A tiny program to put dual-role USB port to host mode
make
The output binary would be usb2host
./usb2host [node 1] [node 2] ...
[node] is the canoncical device-tree node name of a USB dual-role controller used by kernel under sysfs
The program needs write permission to files under /sys/kernel/debug/usb, so you need to run it with root permission.
e.g. fc000000.usb is the node for RK3588(S), so for SBCs using RK3588(S) e.g. OrangePi 5/5B/5+, you should use the following command:
./usb2host fc000000.usb
If no nodes are given, it would search through all available usb debug folders to put all dual-roal usb controllers (those with mode file under their debug folder) into host mode. Usually you don't want this as there would be only one dual-role ports for a single device, which can be named explicitly.
The current ways used in distros to force a dual-role usb controller into host mode uses shell:
echo host > /sys/kernel/debug/usb/fc000000.usb/mode
As > is pure shell syntax, if you want to integrate the above command into a udev rule, or a systemd .service, you'll need to write it like this:
ACTION=="remove", SUBSYSTEM=="typec", RUN+="/usr/bin/sh -c 'echo host > /sys/kernel/debug/usb/fc000000.usb/mode'"
ExecStart=/usr/bin/sh -c 'echo host > /sys/kernel/debug/usb/fc000000.usb/mode'
Starting a full-blown shell just for the sake of writing 4 bytes is very heavy and slows down the bootup speed.
The biggest problem is that it also opens up vulnerability to shell injection and coupliing with shell. This is no different from using system() in C or os.system() or subprocess.call([], shell=True) in Python.
Instead, using this tool, the above examples would become:
ACTION=="remove", SUBSYSTEM=="typec", RUN+="/usr/bin/usb2host fc000000.usb"
ExecStart=/usr/bin/usb2host fc000000.usb