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

From: Jim Baxter <[email-masked]>
Date: Thu, 18 Apr 2013 18:21:30 +0100

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