In a data storage system, a fixture communication channel is created for communications between a fixture and another component, where the fixture is a component of a device driver that provides extended input/output functionality with respect to one or more storage devices. A handle identifies the fixture communication channel, and callbacks are registered with a driver handler and reference callback functions of the fixture. Fixture communication messages each include the handle and a message-specific operation code for an operation to be performed by the fixture, and each is conveyed by (1) issuing a fixture-control input/output control command to the handler that includes the handle and message-specific operation code, and (2) at the handler, using the handle and message-specific operation code to select a callback and invoke a corresponding callback function of the fixture.