Jose did was to extract all model names from the .deb packages Huion and Gaomon provide and auto-generate all libwacom descriptions for all supported devices. Which meant, in one pull request we added around 130 devices. Nice!

As said above, this requires the future kernel 6.10 but you can apply the patches to your current kernel if you want. If you do have one of the newly added devices, please verify the .tablet file for your device and let us know so we can remove the "this is autogenerated" warnings and fix any issues with the file. Some of the new files may now take precedence over the old hand-added ones so over time we'll likely have to merge them. But meanwhile, for a brief moment in time, things may actually work.

[1] fsvo of all but should be all current and past ones provided they were supported by Huions driver
[2] anecdote: in 2011 Jason Gerecke from Wacom and I sat down to and decided on a generic tablet handling library independent of the xf86-input-wacom driver. libwacom was supposed to be that library but it never turned into more than a static description library, libinput is now what our original libwacom idea was.
[3] and XP Pen and UCLogic but we don't yet have a fix for those at the time of writing
[4] names like "HUION PenTablet Pen"...