netdev - Broken TX checksumming offloads

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Date:	Mon, 29 Nov 2010 19:17:42 +0100
From:	Michał Mirosław <mirq-linux@...e.qmqm.pl>
To:	netdev@...r.kernel.org
Cc:	Toshiharu Okada <toshiharu-linux@....okisemi.com>,
	e1000-devel@...ts.sourceforge.net
Subject: Broken TX checksumming offloads

Hi!

Unless I'm horribly mistaken, generic HW checksumming works as follows:
 - driver sets netdev->features & NETIF_F_HW_CSUM to indicate support
   for generic checksumming; if the flag is not set, networking core
   will checksum skb before calling ndo_start_xmit (let's ignore
   other checksumming options for now) and not pass skb with
   skb->ip_summed == CHECKSUM_PARTIAL
 - ndo_start_xmit() should use skb->csum_start and skb->csum_offset
   (or skb->csum) to update checksum in software or instruct HW to do so

Looking at pch_gbe_xmit_frame() and its callee - pch_gbe_tx_queue() it
looks like the driver should set NETIF_F_IP_CSUM instead of NETIF_F_HW_CSUM
feature.

Similar thing happens in ixgbe driver: it sets NETIF_F_HW_CSUM and checks
for skb->ip_summed == CHECKSUM_PARTIAL, but then just warns on protocols
other that TCP and SCTP (see: ixgbe_psum()).

This means that these drivers might send packets with broken checksums
when TX checksumming offload is enabled. I haven't checked other drivers, yet.

Best Regards,
Michał Mirosław
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