RE: [v4,net-next,06/15] net: fec: add fec_enet_rx_queue_xdp() for XDP path

Wei Fang
2026-01-26T02:09:15Z

> > +static int fec_enet_rx_queue_xdp(struct fec_enet_private *fep, int queue,
> > + int budget, struct bpf_prog *prog)
> > +{
>
> [ ... ]
>
> > + switch (act) {
> > + case XDP_PASS:
> > + rxq->stats[RX_XDP_PASS]++;
> > + /* The packet length includes FCS, but we don't want to
> > + * include that when passing upstream as it messes up
> > + * bridging applications.
> > + */
> > + skb = fec_build_skb(fep, rxq, bdp, page, pkt_len);
> > + if (!skb) {
> > + fec_xdp_drop(rxq, &xdp, sync);
> > + trace_xdp_exception(ndev, prog, XDP_PASS);
> > + } else {
> > + napi_gro_receive(&fep->napi, skb);
> > + }
> > + break;
>
> On platforms with FEC_QUIRK_SWAP_FRAME (like i.MX28), the old code path
> would call swap_buffer() after XDP_PASS before building the skb. In the
> old fec_enet_rx_queue(), the flow was:
>
> fec_enet_run_xdp() returns FEC_ENET_XDP_PASS
> -> if (ret != FEC_ENET_XDP_PASS) goto done; // continues
> -> if (need_swap) swap_buffer(data, pkt_len);
> -> fec_build_skb()
>
> With this change, XDP_PASS goes directly to fec_build_skb() without
> calling swap_buffer(). Does this break XDP_PASS on i.MX28 where the DMA
> delivers data in wrong byte order? On such platforms, packets passed to
> the network stack would have incorrect byte order, causing protocol
> parsing failures.
>
> The commit message says "XDP path does not support swap_buffer()" but the
> previous code did support it for XDP_PASS - the swap happened after the
> XDP program returned PASS but before the packet was handed to the stack.

Actually the previous code did not support swap_buffer() for XDP path,
see fec_enet_bpf():

static int fec_enet_bpf(struct net_device *dev, struct netdev_bpf *bpf)
{
[...]
switch (bpf->command) {
case XDP_SETUP_PROG:
/* No need to support the SoCs that require to
* do the frame swap because the performance wouldn't be
* better than the skb mode.
*/
if (fep->quirks & FEC_QUIRK_SWAP_FRAME)
return -EOPNOTSUPP;
[...]
}
}

Therefore, xdp program will not be installed for those platforms which
need swap_buffer(). Because the XDP path and the traditional protocol
stack path were previously mixed in fec_enet_rx_queue(), the logic was
confused, which led to the AI ​​making a misjudgment.

> --
> pw-bot: cr