4.2.11 Use Proper Task Priority Levels

The TPLs provide a mechanism for code to run at a higher priority than application code. One can be running the UEFI Shell, and a UEFI device driver can have a timer event fire and gain control to go poll its device. The TPL_CALLBACK level is typically used for deferred software calls and TPL_NOTIFY is typically used by device drivers. TPL_HIGH_LEVEL is typically used for locks on shared data structures.
Drivers may use events and TPLs if they perform non-blocking I/O. If they perform blocking I/O, then events are not used. They may still use the RaiseTPL() and RestoreTPL() for critical sections.
Driver diagnostics are typically just applications. They do not normally need to use TPLs or events unless the diagnostics is testing the TPL or event mechanisms in EFI. However, there is one exception. If a diagnostic needs to guarantee that EFI's timer interrupt is disabled, then the diagnostic can raise the TPL to TPL_HIGH_LEVEL. If this level is required, it should be done for the shortest possible time interval.
Caution: There are ways in which the platform firmware can be put into an undefined state by misuse of the RaiseTPL() and RestoreTPL() functions.
Caution: Do not misuse the RaiseTPL() service by raising the task priority level too high for an extended period of time. Raising the TPL level above TPLAPPLICATION circumvents the timer tick. This can interfere with other drivers, applications, and other elements that rely on the timer tick. It can cause extreme, and sometimes catastrophic slowing of the system. It can cause other drivers, applications, and other things that rely on the timer tick to fail. Always mirror the raise TPL service with the restore TPL service.