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Date: Sat, 23 Feb 2002 08:34:16 +0100
From: Andi Kleen <ak@suse.de>
To: harish.vasudeva@amd.com
Cc: linux-kernel@vger.kernel.org
Subject: Re: Need some help with IP/TCP Checksum Offload
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On Fri, Feb 22, 2002 at 02:57:22PM -0800, harish.vasudeva@amd.com wrote:
> Hi All,
> 
>  I am trying to offload checksum calculation to my hardware. What i am doing in my driver (kernel 2.4.6) is :
> 
>  dev->features = NETIF_F_IP_CHECKSUM;
> 
>  Then, in my start_xmit( ) routine, i am parsing for the right headers & when i get the IP/TCP header, i print out the checksum & it is already the right checksum. When does the OS/Protocol offload this task? Am i missing something here?

For TX the checksum is only offloaded when you set and support NETIF_F_SG 
as well.  Otherwise the stack has to copy anyways and can compute the 
checksum during the copy operation.  Then with NETIF_F_SG the TX checksumming
will only be used with sendfile(), because that is the only way to do zero 
copy for now.

For RX you should set skb->ip_summed and skb->csum. The hardware checksum
is more useful in this case. 

-Andi
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