usbmon is a mechanism for catching information on the USB bus. This is useful for debugging some kinds of USB problems. The existing usbmon documentation is very good. A rehash of the documentation is below.
Kernel module
USB module can be compiled into kernel statically, or it can be module that can be loaded into kernel. Fedora should have it statically part of the kernel.
# grep USB_MON /boot/config-5.9.* /boot/config-5.9.11-100.fc32.x86_64:CONFIG_USB_MON=y /boot/config-5.9.8-100.fc32.x86_64:CONFIG_USB_MON=y /boot/config-5.9.9-100.fc32.x86_64:CONFIG_USB_MON=y
shows that it was selected to be part of monolitic kernel, rather than module (CONFIG_USB_MON=m). Hence it does not need, or can be loaded into running kernel.
If it was compiled as module, it can be loaded into kernel:
# modprobe usbmon # lsmod | grep usbmon
should list it as module.
If directory /sys/kernel/debug/usb/usbmon/ has files in it, kernel support should be ready.
# ls /sys/kernel/debug/usb/usbmon/ 0s 0u 1s 1t 1u 2s 2t 2u
# ls -l /dev/usbmon* crw-r----- 1 root usbmon 244, 0 Dec 4 19:25 /dev/usbmon0 crw-r----- 1 root usbmon 244, 1 Dec 4 19:25 /dev/usbmon1 crw-r----- 1 root usbmon 244, 2 Dec 4 19:25 /dev/usbmon2
User access rights
In Fedora, debugging users should belong to usbmon group, and additionally wireshark group in order to have access to debugging device files.
# usermod -a -G usbmon tuju
# id tuju uid=1001(tuju) gid=1001(tuju) groups=1001(tuju),10(wheel),18(dialout),135(mock),498(wireshark),497(usbmon),494(vboxusers),48(apache)
[root@wasa ~]# id tuju uid=1001(tuju) gid=1001(tuju) groups=1001(tuju),10(wheel),18(dialout),135(mock),498(wireshark),497(usbmon),494(vboxusers),48(apache)
Capturing USB packets
First, identify which bus number of the device you want to trace. You can find this via lsusb. e.g.
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
gives bus number 2.
To start capturing output do
# cat /sys/kernel/debug/usb/usbmon/2u > /tmp/usbmon_out
replacing 2u with your bus number (i.e. Bus003 -> 3u, Bus008 -> 8u).
This will continue to write to the file until it is terminated. The capture steps now look like
- start capturing the usbmon output to a file - Run your tests with the USB device - stop capturing the usbmon output