RE: [net 2/2] net: fix a bug of dropping FCoE frames when disabling tx ip checksum

On Wed, 2012-03-14 at 00:01 -0700, Jeff Kirsher wrote:

quoted

From: Yi Zou <redacted>

Fix a bug when using 'ethtool -K ethx tx off' to turn off tx ip

checksum,

quoted

FCoE CRC offload should not be impacte. The skb_checksum_help() is

needed

quoted

only if it's not FCoE traffic for ip checksum, regardless of ethtool

toggling

quoted

the tx ip checksum on or off.

[...]

I think the bug is more fundamental, and it's not just a problem for
FCoE.  For the transmit path, the ip_summed values are specified as:

[forwarding]
 *	COMPLETE: the most generic way. Device supplied checksum of _all_
 *	    the packet as seen by netif_rx in skb->csum.
 *	    NOTE: Even if device supports only some protocols, but
 *	    is able to produce some skb->csum, it MUST use COMPLETE,
 *	    not UNNECESSARY.

[locally-generated]
 *	NONE: skb is checksummed by protocol or csum is not required.
 *
 *	PARTIAL: device is required to csum packet as seen by
hard_start_xmit
 *	from skb->csum_start to the end and to record the checksum
 *	at skb->csum_start + skb->csum_offset.

It's implicit that the checksum algorithm for CHECKSUM_PARTIAL is as
specified for TCP/IP.  So none of those is correct when a different
algorithm is to be used.

It seems like we may need another ip_summed value for FCoE, SCTP or any
other protocol with a different checksum algorithm that will be
offloaded.  Maybe allow CHECKSUM_UNNECESSARY to be used on output in
that case?

Ben.

CHECKSUM_UNNECESSARY sounds good to me, if it's ok to be used on the tx path
as well, I think so but I am not 100% sure, it'd resolve this for fcoe and sctp
like, downside is it's a bigger change that requires corresponding changes in
these protocol driver stacks as well.

yi

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Ben Hutchings, Staff Engineer, Solarflare
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