path: root/Documentation/usb
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authorLu Baolu <>2017-03-21 16:01:33 +0800
committerIngo Molnar <>2017-03-21 12:30:17 +0100
commit1b326277798aa820c1043617786609b9bb4bee78 (patch)
tree9315e8a4fabb1d31437b069bb0fb89ea9b01e674 /Documentation/usb
parent57fb47279a04cd53641dc5ae55a6d47e4f32a2fd (diff)
usb/doc: Add document for USB3 debug port usage
Add Documentation/usb/usb3-debug-port.rst. This document includes the guide for using USB3 debug port. Signed-off-by: Lu Baolu <> Acked-by: Thomas Gleixner <> Cc: Greg Kroah-Hartman <> Cc: Linus Torvalds <> Cc: Mathias Nyman <> Cc: Peter Zijlstra <> Cc: Cc: Link: Signed-off-by: Ingo Molnar <>
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+USB3 debug port
+:Author: Lu Baolu <>
+:Date: March 2017
+This is a HOWTO for using the USB3 debug port on x86 systems.
+Before using any kernel debugging functionality based on USB3
+debug port, you need to::
+ 1) check whether any USB3 debug port is available in
+ your system;
+ 2) check which port is used for debugging purposes;
+ 3) have a USB 3.0 super-speed A-to-A debugging cable.
+The xHCI debug capability (DbC) is an optional but standalone
+functionality provided by the xHCI host controller. The xHCI
+specification describes DbC in the section 7.6.
+When DbC is initialized and enabled, it will present a debug
+device through the debug port (normally the first USB3
+super-speed port). The debug device is fully compliant with
+the USB framework and provides the equivalent of a very high
+performance full-duplex serial link between the debug target
+(the system under debugging) and a debug host.
+DbC has been designed to log early printk messages. One use for
+this feature is kernel debugging. For example, when your machine
+crashes very early before the regular console code is initialized.
+Other uses include simpler, lockless logging instead of a full-
+blown printk console driver and klogd.
+On the debug target system, you need to customize a debugging
+kernel with CONFIG_EARLY_PRINTK_USB_XDBC enabled. And, add below
+kernel boot parameter::
+ "earlyprintk=xdbc"
+If there are multiple xHCI controllers in your system, you can
+append a host contoller index to this kernel parameter. This
+index starts from 0.
+Current design doesn't support DbC runtime suspend/resume. As
+the result, you'd better disable runtime power management for
+USB subsystem by adding below kernel boot parameter::
+ "usbcore.autosuspend=-1"
+Before starting the debug target, you should connect the debug
+port to a USB port (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 the debug target, DbC will be detected and
+initialized. After initialization, the debug host should be able
+to enumerate the debug device in debug target. The debug host
+will then bind the debug device with the usb_debug driver module
+and create the /dev/ttyUSB device.
+If the debug device enumeration goes smoothly, you should be able
+to see below kernel messages on the 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, SerialNumber=3
+ [ 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 use any communication program, for example minicom, to
+read and view the messages. Below simple bash scripts can help
+you to check the sanity of the setup.
+.. code-block:: sh
+ ===== start of bash scripts =============
+ #!/bin/bash
+ while true ; do
+ while [ ! -d /sys/class/tty/ttyUSB0 ] ; do
+ :
+ done
+ cat /dev/ttyUSB0
+ done
+ ===== end of bash scripts ===============