本文主要是介绍Linux时间同步(PPS、PTP、chrony)分析笔记，希望对大家解决编程问题提供一定的参考价值，需要的开发者们随着小编来一起学习吧！
1 PPS(pulse per second)
1.1 简介
LinuxPPS provides a programming interface (API) to define in the system several PPS sources.
PPS means "pulse per second" and a PPS source is just a device which provides a high precision signal each second so that an application can use it to adjust system clock time.
A PPS source can be connected to a serial port (usually to the Data Carrier Detect pin) or to a parallel port (ACK-pin) or to a special CPU's GPIOs (this is the common case in embedded systems) but in each case when a new pulse arrives the system must apply to it a timestamp
and record it for userland.
《<kernel_src>/Documentation/pps/pps.txt》
1.2 PPS时间同步信号的输入源
1.2.1 串口信号中的DCD(Data Carrier Detect)信号
1.2.1.1 普通串口
以drivers/tty/serial/amba-pl010.c驱动为例
pl010_int();-> pl010_modem_status();-> uart_handle_dcd_change();-> ld->ops->dcd_change();-> pps_tty_dcd_change();-> pps_event()1.2.1.2 USB串口
以drivers/usb/serial/pl2303.c驱动为例
pl2303_read_int_callback();-> pl2303_update_line_status();-> usb_serial_handle_dcd_change();-> ld->ops->dcd_change();-> pps_tty_dcd_change();-> pps_event()1.2.3 GPIO
1.2.3.1 在设备树中指定使用哪个GPIO
例如
pps {compatible = "pps-gpio";pinctrl-names = "default";pinctrl-0 = <&pinctrl_pps>;gpios = <&gpio1 26 GPIO_ACTIVE_HIGH>;status = "okay";};1.2.3.2 驱动
drivers/pps/clients/pps-gpio.c
pps_gpio_irq_handler();-> pps_event()1.2.4 并口
drivers/pps/clients/pps_parport.c
parport_irq();-> pps_event();1.2.5 PTP
1.2.5.1 判断网卡是否打开PPS功能
/sys/class/ptp/ptp0/pps_available：
This file indicates whether the PTP hardware clock supports a Pulse Per Second to the host CPU.
/sys/class/ptp/ptp0/pps_enable：
This write-only file enables or disables delivery of PPS events to the Linux PPS subsystem.
1.2.5.2 驱动
以Intel的网卡驱动为例
drivers/net/ethernet/intel/igb/igb_main.c
igb_tsync_interrupt();-> ptp_clock_event();-> pps_event();1.3 文件操作接口/dev/ppsX
/** Char device stuff*/static const struct file_operations pps_cdev_fops = {.owner = THIS_MODULE,.llseek = no_llseek,.poll = pps_cdev_poll,.fasync = pps_cdev_fasync,.compat_ioctl = pps_cdev_compat_ioctl,.unlocked_ioctl = pps_cdev_ioctl,.open = pps_cdev_open,.release = pps_cdev_release,
};1.4 在chrony服务中使用PPS作为时间源
在配置文件/etc/chrony/chrony.conf中添加以下内容
refclock SHM 0 poll -2 refid GPS precision 1e-1 offset 0.9999 delay 0.2
refclock PPS /dev/pps0 lock NMEA refid PPS执行"chronyc sources" 命令，出现以下结果表示配置成功
# chronyc sources | grep PPS
#- PPS 0 4 77 16 -309ms[ -309ms] +/- 29ms2 PTP(precise time protocol; IEEE 1588协议)
2.1 简介
IEEE 1588 addresses the clock synchronization requirements of measurement and control systems. The protocol supports system-wide synchronization accuracy in the sub-microsecond range with minimal network and local clock computing resources.
《Intel ® Ethernet Controller I350 Datasheet》P457
2.2 判断网卡是否支持PTP
PTP功能需要网卡硬件支持，可通过ethtool -T xxx来查看
出现以下信息，表示网卡硬件支持PTP
# ethtool -T enp0s31f6
Time stamping parameters for enp0s31f6:
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: 0
Hardware Transmit Timestamp Modes:off (HWTSTAMP_TX_OFF)on (HWTSTAMP_TX_ON)
2.3 在chrony服务中使用PTP作为时间源
在配置文件/etc/chrony/chrony.conf中添加以下内容
refclock PHC /dev/ptp0 poll 2执行"chronyc sources" 命令，出现以下结果表示配置成功
# chronyc sources | grep PHC
#x PHC2 0 2 377 5 +39.6s[ +39.6s] +/- 1139us
缩写：
PHC: PTP hardware clock
2.4 /sys/class/ptp/ptpN/
2.5 扩展知识：虚拟机和物理机之间的时间同步(ptp_kvm)
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