153 lines
5.3 KiB
ReStructuredText
153 lines
5.3 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 controller 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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Serial TTY
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==========
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The DbC support has been added to the xHCI driver. You can get a
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debug device provided by the DbC at runtime.
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In order to use this, you need to make sure your kernel has been
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configured to support USB_XHCI_DBGCAP. A sysfs attribute under
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the xHCI device node is used to enable or disable DbC. By default,
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DbC is disabled::
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root@target:/sys/bus/pci/devices/0000:00:14.0# cat dbc
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disabled
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Enable DbC with the following command::
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root@target:/sys/bus/pci/devices/0000:00:14.0# echo enable > dbc
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You can check the DbC state at anytime::
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root@target:/sys/bus/pci/devices/0000:00:14.0# cat dbc
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enabled
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Connect the debug target to the debug host with a USB 3.0 super-
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speed A-to-A debugging cable. You can see /dev/ttyDBC0 created
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on the debug target. You will see below kernel message lines::
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root@target: tail -f /var/log/kern.log
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[ 182.730103] xhci_hcd 0000:00:14.0: DbC connected
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[ 191.169420] xhci_hcd 0000:00:14.0: DbC configured
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[ 191.169597] xhci_hcd 0000:00:14.0: DbC now attached to /dev/ttyDBC0
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Accordingly, the DbC state has been brought up to::
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root@target:/sys/bus/pci/devices/0000:00:14.0# cat dbc
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configured
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On the debug host, you will see the debug device has been enumerated.
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You will see below kernel message lines::
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root@host: tail -f /var/log/kern.log
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[ 79.454780] usb 2-2.1: new SuperSpeed USB device number 3 using xhci_hcd
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[ 79.475003] usb 2-2.1: LPM exit latency is zeroed, disabling LPM.
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[ 79.475389] usb 2-2.1: New USB device found, idVendor=1d6b, idProduct=0010
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[ 79.475390] usb 2-2.1: New USB device strings: Mfr=1, Product=2, SerialNumber=3
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[ 79.475391] usb 2-2.1: Product: Linux USB Debug Target
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[ 79.475392] usb 2-2.1: Manufacturer: Linux Foundation
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[ 79.475393] usb 2-2.1: SerialNumber: 0001
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The debug device works now. You can use any communication or debugging
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program to talk between the host and the target.
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