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3.8 Events and task priority levels

Events are another type of object that is managed through UEFI services. They provide synchronous or asynchronous call back upon a particular occurrence. They can be created and destroyed and are either in the waiting state or the signaled state. A UEFI image can do any of the following:

  • Create an event.

  • Destroy an event.

  • Check to see if an event is in the signaled state.

  • Wait for an event to be in the signaled state.

  • Request that an event be moved from the waiting state to the signaled state.

UEFI supports polled drivers, not interrupts. Because UEFI does not support interrupts, it can present a challenge to driver writers who are used to an interruptdriven driver model.

The most common use of events by a UEFI driver is the use of timer events that allow drivers to poll a device periodically. The figure below shows the different types of events supported in UEFI, as well as the relationships between those events.

Figure 6-Event types

The following table describes the types of events shown in the preceding figure.

Table 7-Description of event types
Type of events Description
Wait event An event whose notification function is executed whenever the event is checked or waited upon.
Signal event An event whose notification function is scheduled for execution whenever the event goes from the waiting state to the signaled state.
Exit Boot Services event A special type of signal event that is moved from the waiting state to the signaled state when the EFI Boot Service ExitBootServices() is called. This call is the point in time when ownership of the platform is transferred from the firmware to an operating system. The event's notification function is scheduled for execution when ExitBootServices() is called.
Set Virtual Address Map event A special type of signal event that is moved from the waiting state to the signaled state when the UEFI runtime service SetVirtualAddressMap() is called. This call is the point in time when the operating system is making a request for the runtime components of UEFI to be converted from a physical addressing mode to a virtual addressing mode. The operating system provides the map of virtual addresses to use. The event's notification function is scheduled for execution when SetVirtualAddressMap() is called.
Timer event A type of signal event that is moved from the waiting state to the signaled state when at least a specified amount of time has elapsed. Both periodic and one-shot timers are supported. The event's notification function is scheduled for execution when a specific amount of time has elapsed.
Periodic timer event A type of timer event that is moved from the waiting state to the signaled state at a specified frequency. The event's notification function is scheduled for execution when a specific amount of time has elapsed.
One-shot timer event A type of timer event that is moved from the waiting state to the signaled state after the specified time period has elapsed. The event's notification function is scheduled for execution when a specific amount of time has elapsed.

The following three elements are associated with every event:

  • The task priority level (TPL) of the event

  • A notification function

  • A notification context

The notification function for a wait event is executed when the state of the event is checked or when the event is being waited upon. The notification function of a signal event is executed whenever the event transitions from the waiting state to the signaled state.

The notification context is passed into the notification function each time the notification function is executed. The TPL is the priority at which the notification function is executed. The four TPL levels that are defined in UEFI are listed in the table below.

Table 8-Task priority levels defined in UEFI
Task Priority Level Description
TPL_APPLICATION The priority level at which UEFI images are executed.
TPL_CALLBACK The priority level for most notification functions.
TPL_NOTIFY The priority level at which most I/O operations are performed.
TPL_HIGH_LEVEL The priority level for the one timer interrupt supported in UEFI. (Not usable by drivers)

TPLs serve two purposes:

  • Define the priority in which notification functions are executed

  • Create locks