Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.

https://daimajiaoliu.com/daima/569eac9eb656405

每个CPU时钟周期都会对count - 1，当减为0时出发一个时钟中断。
pit_timer_init
https://blog.csdn.net/Rong_Toa/article/details/115350561

The freq parameter shows us how many timer interrupts will be occurred in one second.
```c
hpet_enable 计算了hpet_freq参数，就是后面的时钟中断频率
  clocksource_register_hz
    __clocksource_update_freq_scale
```

freq = FSEC_PER_SEC;
do_div(freq, hpet_period);
hpet_freq = freq;

FSEC_PER_SEC = 1e15  long long

https://www.zhihu.com/column/c_1242535902247120896  参考这个对时钟源修改的实验

现在cpu有高精度定时器HPET，在I/O APIC里面。之前讲启动其他CPU的时候有用到lapic，lapic为一个核心一个，I/O APIC在芯片组里面。之前的32位系统一般通过8259中断控制器，64位一般通过I/O APIC来管理中断以及时钟跟时钟中断。

https://rootw.github.io/2018/02/%E6%97%B6%E9%92%9F%E4%B8%AD%E6%96%AD/

https://blog.csdn.net/lqy971966/article/details/110234641 各种周期的区别

apic.c  __setup_APIC_LVTT函数中修改 TDCR寄存器

https://richardweiyang-2.gitbook.io/kernel-exploring/00-start_from_hardware/06-apic

周期触发模式（Periodic Mode）
周期触发模式中，程序设置一个”初始计数“寄存器（Initial Count），同时Local APIC会将这个数复制到”当前计数“寄存器（Current Count）。Local APIC会将这个数（当前计数）递减，直到减到0为止，这时候将触发一个IRQ（可以理解为触发一次中断），与此同时将当前计数恢复到初始计数寄存器的值，然后周而复始的执行上述逻辑。可以使用这种方法通过Local APIC实现定时按照一定时间间隔触发中断的功能。”当前计数“寄存器中的值的递减速度依赖于当前CPU的主频除以一个叫做”分频寄存器“（“Divide Configuration Register”）的值（换个思路来说就是，多少个tick减少1）。

举例来说，对于一个2.4GHz的CPU使用的CPU总线频率是800MHz（大家说的外频800MHz)，”分频寄存器“（“Divide Configuration Register”）设置的是“除四”（”divide by 4“），并且初始计数（initial count）设置到123456。那么结果就是当前计数（current count）每200M分之1秒减1，大约617.28us触发一个irq(一次中断)，也就是1620.01Hz。


```
  Timer related:

      CCR: Current Count Register
      ICR: Initial Count Register
      DCR: Divide Configuration Register
      Timer: in LVT

  LVT (Local Vector Table):

      Timer
      Local Interrupt
      Performance Monitor Counters
      Thermal Sensor
      Error

  IPI:

      ICR: Interrupt Command Register
      LDR: Logical Destination Register
      DFR: Destination Format Register

  Interrupt State:

      ISR: In-Service Register
      IRR: Interrupt Request Register
      TMR: Trigger Mode Register
```
https://rtoax.blog.csdn.net/article/details/116848809?spm=1001.2101.3001.6650.1&utm_medium=distribute.pc_relevant.none-task-blog-2%7Edefault%7ECTRLIST%7ERate-1-116848809-blog-8112948.pc_relevant_default&depth_1-utm_source=distribute.pc_relevant.none-task-blog-2%7Edefault%7ECTRLIST%7ERate-1-116848809-blog-8112948.pc_relevant_default&utm_relevant_index=2