Naming your serial ports with udev
Brief
UART is still a very common way to communicate with embedded systems, in fact, I would say that it is the most common way. When looking around in my lab, I can see that I have four different development boards/projects connected via UART to my computer. Two of them even use more than one port.
My point is that when you have many serial ports connected to your computer, it can be hard to keep track of which port is which. Especially if you have to unplug and replug them from time to time as the /dev/ttyUSBx or /dev/ttyACMx names can change.
What already exists
Yes, you can keep it consistent by refering to /dev/serial/by-*/, but those names are not very user friendly as you can see:
1$ ls /dev/serial/by-*
2/dev/serial/by-id:
3usb-FTDI_TTL232R-3V3_FT9GF3WL-if00-port0 usb-FTDI_USB_FAST_SERIAL_ADAPTER_FT1DOJH7-if00-port0 usb-FTDI_USB_FAST_SERIAL_ADAPTER_FT4PSCBS-if00-port0
4usb-FTDI_TTL232R-3V3_FTDO721C-if00-port0 usb-FTDI_USB_FAST_SERIAL_ADAPTER_FT1DOJH7-if01-port0 usb-FTDI_USB_FAST_SERIAL_ADAPTER_FT4PSCBS-if01-port0
5
6/dev/serial/by-path:
7pci-0000:80:14.0-usb-0:1.3.1.3.1.1:1.0-port0 pci-0000:80:14.0-usb-0:1.3.3.3.3:1.0-port0 pci-0000:80:14.0-usbv2-0:1.3.1.3.1.3:1.0-port0
8pci-0000:80:14.0-usb-0:1.3.1.3.1.1:1.1-port0 pci-0000:80:14.0-usb-0:1.3.3.3.3:1.1-port0 pci-0000:80:14.0-usbv2-0:1.3.3.3.1:1.0-port0
9pci-0000:80:14.0-usb-0:1.3.1.3.1.3:1.0-port0 pci-0000:80:14.0-usbv2-0:1.3.1.3.1.1:1.0-port0 pci-0000:80:14.0-usbv2-0:1.3.3.3.3:1.0-port0
10pci-0000:80:14.0-usb-0:1.3.3.3.1:1.0-port0 pci-0000:80:14.0-usbv2-0:1.3.1.3.1.1:1.1-port0 pci-0000:80:14.0-usbv2-0:1.3.3.3.3:1.1-port0Those are created by udev rules that comes with the system, you can find it in /usr/lib/udev/rules.d/60-serial.rules on most Linux systems.
Custom udev rules
At least all FTDI based serial ports provide a unique serial number that we can use to identify the device, so we can create udev rules that creates symlinks based on that.
Here is what I came up with.
1ACTION=="remove", GOTO="serial_end"
2SUBSYSTEM!="tty", GOTO="serial_end"
3
4SUBSYSTEMS=="usb", IMPORT{builtin}="usb_id", IMPORT{builtin}="hwdb --subsystem=usb"
5SUBSYSTEMS=="usb-serial", ENV{.ID_PORT}="$attr{port_number}"
6
7# Sanity checks
8IMPORT{builtin}="path_id"
9ENV{ID_BUS}=="", GOTO="serial_end"
10ENV{ID_USB_INTERFACE_NUM}=="", GOTO="serial_end"
11
12# Serial lookup
13ATTRS{serial}=="FT4PSCBS", ENV{LOCATION}="WORKPLACE"
14ATTRS{serial}=="FT1DOJH7", ENV{LOCATION}="LAB"
15
16ENV{.ID_PORT}=="?*", SYMLINK+="tty$env{LOCATION}$env{ID_USB_INTERFACE_NUM}"
17LABEL="serial_end"Add the rule above to a file named e.g. 99-usb-serial-names.rules in /etc/udev/rules.d/ and reload and trigger the rules with the following commands:
1$ sudo udevadm control --reload-rules
2$ sudo udevadm triggerThe rule will create symlinks like /dev/ttyWORKPLACE00 and /dev/ttyLAB01 based on the serial number of the FTDI devices. The critical part is the ATTRS{serial}== lines where you match the serial number of the device and the naming you want.
You can find the serial attribute in sysfs, or just by looking into the kernel log when you plug in the device:
1$ journalctl -fk
2
3usb 5-1.3.3.3.3: new full-speed USB device number 86 using xhci_hcd
4usb 5-1.3.3.3.3: New USB device found, idVendor=0403, idProduct=6010, bcdDevice= 5.00
5usb 5-1.3.3.3.3: New USB device strings: Mfr=1, Product=2, SerialNumber=3
6usb 5-1.3.3.3.3: Product: USB FAST SERIAL ADAPTER
7usb 5-1.3.3.3.3: Manufacturer: FTDI
8usb 5-1.3.3.3.3: SerialNumber: FT4PSCBS
9ftdi_sio 5-1.3.3.3.3:1.0: FTDI USB Serial Device converter detected
10usb 5-1.3.3.3.3: Detected FT2232C/D
11usb 5-1.3.3.3.3: FTDI USB Serial Device converter now attached to ttyUSB0
12ftdi_sio 5-1.3.3.3.3:1.1: FTDI USB Serial Device converter detected
13usb 5-1.3.3.3.3: Detected FT2232C/D
14usb 5-1.3.3.3.3: FTDI USB Serial Device converter now attached to ttyUSB1Result
I now have logical names for my serial ports:
1[17:04:30]marcus@redpanda:~$ ls -al /dev/ttyLAB0*
2lrwxrwxrwx 1 root root 7 19 jan 15.02 /dev/ttyLAB00 -> ttyUSB2
3lrwxrwxrwx 1 root root 7 19 jan 15.02 /dev/ttyLAB01 -> ttyUSB3And can put a small label on my lab setup to know which port is which :)
