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

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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.

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.

-- 
Jani Nikula, Intel Open Source Technology Center
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