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Message-ID: <51702B9A.8070000@mentor.com>
Date: Thu, 18 Apr 2013 18:21:30 +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 18:12, 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
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