netdev - Re: [PATCH] net: fec: remove HW IP header checksum for IPV6 frame

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Date:	Tue, 17 Jun 2014 11:20:25 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Fugang Duan <b38611@...escale.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH] net: fec: remove HW IP header checksum for IPV6 frame

On Tue, Jun 17, 2014 at 04:31:35PM +0800, Fugang Duan wrote:
> The commit 96c50caa5148 (net: fec: Enable IP header hardware checksum)
> enable HW IP header checksum for IPV4 and IPV6, which causes IPV6 TCP/UDP
> cannot work. (The issue is reported by Russell King)
> 
> The reason is that FEC HW IP cannot detect the transmit frame type (can
> detect received frame type), and treats all packets as IPv4, overwrites
> the point in the packet which would be the IPv4 checksum field. Since IPV6
> don't have IP header checksum, so this results in the frame being corrupted.
> 
> The patch just add software detect the current packet type, if it is IPV6
> frame, it don't enable IP header checksum.

I think it /can/ detect the packet type - the hardware showed no signs of
overwriting the checksum field with a non-zero value.  The zeroing of the
bytes seems to come purely from the driver assuming that every partial
checksummed packet is an IPv4 packet:

> @@ -330,7 +335,8 @@ fec_enet_clear_csum(struct sk_buff *skb, struct net_device *ndev)
>  	if (unlikely(skb_cow_head(skb, 0)))
>  		return -1;
>  
> -	ip_hdr(skb)->check = 0;
> +	if (is_ipv4_pkt(skb))
> +		ip_hdr(skb)->check = 0;
>  	*(__sum16 *)(skb->head + skb->csum_start + skb->csum_offset) = 0;
>  
>  	return 0;

So, I will first test whether avoiding this resolves the issue I'm
seeing before deciding whether to test the remainder of the patch.

-- 
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