[PATCH] doc/mlx5: document firmware requirement for flex item

[Khadem Ullah <14pwcse1224@uetpeshawar.edu.pk>]

Some BlueField firmware versions (e.g. 24.30.1004) do not support flex item
definitions using FIELD_MODE_FIXED with size set to 0, such as in 'next_header'.
Attempting to use this configuration may result in the following error:

  'unsupported header length field mode (FIXED)'

This is resolved starting from firmware version 24.38.1002.

The patch adds a note to the mlx5 guide explaining the firmware requirement
for such flex item configurations.

Fixes: 16508bfd0816 ('net/mlx5: doc: add flex item specifications in mlx5 guide')
Cc: stable@dpdk.org
Signed-off-by: Khadem Ullah <14pwcse1224@uetpeshawar.edu.pk>
---
 doc/guides/nics/mlx5.rst | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/doc/guides/nics/mlx5.rst b/doc/guides/nics/mlx5.rst
index c3a350d24e..1b0e3efc41 100644
--- a/doc/guides/nics/mlx5.rst
+++ b/doc/guides/nics/mlx5.rst
@@ -476,6 +476,10 @@ Limitations
   - In flex item configuration, ``next_header.field_base`` value
     must be byte aligned (multiple of 8).
   - Modify field with flex item, the offset must be byte aligned (multiple of 8).
+    .. note::
+     Some firmware versions (e.g. 24.30.1004) do not support
+     ``next_header`` with ``field_mode = FIELD_MODE_FIXED`` and zero size.
+     A newer firmware (e.g. >= 24.38.1002) is required to support this configuration.
 
 - Match on random value:
 
-- 
2.43.0