netdev - Re: [PATCH 0/1] HW checksum offload problem on Xilinx Zynq GEM

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Date:   Fri, 19 Aug 2016 13:00:33 -0700
From:   Tom Herbert <tom@...bertland.com>
To:     David Miller <davem@...emloft.net>
Cc:     helmut.buchsbaum@...il.com, nicolas.ferre@...el.com,
        michal.simek@...inx.com,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        cyrille.pitchen@...el.com
Subject: Re: [PATCH 0/1] HW checksum offload problem on Xilinx Zynq GEM

On Fri, Aug 19, 2016 at 12:28 PM, David Miller <davem@...emloft.net> wrote:
> From: Helmut Buchsbaum <helmut.buchsbaum@...il.com>
> Date: Fri, 19 Aug 2016 15:04:57 +0200
>
>> When working on upgrading the v3.x kernels of our embedded devices
>> to more recent 4.x kernels we noticed some of our proprietary networking
>> stuff is broken. Further investigations brought up an issue with small
>> UDP packets (data payload <= 2), which contained wrong UDP header
>> checksums.
>> We tracked this down to commit 85ff3d87bf2ef1fadcde8553628c60f79806fdb4
>> net/macb: add TX checksum offload feature. It turns out that Zynq's GEM
>> is obviously buggy regarding the UDP checksum calculation of such small
>> UDP packets as long as the UDP checksum field is != 0 *BEFORE* the
>> HW calulation. But since udp_send_skb() *ALWAYS* calculates the UDP header
>> checksum (unless disabled via socket option), this is the usual case.
>> Unfortunately it does not respect the net device feature setting which
>> would leave UDP checksum untouched when checksum offloading is enabled.
>
> Then simply clear the checksum field in the driver, or fix
> udp_send_skb() to do what you claim it is supposed to.
>
> That seems like a much more appropriate fix to me.

Fix the driver to do that. When offloading a UDP checksum the pseudo
header checksum is written into the checksum field-- that's the
interface that everyone else has been using without issue.