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

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Date: Wed, 27 Dec 2023 12:39:21 -0800
From: Doug Berger <opendmb@...il.com>
To: Adrian Cinal <adriancinal1@...il.com>, netdev@...r.kernel.org
Cc: florian.fainelli@...adcom.com, bcm-kernel-feedback-list@...adcom.com
Subject: Re: [PATCH v2] net: bcmgenet: Fix FCS generation for fragmented
 skbuffs

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