linux-surface eraser fixer

On Microsoft Surface devices, the touchscreen generates HID events directly in hardware, which are relayed to the linux HID subsystem via. the IPTS driver. These hardware events contain a bugged / nonstandard proximity report for the eraser of the Surface Pen: When you move the eraser into proximity, IPTS will first report that the pen was moved into proximity, and then report that the eraser moved in. This means that as far as the kernel is concerned, both devices will be active at the same time, as if you would be holding two seperate pens onto the screen.

This behaviour is nonstandard, and as of the latest update to libinput actually unsupported. The eraser events will simply be discarded as long as the pen is still active - which, in the case of IPTS, is always.

The proper solution for this would be to fix the generated HID events, which is impossible, or to process them in the kernel somehow. Since IPTS is not upstream however, it could be a bit difficult to do it correctly.

Until a proper fix can happen, this python daemon will capture all HID events coming from the stylus and forward them to a virtual input device (which then is seen instead of the real stylus by everything else). When it notices that the eraser moved near the screen, it will inject an event, telling the system that the pen moved away from the screen again. Therefor libinput and friends don't see two tools being active at the same time, they only see one.

This should work out of the box for Microsoft Surface devices using IPTS. For other devices (like the Surface Go), it might be neccessary too, depending on whether the issue is in the IPTS hardware, or the Surface Pen itself (which is not tied to IPTS specifically). There is nothing that would stop it from it from working on non-surface devices in theory, but it may or may not actually work.

Quirks

After you suspend and resume the surface, you have to rotate it into normal position before touching the screen. Otherwise GNOME will totally mess up the mapping and then decide to not map it at all and decide to leave the virtual stylus unrotated. This can be fixed by restarting or simply suspending and resuming again, and then following these steps.

Installation

$ git clone https://github.com/StollD/linux-surface-fix-eraser
$ cd linux-surface-fix-eraser
$ sudo cp linux-surface-fix-eraser.py /usr/local/bin/
$ sudo cp linux-surface-fix-eraser.service /etc/systemd/system/
$ sudo systemctl enable --now linux-surface-fix-eraser

You need to install version 1.2.0 of python-evdev or higher. Unfortunately, most distributions seem to package 1.1.2, so you might have to force an update.

$ sudo pip3 install -U evdev