[PATCH v3] doc: add mlx5 PMD flex item documentation

[Gregory Etelson]

Describe firmware configuration requirements.
List mlx5 hardware and PMD limitations.

Signed-off-by: Gregory Etelson <getelson at nvidia.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo at nvidia.com>
Acked-by: Raslan Darawsheh <rasland at nvidia.com>
---
v2: Style fixes.
v3: Update hardware and port support.
---
 doc/guides/nics/mlx5.rst | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/doc/guides/nics/mlx5.rst b/doc/guides/nics/mlx5.rst
index 552e06c0db..7b2647dad6 100644
--- a/doc/guides/nics/mlx5.rst
+++ b/doc/guides/nics/mlx5.rst
@@ -494,6 +494,18 @@ Limitations
     from the reference "Clock Queue" completions,
     the scheduled send timestamps should not be specified with non-zero MSB.
 
+- Flex item API:
+
+  - Firmware configuration: ``FLEX_PARSER_PROFILE_ENABLE=4 PROG_PARSE_GRAPH=1``.
+  - Hardware support: BlueField 2.
+  - Flex item is supported for DPDK ports created over ECPF (Uplink representor, VF representor,
+    SF representor, HPF representor).
+  - Hardware limits ``header_length_mask_width`` up to 6 bits.
+  - Firmware supports 8 global sample fields. Each flex item allocates non-shared sample fields from that pool.
+  - Supported flex item can have 1 IN arc - ``eth`` or ``udp`` and up to 2 OUT arcs - ``ipv4`` or ``ipv6``.
+  - Flex item fields do not participate in RSS hash functions.
+  - In RTE flex item configuration, ``next_header.field_base`` value must be byte aligned (multiple of 8).
+
 Statistics
 ----------
 
@@ -1260,6 +1272,11 @@ Below are some firmware configurations listed.
 
    REAL_TIME_CLOCK_ENABLE=1
 
+- enable dynamic flex parser for RTE flex item::
+
+   FLEX_PARSER_PROFILE_ENABLE=4
+   PROG_PARSE_GRAPH=1
+
 Linux Prerequisites
 -------------------
 
-- 
2.34.0