Hi,

I've just run iperf3 in both directions with the kernel I had on the
board (based on 6.18.0-rc7-net-next+), and stmmac really isn't looking
particularly great - by that I mean, iperf3 *failed* spectacularly.

First, running in normal mode (stmmac transmitting, x86 receiving)
it's only capable of 210Mbps, which is nowhere near line rate.

However, when running iperf3 in reverse mode, it filled the stmmac's
receive queue, which then started spewing PAUSE frames at a rate of
knots, flooding the network, and causing the entire network to stop.
It never recovered without rebooting.

Trying again on 6.19.0-rc4-net-next+,

stmmac transmitting shows the same dire performance:

[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.00   sec  24.2 MBytes   203 Mbits/sec    0    230 KBytes
[  5]   1.00-2.00   sec  25.5 MBytes   214 Mbits/sec    0    230 KBytes
[  5]   2.00-3.00   sec  25.0 MBytes   210 Mbits/sec    0    230 KBytes
[  5]   3.00-4.00   sec  25.5 MBytes   214 Mbits/sec    0    230 KBytes
[  5]   4.00-5.00   sec  25.1 MBytes   211 Mbits/sec    0    230 KBytes
[  5]   5.00-6.00   sec  25.1 MBytes   211 Mbits/sec    0    230 KBytes
[  5]   6.00-7.00   sec  25.7 MBytes   215 Mbits/sec    0    230 KBytes
[  5]   7.00-8.00   sec  25.2 MBytes   212 Mbits/sec    0    230 KBytes
[  5]   8.00-9.00   sec  25.3 MBytes   212 Mbits/sec    0    346 KBytes
[  5]   9.00-10.00  sec  25.4 MBytes   213 Mbits/sec    0    346 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate         Retr
[  5]   0.00-10.00  sec   252 MBytes   211 Mbits/sec    0             sender
[  5]   0.00-10.02  sec   250 MBytes   210 Mbits/sec                  receiver

stmmac receiving shows the same problem:

[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec  64.1 MBytes   537 Mbits/sec
[  5]   1.00-2.00   sec  0.00 Bytes  0.00 bits/sec
[  5]   2.00-3.00   sec  0.00 Bytes  0.00 bits/sec
[  5]   3.00-4.00   sec  0.00 Bytes  0.00 bits/sec
[  5]   4.00-5.00   sec  0.00 Bytes  0.00 bits/sec
[  5]   5.00-6.00   sec  0.00 Bytes  0.00 bits/sec
[  5]   6.00-7.00   sec  0.00 Bytes  0.00 bits/sec
[  5]   7.00-8.00   sec  0.00 Bytes  0.00 bits/sec
[  5]   8.00-9.00   sec  0.00 Bytes  0.00 bits/sec
^C[  5]   9.00-9.43   sec  0.00 Bytes  0.00 bits/sec
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-9.43   sec  0.00 Bytes  0.00 bits/sec                  sender
[  5]   0.00-9.43   sec  64.1 MBytes  57.0 Mbits/sec                  receiver
iperf3: interrupt - the client has terminated

Heh, I was able to reproduce something similar on imx8mp, that has an
imx-dwmac (dwmac 4/5 according to dmesg) :

DUT to x86

Connecting to host 192.168.2.1, port 5201
[  5] local 192.168.2.13 port 54744 connected to 192.168.2.1 port 5201
[ ID] Interval           Transfer     Bitrate         Retr  Cwnd
[  5]   0.00-1.00   sec  0.00 Bytes  0.00 bits/sec    2   1.41 KBytes
[  5]   1.00-2.00   sec  0.00 Bytes  0.00 bits/sec    1   1.41 KBytes

x86 to DUT :

Reverse mode, remote host 192.168.2.1 is sending
[  5] local 192.168.2.13 port 47050 connected to 192.168.2.1 port 5201
[ ID] Interval           Transfer     Bitrate
[  5]   0.00-1.00   sec   112 MBytes   935 Mbits/sec
[  5]   1.00-2.00   sec   112 MBytes   936 Mbits/sec
[  5]   2.00-3.00   sec   112 MBytes   936 Mbits/sec

Nothing as bas as what you face, but there's defintely something going
on there. "good" news is that it worked in v6.19-rc1, I have a bisect
ongoing.

I'll update once I have homed-in on something.

Maxime