Re: [PATCH net v1] Revert "lan743x: trim all 4 bytes of the FCS; not just 2"

[George McCollister]

On Thu, Apr 8, 2021 at 12:23 PM Sven Van Asbroeck <thesven73@xxxxxxxxx> wrote:
>
> From: Sven Van Asbroeck <thesven73@xxxxxxxxx>
>
> This reverts commit 3e21a10fdea3c2e4e4d1b72cb9d720256461af40.
>
> The reverted patch completely breaks all network connectivity on the
> lan7430. tcpdump indicates missing bytes when receiving ping
> packets from an external host:

Can you explain the difference in behavior with what I was observing
on the LAN7431? I'll retest but if this is reverted I'm going to start
seeing 2 extra bytes on the end of frames and it's going to break DSA
with the LAN7431 again.

>
> host$ ping $lan7430_ip
> lan7430$ tcpdump -v
> IP truncated-ip - 2 bytes missing! (tos 0x0, ttl 64, id 21715,
>     offset 0, flags [DF], proto ICMP (1), length 84)
>
> Fixes: 3e21a10fdea3 ("lan743x: trim all 4 bytes of the FCS; not just 2")
> Signed-off-by: Sven Van Asbroeck <thesven73@xxxxxxxxx>
> ---
>
> To: Bryan Whitehead <bryan.whitehead@xxxxxxxxxxxxx>
> To: "David S. Miller" <davem@xxxxxxxxxxxxx>
> To: Jakub Kicinski <kuba@xxxxxxxxxx>
> To: George McCollister <george.mccollister@xxxxxxxxx>
> Cc: UNGLinuxDriver@xxxxxxxxxxxxx
> Cc: netdev@xxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
>
>  drivers/net/ethernet/microchip/lan743x_main.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/net/ethernet/microchip/lan743x_main.c b/drivers/net/ethernet/microchip/lan743x_main.c
> index 1c3e204d727c..dbdfabff3b00 100644
> --- a/drivers/net/ethernet/microchip/lan743x_main.c
> +++ b/drivers/net/ethernet/microchip/lan743x_main.c
> @@ -2040,7 +2040,7 @@ lan743x_rx_trim_skb(struct sk_buff *skb, int frame_length)
>                 dev_kfree_skb_irq(skb);
>                 return NULL;
>         }
> -       frame_length = max_t(int, 0, frame_length - RX_HEAD_PADDING - 4);
> +       frame_length = max_t(int, 0, frame_length - RX_HEAD_PADDING - 2);
>         if (skb->len > frame_length) {
>                 skb->tail -= skb->len - frame_length;
>                 skb->len = frame_length;
> --
> 2.17.1
>

Regards,
George