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-rw-r--r--Documentation/networking/timestamping.txt21
1 files changed, 21 insertions, 0 deletions
diff --git a/Documentation/networking/timestamping.txt b/Documentation/networking/timestamping.txt
index a5c784c89312..5f0922613f1a 100644
--- a/Documentation/networking/timestamping.txt
+++ b/Documentation/networking/timestamping.txt
@@ -162,6 +162,27 @@ SOF_TIMESTAMPING_OPT_CMSG:
option IP_PKTINFO simultaneously.
+SOF_TIMESTAMPING_OPT_TSONLY:
+
+ Applies to transmit timestamps only. Makes the kernel return the
+ timestamp as a cmsg alongside an empty packet, as opposed to
+ alongside the original packet. This reduces the amount of memory
+ charged to the socket's receive budget (SO_RCVBUF) and delivers
+ the timestamp even if sysctl net.core.tstamp_allow_data is 0.
+ This option disables SOF_TIMESTAMPING_OPT_CMSG.
+
+
+New applications are encouraged to pass SOF_TIMESTAMPING_OPT_ID to
+disambiguate timestamps and SOF_TIMESTAMPING_OPT_TSONLY to operate
+regardless of the setting of sysctl net.core.tstamp_allow_data.
+
+An exception is when a process needs additional cmsg data, for
+instance SOL_IP/IP_PKTINFO to detect the egress network interface.
+Then pass option SOF_TIMESTAMPING_OPT_CMSG. This option depends on
+having access to the contents of the original packet, so cannot be
+combined with SOF_TIMESTAMPING_OPT_TSONLY.
+
+
1.4 Bytestream Timestamps
The SO_TIMESTAMPING interface supports timestamping of bytes in a