Providing IoTimer Context Information

The Context pointer passed to IoInitializeTimer identifies a context area where other driver routines, and the IoTimer routine itself, can maintain state about timed operations. The I/O manager passes the Context pointer whenever it calls the IoTimer routine.

Because an IoTimer routine is run at IRQL = DISPATCH_LEVEL, its context area must be in resident, system-space memory. Most drivers that have IoTimer routines use the device extension of the associated device object as a Context-accessible area, but the context can instead be in a controller extension if the driver uses a controller object or in nonpaged pool allocated by the driver.

Follow these guidelines for an IoTimerroutine's context area:

• If the IoTimer routine shares its context area with the driver's ISR, it must use KeSynchronizeExecution to call a SynchCritSection routine that accesses the context area in a multiprocessor-safe manner. For more information, see Using Critical Sections.

• If the IoTimer routine does not share its context area with an ISR, but does share it with another driver routine, the driver must protect the shared context area with an initialized executive spin lock, in order to access the context information in a multiprocessor-safe manner. For more information, see Spin Locks.