netdev - Re: [PATCH v2] net: bcmgenet: Fix FCS generation for fragmented skbuffs

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Date: Thu, 28 Dec 2023 09:10:25 +0100
From: Adrian Cinal <adriancinal1@...il.com>
To: Doug Berger <opendmb@...il.com>
Cc: netdev@...r.kernel.org, florian.fainelli@...adcom.com, 
	bcm-kernel-feedback-list@...adcom.com
Subject: Re: [PATCH v2] net: bcmgenet: Fix FCS generation for fragmented skbuffs

On Wed, 27 Dec 2023 at 21:39, Doug Berger <opendmb@...il.com> wrote:
>
> On 12/27/2023 4:04 AM, Adrian Cinal wrote:
> > The flag DMA_TX_APPEND_CRC was written to the first (instead of the last)
> > DMA descriptor in the TX path, with each descriptor corresponding to a
> > single skbuff fragment (or the skbuff head). This led to packets with no
> > FCS appearing on the wire if the kernel allocated the packet in fragments,
> > which would always happen when using PACKET_MMAP/TPACKET
> > (cf. tpacket_fill_skb() in af_packet.c).
> >
> > Fixes: 1c1008c793fa ("net: bcmgenet: add main driver file")
> > Signed-off-by: Adrian Cinal <adriancinal1@...il.com>
> > ---
> >   drivers/net/ethernet/broadcom/genet/bcmgenet.c | 10 +++++-----
> >   1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/net/ethernet/broadcom/genet/bcmgenet.c b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
> > index 1174684a7f23..df4b0e557c76 100644
> > --- a/drivers/net/ethernet/broadcom/genet/bcmgenet.c
> > +++ b/drivers/net/ethernet/broadcom/genet/bcmgenet.c
> > @@ -2137,16 +2137,16 @@ static netdev_tx_t bcmgenet_xmit(struct sk_buff *skb, struct net_device *dev)
> >               len_stat = (size << DMA_BUFLENGTH_SHIFT) |
> >                          (priv->hw_params->qtag_mask << DMA_TX_QTAG_SHIFT);
> >
> > -             /* Note: if we ever change from DMA_TX_APPEND_CRC below we
> > -              * will need to restore software padding of "runt" packets
> > -              */
> >               if (!i) {
> > -                     len_stat |= DMA_TX_APPEND_CRC | DMA_SOP;
> > +                     len_stat |= DMA_SOP;
> >                       if (skb->ip_summed == CHECKSUM_PARTIAL)
> >                               len_stat |= DMA_TX_DO_CSUM;
> >               }
> > +             /* Note: if we ever change from DMA_TX_APPEND_CRC below we
> > +              * will need to restore software padding of "runt" packets
> > +              */
> >               if (i == nr_frags)
> > -                     len_stat |= DMA_EOP;
> > +                     len_stat |= DMA_TX_APPEND_CRC | DMA_EOP;
> >
> >               dmadesc_set(priv, tx_cb_ptr->bd_addr, mapping, len_stat);
> >       }
> Hmm... this is a little surprising since the documentation is actually
> pretty specific that the hardware signal derived from this flag be set
> along with the SOP signal.
>
> Based on that I think I would prefer the flag to be set for all
> descriptors of a packet rather than just the last, but let me look into
> this a little further.
>
> Thanks for bringing this to my attention,
>      Doug

Hello,

I confirm that it works just as well when the flag is set for all
descriptors rather than just the last. Tested on a BCM2711.

Adrian