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Date:	Tue, 29 May 2007 14:58:08 -0700
From:	Stephen Hemminger <>
To:	Herbert Xu <>
Cc:, Michael Chan <>
Subject: Re: Definition and usage of NETIF_F_HW_SUM?

On Wed, 30 May 2007 07:36:18 +1000
Herbert Xu <> wrote:

> On Tue, May 29, 2007 at 01:58:13PM -0700, Stephen Hemminger wrote:
> > The flag NETIF_F_HW_SUM is being misused. The definition says that the device
> > is capable of checksumming any packet. When in fact from usage it seems to
> > imply that the device is capable of doing IPV6 as well as IPV4.
> That would be a problem.
> > Some devices like 8139too do "fake checksum offloading" because they always have to copy
> > the packet.
> > 
> > Some devices like via-rhine, don't really checksum but if they see CHECKSUM_PARTIAL then they
> > copy. This is bogus, they should just let higher layer do checksum/copy.
> Actually this is OK because if they have to copy it then it's cheaper to
> checksum it there.  Both of these should be able to support all protocols.
> > Devices like e1000, and bnx2 are broken because they assume only TCP/UDP and IPV4/IPV6. 
> > The definition of the flag says other protocols should work, but they probably send the
> > hardware into a state of confusion.
> I just checked e1000 and it's correct as it does use the csum_offset
> when doing TX offload.  However, you're definitely right that bnx2
> seems to be broken.
> > A few devices take a offset, starting point, and insertion point. This looks like
> > the correct model. But no upper layer protocols other than IPV4/IPV6 can do checksum
> > offload at present, so it seems moot.
> I could easily whip up a patch to get GRE to use it for a start :)
> > IMHO the correct solution would be to get rid if NETIF_F_HW_SUM and make a new flag
> > NETIF_F_IPV6_SUM. Devices that can checksum both could do NETIF_F_IPV4_SUM|NETI_F_IPV6_SUM.
> We should definitely keep NETIF_F_HW_SUM for sane hardware such as the
> e1000.  Unfortunately we may just have to invent IPV6_SUM for the broken
> ones.

The Marvell 88e8071 does IPV4 and IPV6 checksum, earlier chips could do arbitrary
checksum. Looks like when they added the TSO6 logic, they made transmit state machine
more protocol dependent.
Stephen Hemminger <>
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