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Subject[PATCH v2 0/1] dwmac-meson8b: Ethernet RGMII TX delay fix
Date
The Ethernet TX performance has been historically bad on Meson8b and
Meson8m2 SoCs because high packet loss was seen. Today I (presumably)
found out why this is: the input clock (which feeds the RGMII TX clock)
has to be at least 4 times 125MHz. With the fixed "divide by 2" in the
clock tree this means that m250_div needs to be at least 2.

With this patch and a 2ns TX delay generated by either the MAC *or* the
PHY this results in improved Ethernet TX performance and no packet loss
anymore:
# iperf3 -c 192.168.1.100
Connecting to host 192.168.1.100, port 5201
[ 5] local 192.168.1.163 port 42636 connected to 192.168.1.100 port 5201
[ ID] Interval Transfer Bitrate Retr Cwnd
[ 5] 0.00-1.00 sec 105 MBytes 878 Mbits/sec 0 609 KBytes
[ 5] 1.00-2.00 sec 106 MBytes 885 Mbits/sec 0 683 KBytes
[ 5] 2.00-3.09 sec 73.7 MBytes 570 Mbits/sec 0 683 KBytes
[ 5] 3.09-4.00 sec 81.9 MBytes 754 Mbits/sec 0 795 KBytes
[ 5] 4.00-5.00 sec 104 MBytes 869 Mbits/sec 0 877 KBytes
[ 5] 5.00-6.00 sec 105 MBytes 878 Mbits/sec 0 877 KBytes
[ 5] 6.00-7.00 sec 68.0 MBytes 571 Mbits/sec 0 877 KBytes
[ 5] 7.00-8.00 sec 80.7 MBytes 676 Mbits/sec 0 877 KBytes
[ 5] 8.00-9.01 sec 102 MBytes 853 Mbits/sec 0 877 KBytes
[ 5] 9.01-10.00 sec 101 MBytes 859 Mbits/sec 0 877 KBytes
- - - - - - - - - - - - - - - - - - - - - - - - -
[ ID] Interval Transfer Bitrate Retr
[ 5] 0.00-10.00 sec 927 MBytes 778 Mbits/sec 0 sender
[ 5] 0.00-10.01 sec 927 MBytes 777 Mbits/sec receiver


The .dts of these boards are still using an incorrect TX delay of 4ns.
This will be fixed in follow-up patches when it's clear whether the MAC
really generates an RX delay and how this can be configured.


Changes since v1 [0]:
- update cover-letter title
- dropped the .dts patches after discussion with Andrew in [1]
- slightly reworded the patch description of patch #1 to indicate that
the goal is to fix the TX delay generated by the MAC, without
suggesting that it's recommended to let the MAC actually generate it.


[0] https://patchwork.kernel.org/cover/11309887/
[1] https://patchwork.kernel.org/patch/11309891/


Martin Blumenstingl (1):
net: stmmac: dwmac-meson8b: Fix the RGMII TX delay on Meson8b/8m2 SoCs

.../net/ethernet/stmicro/stmmac/dwmac-meson8b.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)

--
2.24.1

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