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

From: Helmut Buchsbaum <redacted>
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.