netdev - [RFC] skge csum problems

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Message-ID: <20071224094352.GU8181@ftp.linux.org.uk>
Date:	Mon, 24 Dec 2007 09:43:52 +0000
From:	Al Viro <viro@....linux.org.uk>
To:	netdev@...r.kernel.org
Subject: [RFC] skge csum problems

	Both variants of skge (drivers/net and drivers/net/sk98lin/ resp.)
have the same problem with rx checksums.  They pick checksum from rx
descriptor and use it as-is.  Normally that would be the right thing to
do.  However, skge is told to byteswap descriptors on big-endian boxen.

Checksum is fixed-endian and we want it that way; IOW, what we end up
storing in skb->csum should be fixed-endian as well.  Unless the card
is smart enough to byteswap everything in rx descriptor _except_ the
checksum, we have a trouble - we get a value converted to host-endian
by the general byteswap in descriptor and we must convert it to fixed-endian
ourselves.

FWIW, FreeBSD sk_if sidesteps that mess by not telling the card to 
byteswap, so that's not too informative.  Datasheet on
http://people.freebsd.org/~wpaul/SysKonnect/xmacii_datasheet_rev_c_9-29.pdf
is not clear on what's going on with checksum in byteswapping mode either...

Could somebody with that sucker on a card (all instances I have here are
on-board ones in little-endian boxen) test what's really going on for
big-endian hosts with either driver?
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