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net: fix documentation of struct scm_timestamping 

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>
Acked-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Miroslav Lichvar 2017-05-19 17:52:39 +02:00 committed by David S. Miller
parent aad9c8c470
commit 67953d47bb
1 changed files with 7 additions and 1 deletions
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8
Documentation/networking/timestamping.txt
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@ -322,7 +322,7 @@ struct scm_timestamping {
}; };
The structure can return up to three timestamps. This is a legacy 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]. are passed in ts[0]. Hardware timestamps are passed in ts[2].
ts[1] used to hold hardware timestamps converted to system time. ts[1] used to hold hardware timestamps converted to system time.
@ -331,6 +331,12 @@ a HW PTP clock source, to allow time conversion in userspace and
optionally synchronize system time with a userspace PTP stack such optionally synchronize system time with a userspace PTP stack such
as linuxptp. For the PTP clock API, see Documentation/ptp/ptp.txt. 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. This happens also
on hardware transmit timestamps.
2.1.1 Transmit timestamps with MSG_ERRQUEUE 2.1.1 Transmit timestamps with MSG_ERRQUEUE
For transmit timestamps the outgoing packet is looped back to the For transmit timestamps the outgoing packet is looped back to the