计算机中的 system clock：
system clock is maintained by the kernel of an operating system, and is used to set the tasks and processes. The system clock reports seconds and microseconds since a start point from the system boot up procedure.
计算机中的 Real Time Clock:
RTC is not a physical clock but is an IC(Integrated Circuit) which is present on the motherboard and responsible for timing functioning of the system clock. It has a crystal oscillator.
Real Time Clock is battery backup power clocks so that it tracks the time even while the computer is turned off. The power of a RTC is done by a lithium battery generally. Whenever the motherboard is brought to power, then the battery charges automatically, hence ready for future use.
上面说RTC不是个physical clock，但我认为它应该是经常讲到的hardware clock
ethtool -T <networkInterface> 查看网口是否支持PTP。
以下示例说明eth6网口支持hardware timestamping：
Time stamping parameters for eth6:
Capabilities:
hardware-transmit (SOF_TIMESTAMPING_TX_HARDWARE)
software-transmit (SOF_TIMESTAMPING_TX_SOFTWARE)
hardware-receive (SOF_TIMESTAMPING_RX_HARDWARE)
software-receive (SOF_TIMESTAMPING_RX_SOFTWARE)
software-system-clock (SOF_TIMESTAMPING_SOFTWARE)
hardware-raw-clock (SOF_TIMESTAMPING_RAW_HARDWARE)
PTP Hardware Clock: 1
Hardware Transmit Timestamp Modes:
off (HWTSTAMP_TX_OFF)
on (HWTSTAMP_TX_ON)
Hardware Receive Filter Modes:
none (HWTSTAMP_FILTER_NONE)
all (HWTSTAMP_FILTER_ALL)
以下示例说明enp6s0网口支持software timestamping：
Time stamping parameters for enp6s0:
Capabilities:
software-transmit (SOF_TIMESTAMPING_TX_SOFTWARE)
software-receive (SOF_TIMESTAMPING_RX_SOFTWARE)
software-system-clock (SOF_TIMESTAMPING_SOFTWARE)
PTP Hardware Clock: none
Hardware Transmit Timestamp Modes: none
Hardware Receive Filter Modes: none
介绍有关linux-ptp的信息：建议参考链接
LinuxPTP provides the four user applications - ptp4l, phc2sys, hwstamp_ctl and pmc. The definition and usage of these applications is as follows:
- ptp4l: It is the main program that implements the Precision Time Protocol (PTP) according to IEEE standard 1588 for Linux, as a Boundary Clock (BC) and Ordinary Clock (OC). The ‘slave’ and ‘master’ roles are determined for each port automatically using the BMC
- phc2sys: It is a utility program to synchronize the normal Linux system time to a PTP Hardware Clock - which itself is synchronized by the ptp4l program/application. For phc2sys, the terms ‘slave’ and ‘master’ are not about the PTP, but rather about two local clocks. Usually, the Linux system time is the slave, and the PHC is the master.
- pmc: This is a utility program for sending PTP management queries. The program reads from the standard input actions specified by name and management ID, sends them over the selected transport and prints any received replies.
- hwstamp_ctl: It is just a test program for the SIOCSHWTSTAMP ioctl, used for debugging. The ptp4l program does not need this program to function
The ptp4l program implements the PTP boundary clock and ordinary clock. With hardware time stamping, it is used to synchronize the PTP hardware clock to the master clock, and with software time stamping it synchronizes the system clock to the master clock.
The phc2sys program is needed only with hardware time stamping, for synchronizing the system clock to the PTP hardware clock on the network interface card (NIC).
LinuxPTP 输出内容的意义
~]# ptp4l -i em3 -m
selected em3 as PTP clock
port 1: INITIALIZING to LISTENING on INITIALIZE
port 0: INITIALIZING to LISTENING on INITIALIZE
port 1: new foreign master 00a069.fffe.0b552d-1
selected best master clock 00a069.fffe.0b552d
port 1: LISTENING to UNCALIBRATED on RS_SLAVE
master offset -23947 s0 freq +0 path delay 11350
master offset -28867 s0 freq +0 path delay 11236
master offset -32801 s0 freq +0 path delay 10841
master offset -37203 s1 freq +0 path delay 10583
master offset -7275 s2 freq -30575 path delay 10583
port 1: UNCALIBRATED to SLAVE on MASTER_CLOCK_SELECTED
master offset -4552 s2 freq -30035 path delay 10385
The master offset value is the measured offset from the master in nanoseconds. The s0, s1, s2 strings indicate the different clock servo states: s0 is unlocked, s1 is clock step and s2 is locked. Once the servo is in the locked state (s2), the clock will not be stepped (only slowly adjusted) unless the pi_offset_const option is set to a positive value in the configuration file (described in the ptp4l(8) man page). The adj value is the frequency adjustment of the clock in parts per billion (ppb). The path delay value is the estimated delay of the synchronization messages sent from the master in nanoseconds.
时钟同步，同步的是frequency还是time？
在参考链接中看到一个time jump的概念，对应ptp4l和phc2sys命令的--step-threshold=n的选项。如果new time与old time之间的差距（即time jump）小于n，则修改clock frequency，否则，直接修改time。
timedatectl
可以用于检查当前系统有没有启用NTP。
如果发现有 “NTP service: active”，可以通过运行
timedatectl set-ntp false
来将其关闭。
RedHat PTP参考链接
Linux PTP userdoc
https://www.engineersgarage.com/how_to/how-clock-in-computer-works/
Configuring PTP Using ptp4l - Fedora Docs
