Does anyone know of any good documentation about what goes on in kernel/time/timekeeping.c? I know about the file Documentation/timers/timekeeping.txt, but that's mostly talking about underlying clock sources.
The documentation I'm looking for would answer questions such as: * How is is the kernel's primary wallclock time (xtime) defined? * How are other clocks (boot, monotonic, etc.) derived from xtime? * What is ktime_t and how does it differ from time_t? * What are the invariants involving the timekeeper offsets wall_to_monotonic, offs_real, offs_boot, and offs_tai? * What is the distinction between the regular and the "coarse" clocks? (And what about the "raw" and "alarm" clocks?) * What are the timekeeper "base" variables tkr_mono and tkr_raw? How are they used, and what are the invariants surrounding them? * Which of the functions in timekeeping.c are called at interrupt time? * What are the locking requirements surrounding the timekeeper data structures? * What are the main, suggested, primary calls for fetching the current time? (What about setting it?) * What is the separation of functionality between time.c, timekeeping.c, posix-timers.c, and ntp.c? * What abstractions are intended to be imposed by the inline functions defined in timekeeping.h (current_kernel_time(), etc.)? * How much of this is historical and/or accidental, and what (if any) cleanups and/or improvements to the timekeeping architecture might be in the works? If there's no such documentation, I may try to write something, but I'll likely need some help, as I haven't personally worked out good answers yet for all of the example questions I've posed above.
