usb-kernel-debug — Linux kernel dynamic debug for USB

Kernel dynamic debug shows the Linux side's reasoning that packet capture can't: port resets and their causes, enumeration retries, address (re)assignment, EP halts, xHCI ring/command errors. It applies wherever Linux sits in the link — the rig PC when it is the host, or a Linux gadget peer (dwc2/UDC + gadget modules) when TinyUSB is the host. It cannot see inside the TinyUSB MCU — that is the target-debug skill.

Run this skill's scripts/usb_dyndbg.sh with sudo. It flips the dynamic-debug print flag for an allowlisted set of USB modules only:

# all examples below abbreviate:  sudo .claude/skills/usb-kernel-debug/scripts/usb_dyndbg.sh
sudo usb_dyndbg.sh on  usbcore xhci_hcd   # enable +p; pick modules from `lsusb -t` Driver=
sudo usb_dyndbg.sh status [module]        # list enabled print sites
sudo usb_dyndbg.sh off usbcore xhci_hcd   # ALWAYS turn off when done — very noisy

Allowlisted modules: usbcore xhci_hcd xhci_pci xhci_pci_renesas ehci_hcd ehci_pci ohci_hcd ohci_pci uhci_hcd dwc2 dwc3 cdc_acm usb_storage uas libcomposite udc_core (dwc2/dwc3 + the last two cover a Linux gadget peer's device side).

Workflow

1. sudo usb_dyndbg.sh on usbcore <hcd-module> — usbcore for enumeration/hub logic, plus the controller module (lsusb -t shows the driver per bus). On a gadget peer: dwc2 (or dwc3) + udc_core + libcomposite instead — run on the peer itself (its SSH/serial console); the script is self-contained, copy it over or use the raw dynamic_debug/control writes from the usbmon skill.
2. Reproduce (replug / re-enumerate / rerun the failing test) while following sudo dmesg -w (or grab sudo dmesg | tail afterwards).
3. sudo usb_dyndbg.sh off ... — leaving it on floods the log and skews timing.

On a Linux-PC-host link, pair with the usbmon skill: usbmon for what crossed the bus, dynamic debug for why the kernel reacted. A gadget peer's UDC has no usbmon — pair with usb-sniffer on the wire instead. For a wedged device/bus on the rig PC use the usb-kernel-recover skill.

Requires CONFIG_DYNAMIC_DEBUG and mounted debugfs (standard on distro kernels).