Overview
Linux arch_timer 是内核最重要的基础设施之一，内核术语中，timer 带有前缀arch，可见其地位之高！内核之 arch_timer 提供系统运行的节拍，是其他定时任务的基础，相当于人之心脏！
init view
arch_timer 的启动 log
[ 0.000000] ( swapper/0-0 [00] 20) arch_timer: CPU0: Trapping CNTVCT access
[ 0.000000] ( swapper/0-0 [00] 20) arch_timer: cp15 and mmio timer(s) running at 19.20MHz (virt/virt).
[ 0.000000] ( swapper/0-0 [00] 20) clocksource: arch_sys_counter: mask: 0xffffffffffffff max_cycles: 0x46d987e47, max_idle_ns: 440795202767 ns
[ 0.000006] ( swapper/0-0 [00] 20) sched_clock: 56 bits at 19MHz, resolution 52ns, wraps every 4398046511078ns
[ 0.000014] ( swapper/0-0 [00] 20) arch_counter_register: arch_timer_read_counter()/sched_clock()/arch_timer_read_counter(): 0x6596d05/0x29b5/0x6596d07
[ 0.000047] ( swapper/0-0 [00] 20) clocksource: Switched to clocksource arch_sys_counter
从启动 log 可以看到系统 counter 的晶振是 19MHz，决定了定时精度是 52ns。
start_kernel → time_init
time_init → __arch_timer_setup
time_init → arch_timer_common_init
In arch_counter_register, we will reinitialize arch_timer_read_counter to arch_counter_get_cntvct_stable, then initialize two global variables, which will be used to get time in kernel, we will see them later in this article.
struct clock_data cd
tk_core
timer interrupt
arch_timer_handler_virt → tick_handle_periodic → switch to hrtimer
arch_timer_handler_virt → hrtimer_interrupt
hrtimer_interrupt → tick_sched_timer
hrtimer_interrupt → watchdog_timer_fn
How to get time in kernel
local_clock and sched_clock
ktime_get
