Ubuntu – How to distinguish between identical USB-to-serial adapters

serial portudevusb

I use a number of identical USB-to-serial adapters with my laptop (Ubuntu 9.10). The adapters are manufactured by Sabrent and are built around a Prolific PL2303 IC, as shown by lsusb: 

Bus 001 Device 008: ID 067b:2303 Prolific Technology, Inc. PL2303 Serial Port  
Bus 001 Device 007: ID 067b:2303 Prolific Technology, Inc. PL2303 Serial Port  
Bus 001 Device 006: ID 067b:2303 Prolific Technology, Inc. PL2303 Serial Port

None of the attributes displayed by udevadm seem to be unique to a particular adapter: 

foo@bar:~$ udevadm info --attribute-walk --path=/sys/bus/usb-serial/devices/ttyUSB0

   looking at device
 '/devices/pci0000:00/0000:00:1d.7/usb1/1-4/1-4.1/1-4.1:1.0/ttyUSB0':  
     KERNEL=="ttyUSB0"  
     SUBSYSTEM=="usb-serial"  
     DRIVER=="pl2303"   
     ATTR{port_number}=="0"  

   looking at parent device
 '/devices/pci0000:00/0000:00:1d.7/usb1/1-4/1-4.1/1-4.1:1.0':
     KERNELS=="1-4.1:1.0"  
     SUBSYSTEMS=="usb"  
     DRIVERS=="pl2303"  
     ATTRS{bInterfaceNumber}=="00"  
     ATTRS{bAlternateSetting}==" 0"  
     ATTRS{bNumEndpoints}=="03"  
     ATTRS{bInterfaceClass}=="ff"  
     ATTRS{bInterfaceSubClass}=="00"  
     ATTRS{bInterfaceProtocol}=="00"  
     ATTRS{modalias}=="usb:v067Bp2303d0300dc00dsc00dp00icFFisc00ip00"  
     ATTRS{supports_autosuspend}=="1"  

   looking at parent device
 '/devices/pci0000:00/0000:00:1d.7/usb1/1-4/1-4.1':
     KERNELS=="1-4.1"   
     SUBSYSTEMS=="usb"  
     DRIVERS=="usb"   
     ATTRS{configuration}==""  
     ATTRS{bNumInterfaces}==" 1"  
     ATTRS{bConfigurationValue}=="1"  
     ATTRS{bmAttributes}=="80"  
     ATTRS{bMaxPower}=="100mA"  
     ATTRS{urbnum}=="538"  
     ATTRS{idVendor}=="067b"  
     ATTRS{idProduct}=="2303"  
     ATTRS{bcdDevice}=="0300"  
     ATTRS{bDeviceClass}=="00"  
     ATTRS{bDeviceSubClass}=="00"  
     ATTRS{bDeviceProtocol}=="00"  
     ATTRS{bNumConfigurations}=="1"  
     ATTRS{bMaxPacketSize0}=="64"  
     ATTRS{speed}=="12"  
     ATTRS{busnum}=="1"  
     ATTRS{devnum}=="6"  
     ATTRS{version}==" 1.10"  
     ATTRS{maxchild}=="0"  
     ATTRS{quirks}=="0x0"  
     ATTRS{authorized}=="1"  
     ATTRS{manufacturer}=="Prolific Technology Inc."  
     ATTRS{product}=="USB-Serial Controller"  

     <snip>

 foo@bar:~$ udevadm info --attribute-walk --path=/sys/bus/usb-serial/devices/ttyUSB1

   looking at device
 '/devices/pci0000:00/0000:00:1d.7/usb1/1-4/1-4.5/1-4.5:1.0/ttyUSB1':
     KERNEL=="ttyUSB1"  
     SUBSYSTEM=="usb-serial"  
     DRIVER=="pl2303"  
     ATTR{port_number}=="0"  

   looking at parent device
 '/devices/pci0000:00/0000:00:1d.7/usb1/1-4/1-4.5/1-4.5:1.0':
     KERNELS=="1-4.5:1.0"  
     SUBSYSTEMS=="usb"  
     DRIVERS=="pl2303"  
     ATTRS{bInterfaceNumber}=="00"  
     ATTRS{bAlternateSetting}==" 0"  
     ATTRS{bNumEndpoints}=="03"  
     ATTRS{bInterfaceClass}=="ff"  
     ATTRS{bInterfaceSubClass}=="00"  
     ATTRS{bInterfaceProtocol}=="00"  
     ATTRS{modalias}=="usb:v067Bp2303d0300dc00dsc00dp00icFFisc00ip00"  
     ATTRS{supports_autosuspend}=="1"  

   looking at parent device
 '/devices/pci0000:00/0000:00:1d.7/usb1/1-4/1-4.5':
     KERNELS=="1-4.5"  
     SUBSYSTEMS=="usb"  
     DRIVERS=="usb"  
     ATTRS{configuration}==""  
     ATTRS{bNumInterfaces}==" 1"  
     ATTRS{bConfigurationValue}=="1"  
     ATTRS{bmAttributes}=="80"  
     ATTRS{bMaxPower}=="100mA"  
     ATTRS{urbnum}=="69"  
     ATTRS{idVendor}=="067b"  
     ATTRS{idProduct}=="2303"  
     ATTRS{bcdDevice}=="0300"  
     ATTRS{bDeviceClass}=="00"  
     ATTRS{bDeviceSubClass}=="00"  
     ATTRS{bDeviceProtocol}=="00"  
     ATTRS{bNumConfigurations}=="1"  
     ATTRS{bMaxPacketSize0}=="64"  
     ATTRS{speed}=="12"  
     ATTRS{busnum}=="1"  
     ATTRS{devnum}=="7"  
     ATTRS{version}==" 1.10"  
     ATTRS{maxchild}=="0"  
     ATTRS{quirks}=="0x0"  
     ATTRS{authorized}=="1"  
     ATTRS{manufacturer}=="Prolific Technology Inc."  
     ATTRS{product}=="USB-Serial Controller"  

     <snip>

All of the adapters are plugged into a single USB hub. Since I can't distinguish between the adapters themselves, is there any way I can write a udev rule that fixes the name of each adapter based on which physical port on the hub the adapter is plugged into?

Best Answer

is there any way I can write a udev rule that fixes the name of each adapter based on which physical port on the hub the adapter is plugged into?

Yes there is, as it turns out. Consider the last portion of the device hierarchy shown in the second example above:

looking at parent device '/devices/pci0000:00/0000:00:1d.7/usb1/1-4/1-4.5': KERNELS=="1-4.5"
SUBSYSTEMS=="usb"
DRIVERS=="usb"
ATTRS{configuration}==""
ATTRS{bNumInterfaces}==" 1"
ATTRS{bConfigurationValue}=="1"
ATTRS{bmAttributes}=="80"
ATTRS{bMaxPower}=="100mA"
ATTRS{urbnum}=="69"
ATTRS{idVendor}=="067b"
ATTRS{idProduct}=="2303"
ATTRS{bcdDevice}=="0300"
ATTRS{bDeviceClass}=="00"
ATTRS{bDeviceSubClass}=="00"
ATTRS{bDeviceProtocol}=="00"
ATTRS{bNumConfigurations}=="1"
ATTRS{bMaxPacketSize0}=="64"
ATTRS{speed}=="12"
ATTRS{busnum}=="1"
ATTRS{devnum}=="7" ATTRS{version}==" 1.10" ATTRS{maxchild}=="0" ATTRS{quirks}=="0x0"
ATTRS{authorized}=="1"
ATTRS{manufacturer}=="Prolific Technology Inc."
ATTRS{product}=="USB-Serial Controller"

The name given to this device by the kernel (KERNELS=="1-4.5") indicates that this device is plugged into the fifth port of a hub connected to port four on bus 1 (see this FAQ for more information on how to decode the sysfs usb device hierarchy). With some help from this guide to writing udev rules I came up with the following set of udev rules for my USB-to-serial-port converters:

KERNEL=="ttyUSB*", KERNELS=="1-8.1.5", NAME="ttyUSB0"
KERNEL=="ttyUSB*", KERNELS=="1-8.1.6", NAME="ttyUSB1"
KERNEL=="ttyUSB*", KERNELS=="1-8.1.1", NAME="ttyUSB2"
KERNEL=="ttyUSB*", KERNELS=="1-8.1.2", NAME="ttyUSB3"

These rules have one obvious disadvantage: they assume that all USB-to-serial converters will be plugged into the same hub ("1-8.1.*"). If a USB to serial converter was plugged into another USB port it could be assigned the name "ttyUSB0" which would conflict with the naming scheme described above. However, since I leave all of the converters plugged into the hub I can live with this constraint.

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