@davidrevoy@framapiaf.org I recommend reading my post on understanding HID first, no chance to understand this otherwise.

https://who-t.blogspot.com/2018/12/understanding-hid-report-descriptors.html

The code is simple, but only if you understand all the magic that goes on behind the scenes :)

As it happens, udev-hid-bpf also has a tutorial which may make things more or less confusing: https://libevdev.pages.freedesktop.org/udev-hid-bpf/tutorial.html