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authorMiroslav Lichvar <mlichvar@redhat.com>2017-05-19 18:52:39 +0300
committerDavid S. Miller <davem@davemloft.net>2017-05-21 20:37:32 +0300
commit67953d47bb24e63d209705f745a0de411a4c6578 (patch)
tree4c8eea80b7c24c08766f395e0b3ce7b5045f048b /Documentation
parentaad9c8c470f2a8321a99eb053630ce0e199558d6 (diff)
downloadlinux-67953d47bb24e63d209705f745a0de411a4c6578.tar.xz
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>
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/networking/timestamping.txt8
1 files changed, 7 insertions, 1 deletions
diff --git a/Documentation/networking/timestamping.txt b/Documentation/networking/timestamping.txt
index ce11e3a08c0d..50eb0e554778 100644
--- a/Documentation/networking/timestamping.txt
+++ b/Documentation/networking/timestamping.txt
@@ -322,7 +322,7 @@ 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.
@@ -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
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
For transmit timestamps the outgoing packet is looped back to the