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1 Clock sources, Clock events, sched_clock() and delay timers
10 specific implementations of clock sources, clockevents and several likewise
11 architecture-specific overrides of the sched_clock() function and some
17 sched_clock() is used for scheduling and timestamping, and delay timers
30 n bits which count from 0 to 2^(n-1) and then wraps around to 0 and start over.
34 The clock source shall have as high resolution as possible, and the frequency
35 shall be as stable and correct as possible as compared to a real-world wall
36 clock. It should not move unpredictably back and forth in time or miss a few
37 cycles here and there.
41 and the higher 16 bits in a second bus cycle with the counter bits
46 are various quirks and layers in the timekeeping code for e.g. synchronizing
58 multiply and shift, so in clocksource_cyc2ns() you find:
63 to aid in providing these mult and shift values, such as
65 mult factor from a fixed shift, and clocksource_register_hz() and
66 clocksource_register_khz() which will help out assigning both shift and mult
72 register counts up or down, and the timer clock rate, and then conjure all
79 code knows when the counter will wrap around and can insert the necessary
88 desired time specification value and calculate the values to poke into
92 and register range may be used for the clock event, but it is essentially
95 system, it is ideal (and customary) to have one such event driving timer per
100 idea about translating counters to nanoseconds using mult and shift
101 arithmetic, and you find the same family of helper functions again for
110 In addition to the clock sources and clock events there is a special weak
135 and will likely show up in system benchmarks.
142 The sched_clock() function should be callable in any context, IRQ- and
143 NMI-safe and return a sane value in any context.
145 Some architectures may have a limited set of time sources and lack a nice
176 This is done by declaring a struct delay_timer and assigning the appropriate
177 function pointers and rate settings for this delay timer.