Linux时间同步(PPS、PTP、chrony)分析笔记

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] +/-   29ms

2 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)