netdev - Re: [PATCH net-next v3 1/1] net: fec: Enable imx6 enet checksum acceleration.

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Date:	Thu, 18 Apr 2013 18:08:27 +0100
From:	Jim Baxter <jim_baxter@...tor.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	Frank Li <Frank.Li@...escale.com>,
	Fugang Duan <B38611@...escale.com>, <netdev@...r.kernel.org>,
	Fabio Estevam <festevam@...il.com>,
	Francois Romieu <romieu@...zoreil.com>
Subject: Re: [PATCH net-next v3 1/1] net: fec: Enable imx6 enet checksum acceleration.

On 18/04/13 17:16, Ben Hutchings wrote:
> On Thu, 2013-04-18 at 13:49 +0100, Jim Baxter wrote:
>> On 17/04/13 22:37, Eric Dumazet wrote:
>>> On Wed, 2013-04-17 at 21:07 +0100, Jim Baxter wrote:
>>>
>>>> +			skb_set_transport_header(skb,
>>>> +					ETH_HLEN + ip_hdrlen(skb));
>>>> +			udp_hdr(skb)->check = 0;
>>>> +			break;
>>>> +		case IPPROTO_TCP:
>>>> +			hdr_len = (ETH_HLEN +
>>>> +					(ip_hdr(skb)->ihl << 2) +
>>>> +					sizeof(struct tcphdr));
>>>> +			if (skb->len < hdr_len)
>>>> +				return;
>>>> +			skb_cow_head(skb, hdr_len);
>>>
>>> same here
>> Do I need to call skb_cow_head here, I am not changing the size of the
>> header?
> [...]
> 
> The length passed to skb_cow_head() may be significant when the skb has
> paged fragments.  Since you aren't (yet) implementing scatter-gather,
> that won't happen.  And the headers shouldn't be in paged fragments
> anyway.  I think you can safely use skb_cow_head(skb, 0).
> 
> But you don't actually need to check protocol numbers at all, as the
> kernel already specifies where the checksum should be.
> 
> So I think this function should look like:
> {
> 	if (skb->ip_summed != CHECKSUM_PARTIAL)
> 		return 0;
> 
> 	if (unlikely(skb_cow_head(skb, 0)))
> 		return -1;
> 
> 	*(__sum16 *)(skb->head + skb->csum_start + skb->csum_offset) = 0;
> 	return 0;
> }
> 
> The caller needs to check for the failure, and free the skb
> (kfree_skb()) rather than transmitting it.
> 
> Ben.
> 

Which checksum does skb->csum (skb->csum_start + skb->csum_offset)
relate to?
The network card can generate IP header and protocol (UDP/TCP/ICMP)
checksums as long as the checksums are zeroed?

Jim
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