doc: improve hwtimestamp description

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.

diff --git a/doc/chrony.conf.adoc b/doc/chrony.conf.adoc
index 70e27d409c343f383ba63034670bc8cd8d762a1d..a77363c58ae019f509cb1e989bfd439426424bee 100644 (file)
--- a/doc/chrony.conf.adoc
+++ b/doc/chrony.conf.adoc
@@ -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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