Re: [RESEND PATCH v3 4/4] usb: doc: add document for USB3 debug port usage

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Hi,

On 10/25/2016 05:32 PM, Jani Nikula wrote:
> On Tue, 25 Oct 2016, Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx> wrote:
>> Add Documentation/usb/usb3-debug-port.txt. This document includes
>> the user guide for USB3 debug port.
> It's in reStructuredText, please name it .rst.

Sure.

Best regards,
Lu Baolu

>
> Thanks,
> Jani.
>
>> Cc: linux-doc@xxxxxxxxxxxxxxx
>> Signed-off-by: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
>> ---
>>  Documentation/usb/usb3-debug-port.txt | 89 +++++++++++++++++++++++++++++++++++
>>  1 file changed, 89 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..29ebbf4
>> --- /dev/null
>> +++ b/Documentation/usb/usb3-debug-port.txt
>> @@ -0,0 +1,89 @@
>> +===============
>> +USB3 debug port
>> +===============
>> +
>> +:Author: Lu Baolu <baolu.lu@xxxxxxxxxxxxxxx>
>> +:Date: 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.
>> +
>> +.. code-block:: sh
>> +
>> +	===== 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.

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