101 lines
3.4 KiB
ReStructuredText
101 lines
3.4 KiB
ReStructuredText
===============
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USB3 debug port
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===============
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:Author: Lu Baolu <baolu.lu@linux.intel.com>
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:Date: March 2017
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GENERAL
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=======
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This is a HOWTO for using the USB3 debug port on x86 systems.
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Before using any kernel debugging functionality based on USB3
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debug port, you need to::
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1) check whether any USB3 debug port is available in
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your system;
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2) check which port is used for debugging purposes;
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3) have a USB 3.0 super-speed A-to-A debugging cable.
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INTRODUCTION
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============
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The xHCI debug capability (DbC) is an optional but standalone
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functionality provided by the xHCI host controller. The xHCI
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specification describes DbC in the section 7.6.
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When DbC is initialized and enabled, it will present a debug
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device through the debug port (normally the first USB3
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super-speed port). The debug device is fully compliant with
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the USB framework and provides the equivalent of a very high
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performance full-duplex serial link between the debug target
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(the system under debugging) and a debug host.
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EARLY PRINTK
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============
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DbC has been designed to log early printk messages. One use for
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this feature is kernel debugging. For example, when your machine
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crashes very early before the regular console code is initialized.
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Other uses include simpler, lockless logging instead of a full-
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blown printk console driver and klogd.
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On the debug target system, you need to customize a debugging
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kernel with CONFIG_EARLY_PRINTK_USB_XDBC enabled. And, add below
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kernel boot parameter::
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"earlyprintk=xdbc"
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If there are multiple xHCI controllers in your system, you can
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append a host contoller index to this kernel parameter. This
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index starts from 0.
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Current design doesn't support DbC runtime suspend/resume. As
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the result, you'd better disable runtime power management for
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USB subsystem by adding below kernel boot parameter::
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"usbcore.autosuspend=-1"
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Before starting the debug target, you should connect the debug
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port to a USB port (root port or port of any external hub) on
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the debug host. The cable used to connect these two ports
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should be a USB 3.0 super-speed A-to-A debugging cable.
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During early boot of the debug target, DbC will be detected and
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initialized. After initialization, the debug host should be able
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to enumerate the debug device in debug target. The debug host
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will then bind the debug device with the usb_debug driver module
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and create the /dev/ttyUSB device.
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If the debug device enumeration goes smoothly, you should be able
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to see below kernel messages on the debug host::
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# tail -f /var/log/kern.log
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[ 1815.983374] usb 4-3: new SuperSpeed USB device number 4 using xhci_hcd
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[ 1815.999595] usb 4-3: LPM exit latency is zeroed, disabling LPM.
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[ 1815.999899] usb 4-3: New USB device found, idVendor=1d6b, idProduct=0004
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[ 1815.999902] usb 4-3: New USB device strings: Mfr=1, Product=2, SerialNumber=3
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[ 1815.999903] usb 4-3: Product: Remote GDB
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[ 1815.999904] usb 4-3: Manufacturer: Linux
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[ 1815.999905] usb 4-3: SerialNumber: 0001
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[ 1816.000240] usb_debug 4-3:1.0: xhci_dbc converter detected
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[ 1816.000360] usb 4-3: xhci_dbc converter now attached to ttyUSB0
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You can use any communication program, for example minicom, to
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read and view the messages. Below simple bash scripts can help
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you to check the sanity of the setup.
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.. code-block:: sh
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===== start of bash scripts =============
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#!/bin/bash
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while true ; do
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while [ ! -d /sys/class/tty/ttyUSB0 ] ; do
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:
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done
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cat /dev/ttyUSB0
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done
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===== end of bash scripts ===============
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