Gianfar TCP checksumming broken in 2.6.35+

[David Miller]

From: "Matthew L. Creech" <mlcreech at gmail.com>
Date: Tue, 2 Nov 2010 18:29:08 -0400

> An upgrade from 2.6.34 to 2.6.35 caused networking to stop working on
> my MPC8313-based board.  It turned out that TCP checksums were
> invalid, so I dug through the .35 changelog to try and isolate the
> reason.  The change "tcp: Set CHECKSUM_UNNECESSARY in
> tcp_init_nondata_skb" seems to be the specific one that causes
> breakage - if I revert this one-liner, things work again:
> 
> http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=2e8e18ef52e7dd1af0a3bd1f7d990a1d0b249586
> 
> However, I also noticed that one of my boards was broken while a newer
> prototype (which is very similar, hardware-wise) was not.  It turns
> out they're using 2 different revisions of silicon, so the broken
> board still has a 1.0 version microcontroller.
> 
> Therefore I'm guessing (just a hunch) that the root cause of the
> problem is MPC8313 errata eTSEC12:
> 
> ========
> eTSEC12: Tx IP and TCP/UDP Checksum Generation not supported for some Tx FCB
>          offsets
> Description:
> 	If the Tx FCB (Frame Control Block) 32-byte offset is 0x19, 0x1A, 0x1B,
> 	0x1C, 0x1D, 0x1E or 0x1F, IP and TCP/UDP header checksum generation do
> 	not function properly. The checksum value may be inserted in the wrong
> 	location or not inserted at all.
> 	IP and TCP/UDP header checksum generation is not supported in LINUX
> 	and other systems in which headers are prepended to pre-aligned packet
> 	data, or where the alignment of the Tx FCB cannot be controlled.
> 	This behavior applies to pseudo-header checksum insertion as well as
> 	checksum generation.
> Workaround:
> 	Align Tx FCB to a 16 or 32-byte boundary.
> 	If the alignment of TxFCB is not controllable, set TCTRL[TUCSEN]=0 and
> 	TCTRL[IPCSEN]=0 to disable IP and TCP/UDP header checksum generation.
> Fix plan:
> 	Fixed in Rev 2.0
> ========
> 
> This appears to have been working previously, but doesn't work any
> more.  I'm not familiar enough with Dave's checksum/sk_buff changes to
> figure out whether this errata is to blame, though, or how I should
> fix it if it is.  Presumably there's some alignment magic needed in
> the sk_buff or gfar_add_fcb() to make sure that the microcontroller is
> happy with the FCB offset?
> 
> Any tip on how I can solve this, or at least verify that this errata
> is at fault?  Thanks in advance

Can someone please follow up Matthew to get this bug resolved?  It has
been sitting around for a long time.

I suspect the gianfar driver, for these chip revisions, will need to
do a software checksum when the offset matches the criteria mentioned
in the errata above.