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doc: update description of hwtimestamp directive

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Miroslav Lichvar 2017-06-14 13:18:11 +02:00
parent f2da253bc3
commit 39da10d939
1 changed files with 20 additions and 7 deletions
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27
doc/chrony.conf.adoc
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@ -1853,14 +1853,17 @@ 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_, _SOF_TIMESTAMPING_RX_HARDWARE_, and the filter
modes should have _HWTSTAMP_FILTER_ALL_. When *chronyd* is running, no other
process should be working with the clock on the NIC. If no *hwtimestamp*
directive is specified, *chronyd* will try to use software (kernel)
timestamping. With both hardware and software timestamping there are
some limitations on which packets can be actually timestamped, e.g. transmit
modes should include _HWTSTAMP_FILTER_ALL_ or _HWTSTAMP_FILTER_NTP_ALL_. When
*chronyd* is running, no other process (e.g. a PTP daemon) should be working
with the NIC clock.
+
If the kernel supports software timestamping, it will be enabled for all
interfaces. With both hardware and software timestamping there are some
limitations on which timestamps can be actually used, e.g. transmit
timestamping does not currently work with IPv6 packets using IP options and
hardware receive timestamping does not work with packets from bridged
interfaces. The timestamping used in measurements is indicated in the
hardware timestamping of packets received from bridges, bonds, and other
virtual interfaces, works only on Linux 4.13 and newer. The source of
timestamps (i.e. hardware, kernel, or daemon) is indicated in the
_measurements.log_ file if enabled by the <<log,*log measurements*>> directive,
and the <<chronyc.adoc#ntpdata,*ntpdata*>> report in *chronyc*.
+
@ -1890,6 +1893,16 @@ is 0.
*nocrossts*:::
Some hardware can precisely cross timestamp the NIC clock with the system
clock. This option disables the use of the cross timestamping.
*rxfilter* _filter_:::
This option selects the receive timestamping filter. Possible values are:
_all_, _ntp_, and _none_. The default value is _ntp_, which enables
timestamping of NTP packets (_HWTSTAMP_FILTER_NTP_ALL_) if it is supported, or
timestamping of all packets (_HWTSTAMP_FILTER_ALL_). Setting *rxfilter* to
_all_ forces timestamping of all packets, which can be useful when the NIC
supports both filters and NTP packets are received from or on a non-standard
UDP port (e.g. specified by the *port* directive). Setting *rxfilter* to _none_
disables receive HW timestamping and allows transmit HW timestamping to be
enabled when the NIC supports only PTP-specific receive filters.
::
+
Examples of the directive are: