usb/doc: Add document for USB3 debug port usage

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

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