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

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Date:   Fri, 19 Aug 2016 12:28:39 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     helmut.buchsbaum@...il.com
Cc:     nicolas.ferre@...el.com, michal.simek@...inx.com,
        netdev@...r.kernel.org, cyrille.pitchen@...el.com
Subject: Re: [PATCH 0/1] HW checksum offload problem on Xilinx Zynq GEM

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.