Use of TX offload flags MBUF_FAST_FREE and MULTI_SEGS

[Stephen Hemminger]

On Mon, 6 Oct 2025 16:51:38 +0200
Morten Brørup <mb at smartsharesystems.com> wrote:

> > From: Bruce Richardson [mailto:bruce.richardson at intel.com]
> > Sent: Friday, 3 October 2025 11.18
> > Subject: Minutes of techboard meeting, 2025-10-01  
> 
> > * Use of FAST_FREE and multi-buffer/scattered mbuf flags
> >   - The flags for enabling fast-free and supporting multi-mbuf packets
> > are
> >     now documented incompatible
> >   - Previously they were not defined as incompatible, but that seems to
> >     have been assumed for some usages.
> >   - Techboard discussed how best to resolve this incompatibility with
> >     regards to:
> >     - ensuring correctness
> >     - avoiding major churn to DPDK code
> >     - avoiding churn to end-user code
> >   - Options discussed:
> >     1 change definition back to not have the settings incompatible:
> > this
> >       necessitates checking drivers for correctness
> >     2 keep as explicitly incompatible and report error if both
> > specified:
> >       this could break end-user apps, and requires changes to example
> > apps
> >     3 drop the fast-free flag if multi-segment mbufs are also
> > specified:
> >       "hides" the issue, but probably minimises changes. Would need to
> >       decide whether the dropping of flag done in drivers vs ethdev
> > level.
> >       Pros and cons to both options. Needs clear documenting.
> >   - No firm decision reached, will discuss more over email.  
> 
> IMO, the patch [1] making MBUF_FAST_FREE and MULTI_SEGS explicitly incompatible should be reverted, at least for RC1.
> That will take the project back to the state it was in before we started this discussion.
> And all the examples broken by the patch (because they use both TX offloads) will not need fixing.
> 
> [1]: https://patchwork.dpdk.org/project/dpdk/patch/20250803194218.683318-3-mb@smartsharesystems.com/
> 

There is still code NULL PMD which checks for FAST_FREE with MULTI_SEGS.
I think this commit needs to be reverted as well.

commit 675ddc233d68d90b3858cdb6267dd407c4e85984
Author: Morten Brørup <mb at smartsharesystems.com>
Date:   Wed Jul 30 14:04:51 2025 +0000

    net/null: add fast mbuf release Tx offload
    
    Added fast mbuf release, reusing the existing mbuf pool pointer
    in the queue structure.
    
    Signed-off-by: Morten Brørup <mb at smartsharesystems.com>