[PATCH] doc/mlx5: document modify field limitation on group 0

[Bing Zhao]

Hi,

> -----Original Message-----
> From: Maayan Kashani <mkashani at nvidia.com>
> Sent: Thursday, July 30, 2026 11:45 PM
> To: dev at dpdk.org
> Cc: Maayan Kashani <mkashani at nvidia.com>; Raslan Darawsheh
> <rasland at nvidia.com>; Dariusz Sosnowski <dsosnowski at nvidia.com>; Slava
> Ovsiienko <viacheslavo at nvidia.com>; Bing Zhao <bingz at nvidia.com>; Ori Kam
> <orika at nvidia.com>; Suanming Mou <suanmingm at nvidia.com>; Matan Azrad
> <matan at nvidia.com>
> Subject: [PATCH] doc/mlx5: document modify field limitation on group 0
> 
> Document that on the root table, two sequential MODIFY_FIELD actions where
> the second uses a field modified by the first as src are not
> supported: the second action is handled before the first change is
> applied. Only immediate src types (VALUE/POINTER) are allowed on group 0
> for sync HWS non-template and DV flows.
> 
> Signed-off-by: Maayan Kashani <mkashani at nvidia.com>
> ---
>  doc/guides/nics/mlx5.rst | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/doc/guides/nics/mlx5.rst b/doc/guides/nics/mlx5.rst index
> 9b035b878e0..aa751669444 100644
> --- a/doc/guides/nics/mlx5.rst
> +++ b/doc/guides/nics/mlx5.rst
> @@ -3425,6 +3425,12 @@ Limitations
> 
>  #. In template tables of group 0, the modify action must be fully masked.
> 
> +#. On group 0 (root table), when using sync API, two sequential
> +   ``RTE_FLOW_ACTION_TYPE_MODIFY_FIELD`` actions where the first modifies
> a
> +   packet or metadata field and the second uses that field as ``src`` are
> not
> +   supported. On the root table the second action is handled before the
> first
> +   change is applied, so the source still reads the original field value.
> +
>  #. Modification of an arbitrary place in a packet via
>     the special ``RTE_FLOW_FIELD_START`` field ID is not supported.
> 
> --
> 2.21.0

Acked-by: Bing Zhao <bingz at nvidia.com>