Finding the Kernel Driver(s)

The victim device

$ lsusb 
Bus 010 Device 002: ID 046d:c01e Logitech, Inc. MX518 Optical Mouse
Bus 010 Device 003: ID 051d:0002 American Power Conversion Uninterruptible Power Supply

We're going to try to find out what driver is used for the APC UPS. Note that there are two answers to this question: The driver that the kernel would use, and the driver that is currently in use. Userspace can instruct the kernel to use a different driver (and in the case of my APC UPS, nut has).

Method 1: Using usbutils (easy)

The usbutils package (on Debian, at least) includes a script called usb-devices. If you run it, it outputs information about the devices on the system, including which driver is used:

$ usb-devices
T:  Bus=10 Lev=01 Prnt=01 Port=01 Cnt=02 Dev#=  3 Spd=1.5 MxCh= 0
D:  Ver= 1.10 Cls=00(>ifc ) Sub=00 Prot=00 MxPS= 8 #Cfgs=  1
P:  Vendor=051d ProdID=0002 Rev=01.06
S:  Manufacturer=American Power Conversion
S:  Product=Back-UPS RS 1500 FW:8.g9 .D USB FW:g9 
S:  SerialNumber=XXXXXXXXXXXX  
C:  #Ifs= 1 Cfg#= 1 Atr=a0 MxPwr=24mA
I:  If#= 0 Alt= 0 #EPs= 1 Cls=03(HID  ) Sub=00 Prot=00 Driver=usbfs

Note that this lists the current driver, not the default one. There isn't a way to find the default one.

Method 2: Using debugfs (requires root)

If you have debugfs mounted, the kernel maintains a file in the same format as usb-devices prints out at /sys/kernel/debug/usb/devices; you can view with less, etc. Note that debugfs interfaces are not stable, so different kernel versions may print in a different format, or be missing the file entirely.

Once again, this only shows the current driver, not the default.