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

[Heiner Kallweit <hkallweit1@gmail.com>]

On 08.04.2021 19:23, Sven Van Asbroeck wrote:
> From: Sven Van Asbroeck <thesven73@gmail.com>
> 
> 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:
> 
> 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@gmail.com>
> ---
> 
> To: Bryan Whitehead <bryan.whitehead@microchip.com>
> To: "David S. Miller" <davem@davemloft.net>
> To: Jakub Kicinski <kuba@kernel.org>
> To: George McCollister <george.mccollister@gmail.com>
> Cc: UNGLinuxDriver@microchip.com
> Cc: netdev@vger.kernel.org
> Cc: linux-kernel@vger.kernel.org
> 
>  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;
> 

Can't we use frame_length - ETH_FCS_LEN direcctly here?