[PATCH v4 4/4] doc: add mlx5 datapath tracing feature description

[Viacheslav Ovsiienko <viacheslavo@nvidia.com>]

The mlx5 provides the send scheduling on specific moment of time,
and for the related kind of applications it would be extremely useful
to have extra debug information - when and how packets were scheduled
and when the actual sending was completed by the NIC hardware (it helps
application to track the internal delay issues).

The patch adds the documentation for feature usage.

Signed-off-by: Viacheslav Ovsiienko <viacheslavo@nvidia.com>
---
 doc/guides/nics/mlx5.rst | 78 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 78 insertions(+)

diff --git a/doc/guides/nics/mlx5.rst b/doc/guides/nics/mlx5.rst
index b9843edbd9..1c8fc6f6d4 100644
--- a/doc/guides/nics/mlx5.rst
+++ b/doc/guides/nics/mlx5.rst
@@ -2077,3 +2077,81 @@ where:
 * ``sw_queue_id``: queue index in range [64536, 65535].
   This range is the highest 1000 numbers.
 * ``hw_queue_id``: queue index given by HW in queue creation.
+
+
+Tx datapath tracing
+^^^^^^^^^^^^^^^^^^^
+
+The mlx5 provides the Tx datapath tracing capability with extra debug
+information - when and how packets were scheduled and when the actual
+sending was completed by the NIC hardware. The feature engages the
+existing DPDK datapath tracing capability.
+
+Usage of the mlx5 Tx datapath tracing:
+
+#. Build DPDK application with enabled datapath tracking
+
+   * The meson option should be specified: ``--enable_trace_fp=true``
+   * The c_args should be specified:  ``-DALLOW_EXPERIMENTAL_API``
+
+   .. code-block:: console
+
+      meson configure --buildtype=debug -Denable_trace_fp=true
+         -Dc_args='-DRTE_LIBRTE_MLX5_DEBUG -DRTE_ENABLE_ASSERT -DALLOW_EXPERIMENTAL_API' build
+
+      meson configure --buildtype=release -Denable_trace_fp=true
+         -Dc_args='-DRTE_ENABLE_ASSERT -DALLOW_EXPERIMENTAL_API' build
+
+#. Configure the NIC
+
+   If the sending completion timings are important the NIC should be configured
+   to provide realtime timestamps, the ``REAL_TIME_CLOCK_ENABLE`` NV settings
+   parameter should be configured as TRUE.
+
+   .. code-block:: console
+
+      mlxconfig -d /dev/mst/mt4125_pciconf0 s REAL_TIME_CLOCK_ENABLE=1
+
+#. Run application with EAL parameters configuring the tracing in mlx5 Tx datapath
+
+    * ``--trace=pmd.net.mlx5.tx`` - the regular expression enabling the tracepoints
+      with matching names at least "pmd.net.mlx5.tx" must be enabled to gather all
+      events needed to analyze mlx5 Tx datapath and its timings. By default all
+      tracepoints are disabled.
+
+#. Store the file with gathered tracing information
+
+#. Install or build the ``babeltrace2`` package
+
+   The gathered trace data can be analyzed with a developed Python script.
+   To parse the trace, the data script uses the ``babeltrace2`` library.
+   The package should be either installed or built from source code as
+   shown below.
+
+   .. code-block:: console
+
+      git clone https://github.com/efficios/babeltrace.git
+      cd babeltrace
+      ./bootstrap
+      ./configure -help
+      ./configure --disable-api-doc --disable-man-pages
+                  --disable-python-bindings-doc --enable-python-plugins
+                  --enable-python-binding
+
+#. Run analyzing scrypt (in Python) to combine related events (packet firing and
+   completion) and see the output in human-readable view
+
+   The analyzing script is located in the folder: ``./drivers/net/mlx5/tools``
+   It requires Python3.6, ``babeltrace2`` packages and it takes the only parameter
+   of trace data file.
+
+   .. code-block:: console
+
+      ./mlx5_trace.py /var/log/rte-2023-01-23-AM-11-52-39
+
+#. Interpreting the Script Output Data
+
+   All the timings are given in nanoseconds.
+   The list of Tx bursts per port/queue is presented in the output.
+   Each list element contains the list of built WQEs with specific opcodes, and
+   each WQE contains the list of the encompassed packets to send.
-- 
2.18.1