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

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Date:	Thu, 18 Apr 2013 22:27:22 +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 18:21, Jim Baxter wrote:
> On 18/04/13 18:12, Ben Hutchings wrote:
>> On Thu, 2013-04-18 at 18:08 +0100, Jim Baxter wrote:
>>> 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?
>>
>> TCP or UDP.
>>
>>> The network card can generate IP header and protocol (UDP/TCP/ICMP)
>>> checksums as long as the checksums are zeroed?
>>
>> You don't need to offload the IPv4 header checksum.  TX checksum offload
>> will not be requested for ICMP.
> 
> So the kernel only offloads TCP and UDP checksum's. All other transport
> protocols (ICMP, ICMPIPV6 etc.) will be done by the kernel.
> 
>>
>> Ben.
>>
> 
> Thanks, though I saw you replied to my question before I could send it.
> 
> It sounds like there is no reason wasting CPU cycles clearing the IP
> header checksum, so I can keep the code efficient.
> 
> Jim
> 

Does anybody know if the kernel uses checksum offloading for IPV6, if I
enable the feature NETIF_F_HW_CSUM, I only receive CHECKSUM_NONE when
the protocol is 0x86DD?

I realise there is no IP header checksum, but I thought the protocols
still use a checksum.

I am using iperf with the -V option in UDP mode, which offloads for IPV4.


Thank you,
Jim
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