## with collision detection enabled

iperf3 normal
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec  5.54 MBytes  4.65 Mbits/sec    0             sender
[  5]   0.00-10.01  sec  5.40 MBytes  4.53 Mbits/sec                  receiver

iperf3 reverse
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec   929 KBytes   761 Kbits/sec  293             sender
[  5]   0.00-10.00  sec   830 KBytes   680 Kbits/sec                  receiver


## with collision detection disabled

iperf3 normal
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec  6.39 MBytes  5.36 Mbits/sec    0             sender
[  5]   0.00-10.04  sec  6.19 MBytes  5.17 Mbits/sec                  receiver

iperf3 reverse
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.27  sec  1.10 MBytes   897 Kbits/sec  268             sender
[  5]   0.00-10.00  sec  1.01 MBytes   843 Kbits/sec                  receiver

# Conclusions

The arm system running the lan865x macphy uses a not yet mainlined driver, see
https://lore.kernel.org/all/20231023154649.45931-1-Parthiban.Veerasooran@microchip.com/ (local)

The lan865x driver crashed out every once in a while on reverse mode, there
is definetly something biased in the driver for tx over rx.

Then again it's not accepted yet.

Disabling collision detection seemes to have an positive effect.
Slightly higher speeds and slightly fewer retransmissions.

I would want to first understand why there is such a big difference
with the direction. Is it TCP backing off because of the packet loss?
Or is there some other problem.

Maybe try with UDP streams, say with a bandwidth of 5Mbps. Do you
loose 4Mbps in one direction? Or a much smaller number of packets.

Are there any usable statistics? FCS errors?

    Andrew