[dpdk-dev] [PATCH] doc: update timestamp validity for latency measurement

[Reshma Pattan <reshma.pattan@intel.com>]

Updated the doc on how packets are marked to identify
their timestamp as valid and considered for latency
measurement.

Suggested-by: Bao-Long Tran <longtb5@viettel.com.vn>
Signed-off-by: Reshma Pattan <reshma.pattan@intel.com>
---
This change is relevant to the below patch and must be applied
after that.
http://patches.dpdk.org/patch/45328/
---
 doc/guides/prog_guide/metrics_lib.rst | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/doc/guides/prog_guide/metrics_lib.rst b/doc/guides/prog_guide/metrics_lib.rst
index 5cbe17c90..e68e4e743 100644
--- a/doc/guides/prog_guide/metrics_lib.rst
+++ b/doc/guides/prog_guide/metrics_lib.rst
@@ -271,3 +271,12 @@ de-initialise the latency library.
 .. code-block:: c
 
     rte_latencystats_uninit();
+
+Timestamp and latency calculation
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The Latency stats library marks the time in the timestamp field of the
+mbuf for the ingress packets and sets the ``PKT_RX_TIMESTAMP`` flag of
+``ol_flags`` for the mbuf to indicate the marked time as a valid one.
+At the egress, the mbufs with the flag set are considered having valid
+timestamp and are used for the latency calculation.
-- 
2.17.1