set USB3 back to USB2

00 - Brief

One of our standard hardware classes doesn't handle a large number (32) of daisy-chained USB Serials well, it exhausts lanes.
As a workaround we set the capabilities of the USB port in question back to lower standards, which seems to do what we want.

40-set_usb2.rules

# /etc/udev/rules.d/
# udev rules to go with set_usb2
ACTION=="add", SUBSYSTEM=="usb", ATTR{idVendor}=="1a40", ATTR{idProduct}=="0201", TAG+="systemd", ENV{SYSTEMD_WANTS}="set_usb2.service"

set_usb2.service

# /etc/systemd/system/
[Unit]
Description=switch USB for serials to USB2

[Service]
Type=oneshot
RemainAfterExit=no
ExecStart=/usr/local/bin/set_usb2.sh

[Install]
WantedBy=multi-user.target

set_usb2.sh

#!/bin/bash
# this goes in /usr/local/bin/
[ -h /sys/class/tty/ttyUSB0 ] || exit 1
pci_id=$( /usr/bin/realpath /sys/class/tty/ttyUSB0 | awk -F'/' '{print gensub(/....:(.*)/, "\\1","1",$5)}' )
usb_id=$( lspci -n| awk -v id=$pci_id '$0 ~ id {print $3}' )
/usr/bin/setpci -H1 -d $usb_id d0.l=0