[enhancement] Chrony config doesn't reflect hardware timestamping capability

[Mark Shuttleworth]

Hardware timestamping makes chrony syncing much more accurate, especially between machines on the same segment.

It's easy to see if a network interface supports hardware timestamping, with 'ethtool -T interface' looking for TX_HARDWARE and RX_HARDWARE capabilities:

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)
Hardware Receive Filter Modes:
 none (HWTSTAMP_FILTER_NONE)
 all (HWTSTAMP_FILTER_ALL)
 ptpv1-l4-sync (HWTSTAMP_FILTER_PTP_V1_L4_SYNC)
 ptpv1-l4-delay-req (HWTSTAMP_FILTER_PTP_V1_L4_DELAY_REQ)
 ptpv2-l4-sync (HWTSTAMP_FILTER_PTP_V2_L4_SYNC)
 ptpv2-l4-delay-req (HWTSTAMP_FILTER_PTP_V2_L4_DELAY_REQ)
 ptpv2-l2-sync (HWTSTAMP_FILTER_PTP_V2_L2_SYNC)
 ptpv2-l2-delay-req (HWTSTAMP_FILTER_PTP_V2_L2_DELAY_REQ)
 ptpv2-event (HWTSTAMP_FILTER_PTP_V2_EVENT)
 ptpv2-sync (HWTSTAMP_FILTER_PTP_V2_SYNC)
 ptpv2-delay-req (HWTSTAMP_FILTER_PTP_V2_DELAY_REQ)

If an interface supports timestamping, that can be enabled in chrony.conf with the hwtimestamping directive:

hwtimestamp eth0
hwtimestamp eth1

It's possible to use * to set hwtimestamp for all interfaces that support it:

hwtimestamp *

When a server supports timestamping you can get the accuracy improvement with the xleave directive:

server doorway.mallards minpoll 2 maxpoll 4 polltarget 30 xleave