===============
USB3 debug port
===============

:Author: Lu Baolu <baolu.lu@linux.intel.com>
:Date: March 2017

GENERAL
=======

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.

INTRODUCTION
============

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.

EARLY PRINTK
============

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 ===============