Touch-screen Style Remotes

Cheap page-turner rings and remotes made for scrolling short videos on a phone (“TikTok rings”) do not send key presses. They present themselves as a touch screen and send every button as a simulated finger gesture: BTN_TOUCH down, a few ABS_X/ABS_Y positions, BTN_TOUCH up. Two things are needed before such a device can be used like a normal remote.

Driver binding

The HID descriptor of such a device carries a digitizer contact ID, so the kernel files it under the multitouch HID group (hid:b0005g0002… in its modalias) and leaves it to hid-multitouch. Kobo kernels do not ship that driver (CONFIG_HID_MULTITOUCH is not set), so the device connects, appears on the HID bus without a driver link, and never gets a /dev/input/eventN node.

InputDeviceHandler:_bindUnclaimedHidDevices() in src/lib/bluetooth/input_device_handler.lua writes the device’s <bus> <vendor> <product> ID to /sys/bus/hid/drivers/hid-generic/new_id. Dynamic IDs match any HID group, so hid-generic claims the device and the input node appears. The ID stays registered until reboot; reconnects of the same device are bound by the kernel on its own.

The binding is deliberately narrow. A device is only handed to hid-generic when all of these hold:

• it is on the Bluetooth bus (0005);
• it has no driver;
• it is in a multitouch HID group (0002 or 0100), the only case in which a HID device the user paired stays driverless on a Kobo;
• when the plugin is opening a specific paired device, the device’s HID_UNIQ (its Bluetooth address, reported with reversed byte order on MTK devices) matches that device.

A list of approved vendor/product IDs was considered and rejected: the WX02 ring, for instance, reports Apple’s 05AC:0220, an ID shared with unrelated products, so IDs identify neither the product nor its behaviour.

Gesture to key translation

BluetoothInputReader in src/lib/bluetooth/bluetooth_input_reader.lua folds a touch contact into one synthetic key press and release, using key codes above the kernel’s KEY_MAX (767) so they never collide with a real key:

A swipe fires as soon as the movement exceeds 5% of the largest coordinate seen so far, not when the finger lifts: the release only arrives several reports later, which would make page turns feel slow. A tap is decided at release. One gesture is fired per contact. The digitizer tool and contact buttons (BTN_TOOL_*, BTN_TOUCH) and any key sent in the same report as the touch button are swallowed; plain key events from ordinary remotes pass through unchanged.

Known device

The WX02 ring (“wxlj-02”, classic Bluetooth, three buttons) sends swipe down and swipe up for its next/previous buttons and a tap for the middle button, which also sends KEY_116 (power) on its first press after power-on. Its touch reports are one byte shorter than its descriptor declares; Linux pads short reports, so they work on the Kobo.