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Xue USB 3 Debugger

Xue is a cross-platform driver for the USB 3 Debug Capability (DbC). It is a header-only library so that it may be easily included into various environments. The goal of Xue is to provide a high-bandwidth debugger that can be used with two standard mobile/laptop devices without the need for legacy UART hardware.

Hardware Requirements

To use xue, you need two machines: a target and a host. The target (the one you're debugging) must have a USB 3 xHCI host controller that implements the Debug Capability as described in the xHCI specification. So far the following Intel (PCI vendor 0x8086) host controllers have been tested to work:

  • Z370 (PCI device 0xA2AF)
  • Z390 (PCI device 0xA36D)
  • Wildcat Point-LP (PCI device 0x9CB1)
  • Sunrise Point-LP (PCI device 0x9D2F)
  • Cannon Point-LP (PCI device 0x9DED)

Devices not listed here will likely work after you add the appropriate #define XUE_XHC_DEV_* to xue.h. If you have a device not listed above, and/or it does not work after adding the #define, please open a merge request so support can be added.

The host (the machine you're viewing the target output from) only needs one Super-Speed port. The debug cable that connects the target to the host is an A/A crossover cable that can be purchased here. The final requirement is that the cable must be connected directly to a root Super-Speed port on the target, i.e., it will not work through a hub.

Debug Targets

Xue is officially supported to run from the following environments:

NOTE: To use the Xen example above, pass 'console=dbgp dbgp=xue' on the Xen command line

Xue requires a set of operations to properly communicate with the DbC hardware. These operations are defined in struct xue_ops in xue.h. Xue provides default implementations of these operations for each of the four systems above. However, you may override each of these defaults in order to adapt xue to your target system. To use xue in your own project, follow these steps:

  1. Ensure include/xue.h is in your include path
  2. Allocate and clear a struct xue and struct xue_ops
  3. If needed, implement system-specific xue_ops and initialize the previously allocated struct xue_ops with them
  4. Optionally allocate a structure for storing system-specific state. This may be used in conjunction with the operations defined in 3
  5. Open xue by passing the addresses of the above structures to xue_open
  6. Write data to xue with xue_write

Any NULL members of the struct xue_ops passed to xue_open will be set to the xue_sys_* defaults defined for that system. If a member is not NULL, then xue assumes it is a user-defined override and will simply call it.

Debug Hosts

Xue itself runs on the target machine and sends any data provided to xue_write to the debug host, which can be either Linux or Windows. Each host platform has its own instructions outlined below you should follow in order to read data from xue.

Linux

Xue presents itself as the xhci_dbc device over USB. This means that Linux will bind the xhci_dbc driver to the device and will create a /dev/ttyUSBx file that can be read. You can read this file like any other serial device, however the scripts/read.sh is provided and recommended as it handles common things like disconnects that may occur during development.

Windows 10

If your debug host is Windows 10, then you can run python scripts/read.py from a terminal to read the target's output, but there are a few steps you need to do before the script will work:

  • Install zadig
  • Run zadig as admin
  • Click 'Device' -> 'Create New Device'
  • Enter 'Xue DbC Device' in the top text box
  • Enter '1D6B' in the left text box of USB ID
  • Enter '0010' in the middle text box of USB ID
  • Click 'Install Driver'

This will install the WinUSB driver and bind the DbC device to it whenever the debug cable is connected and the DbC enabled.

NOTE: The DbC is considered 'enabled' after a successful call to xue_open.

After WinUSB is installed, setup the python environment:

  • Install the latest 64-bit python for Windows
    • Select "Add Python to PATH"
    • Select "Install Now"
  • Install 7zip and libusb
  • Extract libusb with 7zip
  • Copy <path\to\extracted\libusb>\MS64\dll\libusb-1.0.dll to C:\Windows\System32\
  • Run pip install pyusb

Now you should be able to run python scripts\read.py from a terminal to read the output from the debug target.

Known Limitations

  • Does not run from Windows 10. Based on earlier experiments, Windows does not allow multiple drivers to map in a given PCI MMIO region simultaneously. This prevents Xue from mapping in the DbC's registers because Window's xHCI driver already owns the mapping. If anyone knows a workaround for this, please share.

  • The DbC is subject to USB host controller resets. This means if any other code resets the host controller, the DbC is reset as well. This means that if you xue_open from EFI, the DbC will be reset. Xue checks if this occured before each write, and will re-initialize the device if it has been reset, but this could lead to data loss depending on when the host controller was reset.

  • The DbC is subject to DMA remapping. If the USB host controller is being remapped by an IOMMU, then the default xue_sys_* functions provided in xue.h may not work.

Testing

To run the unit tests, pass -DBUILD_TESTS=ON to cmake. Then run make test. This runs the actual test and performs static analysis of the resulting binary with clang-tidy.

Documentation

The portion of the code intended to be "public" is documented with doxygen. You can generate the docs with doxygen .doxygen.txt from the source root. That said, most of the "internal" code is documented in xue.h as well.