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Message-ID: <1342508968.2626.148.camel@edumazet-glaptop>
Date: Tue, 17 Jul 2012 09:09:28 +0200
From: Eric Dumazet <eric.dumazet@...il.com>
To: Andy Cress <andy.cress@...kontron.com>
Cc: netdev@...r.kernel.org, Zhong Hongbo <hongbo.zhong@...driver.com>
Subject: Re: [PATCH 1/4] pch_gbe: Fix the checksum fill to the error
location
On Mon, 2012-07-16 at 13:03 -0700, Andy Cress wrote:
> Author: Zhong Hongbo <hongbo.zhong@...driver.com>
>
> Due to some unknown hardware limitations the pch_gbe hardware cannot
> calculate checksums when the length of network package is less
> than 64 bytes, where we will surprisingly encounter a problem of
> the destination IP incorrectly changed.
>
> When forwarding network packages at the network layer the IP packages
> won't be relayed to the upper transport layer and analyzed there,
> consequently, skb->transport_header pointer will be mistakenly remained
> the same as that of skb->network_header, resulting in TCP checksum
> wrongly
> filled into the field of destination IP in IP header.
>
> We can fix this issue by manually calculate the offset of the TCP
> checksum
> and update it accordingly.
>
> We would normally use the skb_checksum_start_offset(skb) here, but in
> this
> case it is sometimes -2 (csum_start=0 - skb_headroom=2 => -2), hence the
> manual calculation.
>
> Signed-off-by: Zhong Hongbo <hongbo.zhong@...driver.com>
> Merged-by: Andy Cress <andy.cress@...kontron.com>
Hmm... I fail to understand why you care about NIC doing checksums,
while pch_gbe_tx_queue() make a _copy_ of each outgoing
packets.
There _must_ be a way to avoid most of these copies (ie not touching
payload), only mess with the header to insert these 2 nul bytes ?
/* [Header:14][payload] ---> [Header:14][paddong:2][payload] */
So at device setup : dev->needed_headroom = 2;
and in xmit,
if (skb_headroom(skb) < 2) {
struct sk_buff *skb_new;
skb_new = skb_realloc_headroom(skb, 2);
if (!skb_new) { handle error }
consume_skb(skb);
skb = skb_new;
}
ptr = skb_push(skb, 2);
memmove(ptr, ptr + 2, ETH_HLEN);
ptr[ETH_HLEN] = 0;
ptr[ETH_HLEN + 1] = 0;
--
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