When iicbus is attached as child of controller that have an ACPI handle, it scans all ACPI children for "I2C Serial Bus Connection Resource Descriptor" described in section 19.6.57 of ACPI specs.
If such a descriptor is found, I2C child is added to iicbus, its I2C address and ACPI handle are added to child ivars and all newbus resources including IRQs are copied from ACPI-hosted device to IICbus-hosted one .
This effectively makes a copy of existing ACPI device on top of I2C bus that allows to implement child device identification through ACPI HID/CID comparison
The dummy acpi_iicdev driver is attached to ACPI child device to block possible attachment of other drivers and to enable suspend/resume services (_PS method calls) provided by acpi bus.
 currently it is restricted to Designware I2C controllers only.
 I2C bus speed is calculated as well. It is equal to speed of slowest device among the children.