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doc: improve hwtimestamp description

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Latest versions of ethtool print only the shorter lower-case names of
capabilities and filters. Explain that chronyd doesn't synchronize the
PHC and refer to the new vclock feature of the kernel, which should be
used by applications that need a synchronized PHC (e.g. ptp4l and
phc2sys) in order to not interfere with chronyd.
This commit is contained in:
Miroslav Lichvar 2022-05-19 10:09:08 +02:00
parent 46030d9d3e
commit f41d09e19f
1 changed files with 16 additions and 8 deletions
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24
doc/chrony.conf.adoc
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@ -2463,13 +2463,20 @@ be enabled by the *xleave* option in the <<server,*server*>> or the
+
This directive is supported on Linux 3.19 and newer. The NIC must support HW
timestamping, which can be verified with the *ethtool -T* command. The list of
capabilities should include _SOF_TIMESTAMPING_RAW_HARDWARE_,
_SOF_TIMESTAMPING_TX_HARDWARE_, and _SOF_TIMESTAMPING_RX_HARDWARE_. Receive
filter _HWTSTAMP_FILTER_ALL_, or _HWTSTAMP_FILTER_NTP_ALL_, is necessary for
timestamping of received NTP packets. Timestamping of packets received on bridged
and bonded interfaces is supported on Linux 4.13 and newer. When *chronyd* is
running, no other process (e.g. a PTP daemon) should be working with the NIC
clock.
capabilities should include _hardware-raw-clock_, _hardware-transmit_, and
_hardware-receive_. The receive filter _all_, or _ntp_, is necessary for
timestamping of received NTP packets. Timestamping of packets received on
bridged and bonded interfaces is supported on Linux 4.13 and newer. If HW
timestamping does not work for received packets, *chronyd* will use kernel
receive timestamps instead. Transmit-only HW timestamping can still be useful
to improve stability of the synchronisation.
+
*chronyd* does not synchronise the NIC clock. It assumes the clock is running
free. Multiple instances of *chronyd* can use the same interface with enabled
HW timestamping. Applications which need HW timestamping with a synchronised
clock (e.g. a PTP daemon) should use a virtual clock running on top of the
physical clock created by writing to _/sys/class/ptp/ptpX/n_vclocks_. This
feature is available on Linux 5.14 and newer.
+
If the kernel supports software timestamping, it will be enabled for all
interfaces. The source of timestamps (i.e. hardware, kernel, or daemon) is
@ -2527,7 +2534,8 @@ is selected by default. Some NICs can timestamp PTP packets only. By default,
they will be configured with the _none_ filter and expected to provide hardware
timestamps for transmitted packets only. Timestamping of PTP packets is useful
with NTP-over-PTP enabled by the <<chrony.conf.adoc#ptpport,*ptpport*>>
directive. Forcing timestamping of all packets with the _all_ filter could be
directive, or when another application is receiving PTP packets on the
interface. Forcing timestamping of all packets with the _all_ filter could be
useful if the NIC supported both the _all_ and _ntp_ filters, and it should
timestamp both NTP and PTP packets, or NTP packets on a different UDP port.
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