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This merge request adds a new driver into the kernel to allow for the userspace to custom control the haptic effects of the apple MTP trackpad and follows the convention for haptic touchpad functionality as specified in [1].
Set the INPUT_PROP_PRESSUREPAD property within the MTP trackpad device indicating that this device is a haptic touchpad, and set the FF_HAPTIC capability to indicate that force feedback haptic effects can be uploaded to the trackpad [2]. Userspace can thus choose to upload force feedback effects to the touchpad, which would trigger it to switch into "Host controlled" mode, where the touchpad will no longer click unless signalled to do so by userspace.
This patch uses an unconventional means of doing so, by writing a separate loadable module for the haptic actuator. This is because the haptic actuator is considered a separate device on MTP trackpads, whereas haptics for HUTRR63 (And the other kernel drivers) considers the haptic actuator and the touchpad to be the very same device. Therefore, to conform to the standardized interface in [1] this patch creates a separate driver for the separate HID device, and is designed such that commands to the actuator are sent through the touchpad.
This changeset is relevant because allowing userspace control of the haptics allows for user control of:
1. Click feedback intensity
2. Sensitivity of when clicks trigger
3. Configuring "Force Touch"-like deep click functionality
The specific hardware commands and their fields have been reverse engineered on my own machine (Macbook Pro 2023 M2 Max) using the m1n1 hypervisor. The changes have not been tested on any other machine.
See also [3] where discussions were previously had on some of the specifics of the hardware
[1] https://lore.kernel.org/all/20250818-support-forcepads-v3-0-e4f9ab0add84@google.com/T/#m12f9be6691015fb9b2e834248c368e5f90
[2] https://docs.kernel.org/input/event-codes.html#input-prop-pressurepad
[3] https://gitlab.freedesktop.org/libinput/libinput/-/work_items/1027