The system assigns an instance number to each device. The driver might not reliably predict the value of the instance number assigned to a particular device. The driver should retrieve the particular instance number that has been assigned by calling ddi_get_instance(9F).
Instance numbers represent the system's notion of devices. Each dev_info, that is, each node in the device tree, for a particular driver is assigned an instance number by the kernel. Furthermore, instance numbers provide a convenient mechanism for indexing data specific to a particular physical device. The most common use of instance numbers is ddi_get_soft_state(9F), which uses instance numbers to retrieve soft state data for specific physical devices.
For pseudo devices, that is, the children of pseudo nexuses, the instance numbers are defined in the driver.conf(4) file using the instance property. If the driver.conf file does not contain the instance property, the behavior is undefined. For hardware device nodes, the system assigns instance numbers when the device is first seen by the OS. The instance numbers persist across system reboots and OS upgrades.