The I2C HID support is a conglomerate of the following basic mechanisms.
Intel PCH GPIO support for the touchpad interrupt pin
PCH GPIO is a platform device (e.g., INT34C5) with a spec-defined configuration that can be retrieved from ACPI tables.
Intel LPSS I2C (master) support
The I2C master controller is a (pseudo-)PCI device, mostly with unsufficiently initialized BARs.
I2C HID driver
The actual HID device (CID PNP0C50) is accessible via the mechanisms above and its configuration can be retrieved from ACPI resources and device-specific methods.
The I2C HID support is a conglomerate of the following basic mechanisms.