Subject: Re: [PATCH v2 01/15] net: dsa: Don't set skb->protocol on outgoing tagged packets

From: Florian Fainelli <[email-masked]>
Date: Tue, 28 Oct 2014 12:19:21 -0700

On 10/26/2014 09:52 AM, Guenter Roeck wrote:
> Setting skb->protocol to a private protocol type may result in warning
> messages such as
> 	e1000e 0000:00:19.0 em1: checksum_partial proto=dada!
> 
> This happens if the L3 protocol is IP or IPv6 and skb->ip_summed is set
> to CHECKSUM_PARTIAL. Looking through the code, it appears that changing
> skb->protocol for transmitted packets is not necessary and may actually
> be harmful. Drop it.

You could emphasize that overriding skb->protocol prevents purposely
unmodified (from a DSA perspective) network drivers from properly
setting up their transmit checksum offload pointers since they inspect
skb->protocol to setup the IPv4 header or IPv6 header pointers.

> 
> Signed-off-by: Guenter Roeck <[email-masked]>
> ---
> v2:
> - No change
> 
>  net/dsa/tag_dsa.c     | 2 --
>  net/dsa/tag_edsa.c    | 2 --
>  net/dsa/tag_trailer.c | 2 --
>  3 files changed, 6 deletions(-)
> 
> diff --git a/net/dsa/tag_dsa.c b/net/dsa/tag_dsa.c
> index ce90c8b..2dab270 100644
> --- a/net/dsa/tag_dsa.c
> +++ b/net/dsa/tag_dsa.c
> @@ -63,8 +63,6 @@ static netdev_tx_t dsa_xmit(struct sk_buff *skb, struct net_device *dev)
>  		dsa_header[3] = 0x00;
>  	}
>  
> -	skb->protocol = htons(ETH_P_DSA);
> -
>  	skb->dev = p->parent->dst->master_netdev;
>  	dev_queue_xmit(skb);
>  
> diff --git a/net/dsa/tag_edsa.c b/net/dsa/tag_edsa.c
> index 94fcce7..9aeda59 100644
> --- a/net/dsa/tag_edsa.c
> +++ b/net/dsa/tag_edsa.c
> @@ -76,8 +76,6 @@ static netdev_tx_t edsa_xmit(struct sk_buff *skb, struct net_device *dev)
>  		edsa_header[7] = 0x00;
>  	}
>  
> -	skb->protocol = htons(ETH_P_EDSA);
> -
>  	skb->dev = p->parent->dst->master_netdev;
>  	dev_queue_xmit(skb);
>  
> diff --git a/net/dsa/tag_trailer.c b/net/dsa/tag_trailer.c
> index 115fdca..e268f9d 100644
> --- a/net/dsa/tag_trailer.c
> +++ b/net/dsa/tag_trailer.c
> @@ -57,8 +57,6 @@ static netdev_tx_t trailer_xmit(struct sk_buff *skb, struct net_device *dev)
>  	trailer[2] = 0x10;
>  	trailer[3] = 0x00;
>  
> -	nskb->protocol = htons(ETH_P_TRAILER);
> -
>  	nskb->dev = p->parent->dst->master_netdev;
>  	dev_queue_xmit(nskb);
>  
>