Add Documentation/usb/usb3-debug-port.txt. This document includes
the user guide for USB3 debug port.

Signed-off-by: Lu Baolu <>
 Documentation/usb/usb3-debug-port.txt | 87 +++++++++++++++++++++++++++++++++++
 1 file changed, 87 insertions(+)
 create mode 100644 Documentation/usb/usb3-debug-port.txt

diff --git a/Documentation/usb/usb3-debug-port.txt 
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+                  USB3 debug port
+         Lu Baolu <>
+              Last-updated: October 2016
+This is a HOWTO for using USB3 debug port on x86 systems.
+Before using any kernel debugging functionalities based on USB3
+debug port, you need to check 1) whether debug port is supported
+by the xHCI host, 2) which port is used for debugging purpose
+(normally the first USB3 root port). You must have a USB 3.0
+super-speed A-to-A debugging cable to connect the debug target
+with a debug host. In this document, a debug target stands for
+the system under debugging; while, a debug host stands for a
+stand-alone system that is able to talk to the debugging target
+through the USB3 debug port.
+On debug target system, you need to customize a debugging kernel
+with CONFIG_EARLY_PRINTK_XDBC enabled. And add below kernel boot
+    "earlyprintk=xdbc"
+If there are multiple xHCI controllers in the system, you can
+append a host contoller index to this kernel parameter. This
+index is started from 0.
+If you are going to leverage the keep option defined by the
+early printk framework to keep the boot console alive after
+early boot, you'd better add below kernel boot parameter.
+    "usbcore.autosuspend=-1"
+On debug host side, you don't need to customize the kernel, but
+you need to disable usb subsystem runtime power management by
+adding below kernel boot parameter.
+    "usbcore.autosuspend=-1"
+Before starting the debug target, you should connect the debug
+port on debug target with a root port or port of any external hub
+on the debug host. The cable used to connect these two ports
+should be a USB 3.0 super-speed A-to-A debugging cable.
+During early boot of debug target, DbC (the debug engine for USB3
+debug port) hardware gets initialized. Debug host should be able
+to enumerate the debug target as a debug device. Debug host will
+then bind the debug device with the usb_debug driver module and
+create the /dev/ttyUSB0 device.
+If device enumeration goes smoothly, you should be able to see
+below kernel messages on debug host.
+# tail -f /var/log/kern.log
+[ 1815.983374] usb 4-3: new SuperSpeed USB device number 4 using xhci_hcd
+[ 1815.999595] usb 4-3: LPM exit latency is zeroed, disabling LPM.
+[ 1815.999899] usb 4-3: New USB device found, idVendor=1d6b, idProduct=0004
+[ 1815.999902] usb 4-3: New USB device strings: Mfr=1, Product=2, 
+[ 1815.999903] usb 4-3: Product: Remote GDB
+[ 1815.999904] usb 4-3: Manufacturer: Linux
+[ 1815.999905] usb 4-3: SerialNumber: 0001
+[ 1816.000240] usb_debug 4-3:1.0: xhci_dbc converter detected
+[ 1816.000360] usb 4-3: xhci_dbc converter now attached to ttyUSB0
+You can run below bash scripts on debug host to read the kernel
+log sent from debug target.
+===== start of bash scripts =============
+while true ; do
+       while [ ! -d /sys/class/tty/ttyUSB0 ] ; do
+               :
+       done
+       cat /dev/ttyUSB0 >> xdbc.log
+===== end of bash scripts ===============
+You should be able to see the early boot message in xdbc.log.

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