why udevadm trigger need /sys rw?

Question

look at systemd-udev-trigger unit file , we can see If we want to start systemd-udev-trigger service we need /sys rw, and I checked that command /usr/bin/udevadm trigger --type=subsystems --action=add ; /usr/bin/udevadm trigger --type=devices --action=add alse need /sys rw

#systemctl cat  systemd-udev-trigger
# /usr/lib/systemd/system/systemd-udev-trigger.service
#  This file is part of systemd.
#
#  systemd is free software; you can redistribute it and/or modify it
#  under the terms of the GNU Lesser General Public License as published by
#  the Free Software Foundation; either version 2.1 of the License, or
#  (at your option) any later version.

[Unit]
Description=udev Coldplug all Devices
Documentation=man:udev(7) man:systemd-udevd.service(8)
DefaultDependencies=no
Wants=systemd-udevd.service
After=systemd-udevd-kernel.socket systemd-udevd-control.socket systemd-hwdb-update.service
Before=sysinit.target
ConditionPathIsReadWrite=/sys

[Service]
Type=oneshot
RemainAfterExit=yes
ExecStart=/usr/bin/udevadm trigger --type=subsystems --action=add ; /usr/bin/udevadm trigger --type=devices --action=add

Answer 1

The point of udevadm trigger is to tell the kernel to send events for all the devices that are present. It does that by writing to /sys/devices/*/*/uevent. This requires sysfs to be mounted read-write on /sys.