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

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Date:   Tue, 23 Aug 2016 07:39:00 +0200
From:   Helmut Buchsbaum <helmut.buchsbaum@...il.com>
To:     Tom Herbert <tom@...bertland.com>,
        David Miller <davem@...emloft.net>
Cc:     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 08/19/2016 10:00 PM, Tom Herbert wrote:
> 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.
>
Thanks for pointing me in the right direction. I'm just verifying a 
patch update I'll provide soon!