@@ -321,7 +321,7 @@ struct scm_timestamping {
};
The structure can return up to three timestamps. This is a legacy
-feature. Only one field is non-zero at any time. Most timestamps
+feature. At least one field is non-zero at any time. Most timestamps
are passed in ts[0]. Hardware timestamps are passed in ts[2].
ts[1] used to hold hardware timestamps converted to system time.
@@ -330,6 +330,12 @@ a HW PTP clock source, to allow time conversion in userspace and
optionally synchronize system time with a userspace PTP stack such
as linuxptp. For the PTP clock API, see Documentation/ptp/ptp.txt.
+Note that if the SO_TIMESTAMP or SO_TIMESTAMPNS option is enabled
+together with SO_TIMESTAMPING using SOF_TIMESTAMPING_SOFTWARE, a false
+software timestamp will be generated in the recvmsg() call and passed
+in ts[0] when a real software timestamp is missing. For this reason it
+is not recommended to combine SO_TIMESTAMP(NS) with SO_TIMESTAMPING.
+
2.1.1 Transmit timestamps with MSG_ERRQUEUE
For transmit timestamps the outgoing packet is looped back to the
The scm_timestamping struct may return multiple non-zero fields, e.g. when both software and hardware RX timestamping is enabled, or when the SO_TIMESTAMP(NS) option is combined with SCM_TIMESTAMPING and a false software timestamp is generated in the recvmsg() call in order to always return a SCM_TIMESTAMP(NS) message. CC: Richard Cochran <richardcochran@gmail.com> CC: Willem de Bruijn <willemb@google.com> Signed-off-by: Miroslav Lichvar <mlichvar@redhat.com> --- Documentation/networking/timestamping.txt | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)