Read common descriptors from sysfs, when available

Running Debian Linux 11 with Python 3.10 and libusb1. I am trying to connect to an LG UltraFine monitor via USB-C.

It works via lsusb (running as my user):

$ lsusb -D /dev/bus/usb/003/010
Device: ID 043e:9a39 LG Electronics USA, Inc. LG Monitor Controls
Couldn't open device, some information will be missing
Device Descriptor:
  bLength                18
  bDescriptorType         1
  bcdUSB               2.00
  bDeviceClass          239 Miscellaneous Device
  bDeviceSubClass         2 
  bDeviceProtocol         1 Interface Association
  bMaxPacketSize0        64
  idVendor           0x043e LG Electronics USA, Inc.
  idProduct          0x9a39 
  bcdDevice            4.11
  iManufacturer           1 LG Electronics Inc.
  iProduct                3 LG Monitor Controls
  iSerial                 4 107NTYTAD986
  bNumConfigurations      1

but not via pyusb:

$ python -c 'import usb.core; d = usb.core.find(idVendor=0x043e, idProduct=0x9a39); print(d)'
DEVICE ID 043e:9a39 on Bus 003 Address 010 =================
 bLength                :   0x12 (18 bytes)
 bDescriptorType        :    0x1 Device
 bcdUSB                 :  0x200 USB 2.0
 bDeviceClass           :   0xef Miscellaneous
 bDeviceSubClass        :    0x2
 bDeviceProtocol        :    0x1
 bMaxPacketSize0        :   0x40 (64 bytes)
 idVendor               : 0x043e
 idProduct              : 0x9a39
 bcdDevice              :  0x411 Device 4.11
 iManufacturer          :    0x1 Error Accessing String
 iProduct               :    0x3 Error Accessing String
 iSerialNumber          :    0x4 Error Accessing String
 bNumConfigurations     :    0x1

$ ls -lah /dev/bus/usb/003/010
crw-rw-r-- 1 root root 189, 265 Sep 28 07:48 /dev/bus/usb/003/010

The device file is world readable.

The strange thing is that pyusb and also lsusb try to open the device with write permissions. I wonder why those are necessary to list something.

I can also do

$ cat /sys/bus/usb/devices/3-6.3/manufacturer
LG Electronics Inc.

or

$ cat /sys/bus/usb/devices/3-6.3/product 
LG Monitor Controls
$ ll /sys/bus/usb/devices/3-6.3/product 
-r--r--r-- 1 root root 4096 Sep 28 07:48 /sys/bus/usb/devices/3-6.3/product

I would expect pyusb to be able to show the vendor and manufacturer similar to lsusb without having write permissions (which are not necessary).

[ronny-rentner]

Thanks for the direction, will do it like this.

[jonasmalacofilho]

Can we reopen the ticket for this?

Done, but I also renamed for accuracy.

PS: I would probably not replace any existing logic but just whenever there is “Error Accessing String” do an extra step and try to find the string through /sys/bus/usb Sounds pretty straight forward to me.

Actually, it should probably go in the Device.manufacturer and similar properties, not in Device.__str__ (which may then be reworked to use the new feature).

PyUSB is not a lsusb clone: sure, this feature will improve Device.__str__, but it’s also necessary (and, arguably, more important) to make the data available for callers using PyUSB for something more than print(dev).

And the preference order should probably be: locally cached value > (on Linux:) sysfs value > GET_DESCRIPTOR transfer.