Input and Output Protocol

Physical Dial Input

The driver reads the Surface Dial System Multi Axis evdev device.

Find it with:

rg -n -C 8 "Surface Dial System Multi Axis" /proc/bus/input/devices

Then inspect raw data:

sudo evtest /dev/input/eventXX

Expected raw events:

EV_REL / REL_DIAL: rotation
EV_KEY / BTN_0: button press and release
EV_SYN and EV_MSC: ignored framing/scancode events

The Surface Dial System Control node is not used for scrolling. It exposes sleep/wakeup-style keys, not rotation.

Virtual Touchpad

The multitouch output device is named:

Surface Dial Renewed Touchpad

It exposes a type-B multitouch touchpad with slots. On gesture start it creates two active tracking IDs. On movement it moves both slots along the Y axis and updates the single-touch compatibility ABS_Y value.

Important implementation details:

Two fingers are used because libinput interprets that as scroll.
INPUT_PROP_POINTER is enabled.
INPUT_PROP_BUTTONPAD is intentionally not enabled.
BTN_LEFT is intentionally not exposed.
Pressure/touch-major/tool-width axes are exposed to look more like a real touchpad.

The lack of buttonpad/left-button capability reduces accidental tap/click interpretation when a scroll gesture is short.

Gesture Lifecycle

multitouch_start

Creates slot 0 and slot 1 with tracking IDs, positions, pressure, touch major, and two-finger tool state.

multitouch_move

Moves both slots to the same new Y position. The driver keeps an accumulated mt_position relative to MT_BASELINE.

multitouch_settle

Emits one final stationary frame before ending. This reduces the chance that the final movement is interpreted as a flick.

multitouch_end

Sets both tracking IDs to -1, releases BTN_TOUCH and tool keys, and reports zero pressure.

Coordinate Model

MT_BASELINE is 10_000, with coordinates from 0 to 20_000.

The driver recenters the synthetic gesture if mt_position exceeds MT_RECENTER_THRESHOLD. This avoids running into coordinate limits during long scrolls.

Virtual Wheel

The fallback wheel device is named:

Surface Dial Renewed Wheel

It emits both:

REL_WHEEL
REL_WHEEL_HI_RES

Wheel mode is implemented but has not received the same tuning as multitouch mode.

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