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.