Add Documentation/usb/usb3-debug-port.txt. This document includes the user guide for USB3 debug port.
Cc: linux-...@vger.kernel.org Signed-off-by: Lu Baolu <baolu...@linux.intel.com> --- Documentation/usb/usb3-debug-port.txt | 87 +++++++++++++++++++++++++++++++++++ 1 file changed, 87 insertions(+) create mode 100644 Documentation/usb/usb3-debug-port.txt diff --git a/Documentation/usb/usb3-debug-port.txt b/Documentation/usb/usb3-debug-port.txt new file mode 100644 index 0000000..df5ce27 --- /dev/null +++ b/Documentation/usb/usb3-debug-port.txt @@ -0,0 +1,87 @@ + USB3 debug port + + Lu Baolu <baolu...@linux.intel.com> + + Last-updated: October 2016 + +GENERAL +======= + +This is a HOWTO for using USB3 debug port on x86 systems. + +Before using any kernel debugging functionalities based on USB3 +debug port, you need to check 1) whether debug port is supported +by the xHCI host, 2) which port is used for debugging purpose +(normally the first USB3 root port). You must have a USB 3.0 +super-speed A-to-A debugging cable to connect the debug target +with a debug host. In this document, a debug target stands for +the system under debugging; while, a debug host stands for a +stand-alone system that is able to talk to the debugging target +through the USB3 debug port. + +EARLY PRINTK +============ + +On debug target system, you need to customize a debugging kernel +with CONFIG_EARLY_PRINTK_XDBC enabled. And add below kernel boot +parameter. + + "earlyprintk=xdbc" + +If there are multiple xHCI controllers in the system, you can +append a host contoller index to this kernel parameter. This +index is started from 0. + +If you are going to leverage the keep option defined by the +early printk framework to keep the boot console alive after +early boot, you'd better add below kernel boot parameter. + + "usbcore.autosuspend=-1" + +On debug host side, you don't need to customize the kernel, but +you need to disable usb subsystem runtime power management by +adding below kernel boot parameter. + + "usbcore.autosuspend=-1" + +Before starting the debug target, you should connect the debug +port on debug target with a 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 debug target, DbC (the debug engine for USB3 +debug port) hardware gets initialized. Debug host should be able +to enumerate the debug target as a debug device. Debug host will +then bind the debug device with the usb_debug driver module and +create the /dev/ttyUSB0 device. + +If device enumeration goes smoothly, you should be able to see +below kernel messages on 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 run below bash scripts on debug host to read the kernel +log sent from debug target. + +===== start of bash scripts ============= +#!/bin/bash + +while true ; do + while [ ! -d /sys/class/tty/ttyUSB0 ] ; do + : + done + cat /dev/ttyUSB0 >> xdbc.log +done +===== end of bash scripts =============== + +You should be able to see the early boot message in xdbc.log. -- 2.1.4