Updated: May 06, 2022

Information for kernel callouts

The callout area is CPU-independent. It is defined by the callout_entry data structure, which includes the following members:
A pointer to the reboot callout function. This callout function is invoked in two scenarios:
  • A “normal” reboot, which is done based on a user request (e.g., through sysmgr_reboot()) and is therefore expected
  • An “abnormal” reboot, which is done to recover from an unexpected system failure and is therefore unexpected

For more details and a code sample, see System reset.

Deprecated. This field is set to NULL.
A pointer to a callout that sets the period for the system tick timer and enables it. The callout is called first during procnto initialization to make the timer start generating system ticks. It is also called when the system tick period is changed (via ClockPeriod()), when entering or leaving tickless operations, and by high-resolution software timers.

For details on what exactly this function does, see the timer_load() description in “Timer and clock”.

A pointer to a callout used to manually reload the timer with the same period for the next fire. If a new period needs to be set, timer_load() will be used instead.

For more information, see the timer_reload() description in “Timer and clock”.

A pointer to a callout that is used to determine when the next interrupt is expected. It's a count-up value from 0 (since “the last interrupt”) to the current time, to tell how far the timer has gone.

For examples of using this callout, see the timer_value() description in “Timer and clock”.

This field is an array that has two entries:
  • debug[0] — used for informational purposes
  • debug[1] — currently unused; set this field to NULL

Only the first entry is discussed in this guide. This entry points to a callout that prints out internal debugging information to a device. For information about the specific types of kernel debug callouts, see the Kernel debug entry in the “Kernel Callouts” chapter.

Not used. This field is set to NULL.
This field must be either:
  • NULL, indicating that no custom kernel call callout is provided
  • non-NULL, and pointing to valid code provided by the startup program, indicating that a custom kernel call callout is provided. For information about the signature and a code sample for a custom callout, see the Custom entry in the “Kernel Callouts” chapter.

The callout_entry structure includes different versions of all these fields for 32-bit and 64-bit systems.