RE: [PATCH V3 net-next] net: fec: add XDP_TX feature support

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The FEC of i.MX8MP-EVK has dma_coherent=false, and as I mentioned
above, I did not see an obvious difference in the performance. :(

That is surprising - given the results.

(see below, lack of perf/diff might be caused by Ethernet flow-control).

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The result of the current modification.
root@imx8mpevk:~# ./xdp2 eth0
proto 17:     260180 pkt/s

These results are*significantly*  better than reported in patch-1.
What happened?!?

The test environment is slightly different, in patch-1, the FEC port
was directly connected to the port of another board. But in the latest
test, the ports of the two boards were connected to a switch, so the
ports of the two boards are not directly connected.

Hmm, I've seen this kind of perf behavior of direct-connected or via switch
before. The mistake I made was, that I had not disabled Ethernet flow-control.
The xdp2 XDP_TX program will swap the mac addresses, and send the packet
back to the packet generator (running pktgen), which will get overloaded
itself and starts sending Ethernet flow-control pause frames.

Command line to disable:
  # ethtool -A eth0 rx off tx off

Can I ask/get you to make sure that Ethernet flow-control is disabled (on
both generator and DUT (to be on safe-side)) and run the test again?

The flow-control was not disabled before, so according to your suggestion,
I disable the flow-control on the both boards and run the test again, the
performance is slightly improved, but still can not see a clear difference
between the two methods. Below are the results.

Result: use "sync_dma_len" method
root@imx8mpevk:~# ./xdp2 eth0
proto 17:     258886 pkt/s
proto 17:     258879 pkt/s
proto 17:     258872 pkt/s
proto 17:     258312 pkt/s
proto 17:     258926 pkt/s
proto 17:     259057 pkt/s
proto 17:     258437 pkt/s
proto 17:     259242 pkt/s
proto 17:     258665 pkt/s
proto 17:     258779 pkt/s
proto 17:     259209 pkt/s

Result: use the current method
root@imx8mpevk:~# ./xdp2 eth0
proto 17:     258752 pkt/s
proto 17:     258672 pkt/s
proto 17:     258317 pkt/s
proto 17:     258787 pkt/s
proto 17:     258757 pkt/s
proto 17:     258542 pkt/s
proto 17:     258829 pkt/s
proto 17:     258480 pkt/s
proto 17:     258859 pkt/s
proto 17:     258918 pkt/s
proto 17:     258782 pkt/s
proto 17:     259086 pkt/s
proto 17:     258337 pkt/s