Driver Soft State Management

To assist device driver writers in allocating state structures, the Solaris 9 DDI/DKI provides a set of memory management routines called software state management routines (also known as the soft state routines). These routines dynamically allocate, retrieve, and destroy memory items of a specified size, and hide the details of list management. An instance number identifies the desired memory item. This number can be (and usually is) the instance number assigned by the system.

Drivers will typically allocate a soft state structure for each device instance that attaches to the driver by calling ddi_soft_state_zalloc(9F), passing the instance number of the device. Because there cannot be two device nodes with the same instance number, ddi_soft_state_zalloc(9F) will fail if an allocation already exists for a given instance number.

A driver's character or block entry point (cb_ops(9S)) will reference a particular soft state structure by first decoding the device's instance number from the dev_t argument that is passed to the entry point function. The driver then calls ddi_get_soft_state(9F), passing the per-driver soft state list and the instance number that was derived. If ddi_get_soft_state(9F) returns a NULL value, the driver should treat this as if the device does not exist and return the appropriate code.

See Creating Minor Device Nodes for additional information on how instance numbers and device numbers, or dev_t's, are related.