Fixing MBUF_FAST_FREE TX offload requirements?

["Morten Brørup" <mb@smartsharesystems.com>]

Dear NIC driver maintainers (CC: DPDK Tech Board),

The DPDK Tech Board has discussed that patch [1] (included in DPDK 25.07) extended the documented requirements to the RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE offload.
These changes put additional limitations on applications' use of the MBUF_FAST_FREE TX offload, and made MBUF_FAST_FREE mutually exclusive with MULTI_SEGS (which is typically used for jumbo frame support).
The Tech Board discussed that these changes do not reflect the intention of the MBUF_FAST_FREE TX offload, and wants to fix it.
Mainly, MBUF_FAST_FREE and MULTI_SEGS should not be mutually exclusive.

The original RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE requirements were:
When set, application must guarantee that
1) per-queue all mbufs come from the same mempool, and
2) mbufs have refcnt = 1.

The patch added the following requirements to the MBUF_FAST_FREE offload, reflecting rte_pktmbuf_prefree_seg() postconditions:
3) mbufs are direct,
4) mbufs have next = NULL and nb_segs = 1.

Now, the key question is:
Can we roll back to the original two requirements?
Or do the drivers also depend on the third and/or fourth requirements?

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Drivers freeing mbufs directly to a mempool should use the new rte_mbuf_raw_free_bulk() instead of rte_mempool_put_bulk(), so the preconditions for freeing mbufs directly into a mempool are validated in mbuf debug mode (with RTE_LIBRTE_MBUF_DEBUG enabled).
Similarly, rte_mbuf_raw_alloc_bulk() should be used instead of rte_mempool_get_bulk().
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PS: The feature documentation [2] still reflects the original requirements.

[1]: https://github.com/DPDK/dpdk/commit/55624173bacb2becaa67793b71391884876673c1
[2]: https://elixir.bootlin.com/dpdk/v25.07/source/doc/guides/nics/features.rst#L125


Venlig hilsen / Kind regards,
-Morten Brørup