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

[Dariusz Sosnowski]

+ mlx5 maintainers

On Tue, Jul 08, 2025 at 07:49:21AM -0400, Khadem Ullah wrote:
> 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.

Could you please share the DPDK version you used and
a code sample with flex item configuration?

I checked internally and I see no change which would trigger
a change in behavior regarding 'unsupported header length field mode (FIXED)' error message.

> 
> 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 at dpdk.org
> Signed-off-by: Khadem Ullah <14pwcse1224 at 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
> 

Best regards,
Dariusz Sosnowski