Re: [PATCH net 2/2] bnxt_en: Pad short SW USO segments to BNXT_MIN_PKT_SIZE

[Joe Damato <joe@dama.to>]

On Thu, Oct 08, 2026 at 10:26:06PM -0700, Michael Chan wrote:
> On Thu, Oct 8, 2026 at 12:11 PM Joe Damato <joe@dama.to> wrote:
> 
> > diff --git a/drivers/net/ethernet/broadcom/bnxt/bnxt_gso.c b/drivers/net/ethernet/broadcom/bnxt/bnxt_gso.c
> > index ef04c9d08066..f87e02ef3033 100644
> > --- a/drivers/net/ethernet/broadcom/bnxt/bnxt_gso.c
> > +++ b/drivers/net/ethernet/broadcom/bnxt/bnxt_gso.c
> > @@ -82,11 +82,14 @@ int bnxt_sw_udp_gso_xmit(struct bnxt *bp, struct bnxt_tx_ring_info *txr,
> >         if (unlikely(num_segs <= 1))
> >                 goto drop;
> >
> > +       if (unlikely(bnxt_sw_gso_pad_len(hdr_len, mss)))
> > +               goto drop;
> > +
> 
> This looks redundant since you already have the same check in
> bnxt_features_check().

It's redundant for skbs that go through validate_xmit_skb(), but generic XDP
doesn't AFAIU. Not sure how else to handle this case, but open to suggestions.

If you agree and are OK with dropping these, then I could add a comment
clarifying this in the v2. LMK what you think.

> >         /* Upper bound on the number of descriptors needed.
> >          *
> >          * Each segment uses 1 long BD + 1 ext BD + payload BDs, which is
> >          * at most num_segs + nr_frags (each frag boundary crossing adds at
> > -        * most 1 extra BD).
> > +        * most 1 extra BD). The last segment may need 1 pad BD.
> >          */
> >         bds_needed = 3 * num_segs + skb_shinfo(skb)->nr_frags + 1;
> 
> I don't quite understand why we don't need to add 1 more for possible
> padding here.

Re-reading the code, I think the +1 was never needed in the first place.
Payload BDs come from at most 1 + nr_frags DMA regions. Going from one region
to the next adds at most one BD on top of one BD per segment. There are
nr_frags boundaries at most, so I think the math is:

  2*num_segs + num_segs + nr_frags

which simplifies to 3*num_segs + nr_frags.

I would appreciate a double check on the math :) but I think the +1 from the
original code was actually unnecessary and so for this change we can use it
for the pad BD.

Shockingly: it seems sashiko didn't find any issues (?). If you agree with the
above I can respin to add the comment for the generic XDP path and re-send
later today.

Thanks,
Joe