Callback Functions

Certain DDI mechanisms provide a callback mechanism. DDI functions provide a mechanism for scheduling a callback when a condition is met. Conditions for which callback functions are used include:

• When a transfer has completed

• When a resource might have become available

• When a time-out period has expired

In some sense, callback functions are similar to entry points—interrupt handlers, for example. DDI functions that allow callbacks expect the callback function to perform certain tasks. In the case of DMA routines, a callback function must return a value indicating whether the callback function needs to be rescheduled in case of a failure.

Callback functions execute as a separate interrupt thread and must handle all the usual multithreading issues.

A driver must cancel all scheduled callback functions before detaching a device.