Device Instances and Instance Numbers

The system assigns an instance number to each device. The driver may 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 usage for this is ddi_get_soft_state(9F), which uses an instance number to retrieve soft state data for a particular physical device.

For pseudo devices (children of the pseudo nexus), the instance numbers are defined in the driver.conf(4) file via the instance property. If the driver.conf(4) 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.