Re: [PATCH 1/2] ethdev: support selective Rx data

[Stephen Hemminger <stephen@networkplumber.org>]

On Mon, 2 Feb 2026 18:09:02 +0200
Gregory Etelson <getelson@nvidia.com> wrote:

> In some cases application does not need to receive entire packet
> from port hardware.
> If application could receive required Rx data only and safely discard
> the rest of Rx packet data, that could improve port performance by
> reducing PCI bandwidth and application memory consumption.
> 
> Selective Rx data allows application to receive
> only pre-configured packet segments and discard the rest.
> For example:
> - Deliver the first N bytes only.
> - Deliver the last N bytes only.
> - Deliver N1 bytes from offset Off1 and N2 bytes from offset Off2.
> 
> Selective Rx data is implemented on-top of the existing Rx
> BUFFER_SPLIT functionality:
> - The rte_eth_rxseg_split will use the NULL mempool for data segments
> that should be discarded.
> - PMD will not create MBUF segments if no data was read.
> 
> For example: Deliver Ethernet header only
> 
> Rx queue segments configuration:
> struct rte_eth_rxseg_split split[2] = {
>     {
>         .mp = <some mempool>,
>         .length = sizeof(struct rte_ether_hdr)
>     },
>     {
>         .mp = NULL, /* discard data */
>         .length = <MTU>
>     }
> };
> 
> Received MBUF configuration:
> mbuf[0].pkt_len = sizeof(struct rte_ether_hdr);
> mbuf[0].data_len = sizeof(struct rte_ether_hdr);
> mbuf[0].next = NULL; /* The next segment did not deliver data */

And nb_segs should be 1?
And mbuf must still pass sanity check?

> 
> After selective Rx, the mbuf packet length reflects only the
> existing data that was actually received, and can be less than the
> original wire packet length.
> 
> A PMD activates the selective Rx data capability by setting the
> rte_eth_rxseg_capa.selective_read bit.
> 
> Signed-off-by: Gregory Etelson <getelson@nvidia.com>
> ---

Need documentation updates as well.
At a minimum:
  - entry in nic/guides/features/default.ini
  - update to the ethdev documentation
  - release note
It would also be good if one or more examples used split but
that can wait.