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Commit 90b2a01e authored by Lu Baolu's avatar Lu Baolu Committed by Jonathan Corbet
Browse files

usb: doc: Update document for USB3 debug port usage 



Update Documentation/driver-api/usb/usb3-debug-port.rst. This update
includes the guide for using xHCI debug capability based TTY serial
link.

Cc: Mathias Nyman <mathias.nyman@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: default avatarLu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
parent 481f2720

Documentation/driver-api/usb/usb3-debug-port.rst

+52 −0
Original line number Diff line number Diff line
@@ -98,3 +98,55 @@ you to check the sanity of the setup. 
	cat /dev/ttyUSB0

	done

	===== end of bash scripts ===============



Serial TTY

==========



The DbC support has been added to the xHCI driver. You can get a

debug device provided by the DbC at runtime.



In order to use this, you need to make sure your kernel has been

configured to support USB_XHCI_DBGCAP. A sysfs attribute under

the xHCI device node is used to enable or disable DbC. By default,

DbC is disabled::



	root@target:/sys/bus/pci/devices/0000:00:14.0# cat dbc

	disabled



Enable DbC with the following command::



	root@target:/sys/bus/pci/devices/0000:00:14.0# echo enable > dbc



You can check the DbC state at anytime::



	root@target:/sys/bus/pci/devices/0000:00:14.0# cat dbc

	enabled



Connect the debug target to the debug host with a USB 3.0 super-

speed A-to-A debugging cable. You can see /dev/ttyDBC0 created

on the debug target. You will see below kernel message lines::



	root@target: tail -f /var/log/kern.log

	[  182.730103] xhci_hcd 0000:00:14.0: DbC connected

	[  191.169420] xhci_hcd 0000:00:14.0: DbC configured

	[  191.169597] xhci_hcd 0000:00:14.0: DbC now attached to /dev/ttyDBC0



Accordingly, the DbC state has been brought up to::



	root@target:/sys/bus/pci/devices/0000:00:14.0# cat dbc

	configured



On the debug host, you will see the debug device has been enumerated.

You will see below kernel message lines::



	root@host: tail -f /var/log/kern.log

	[   79.454780] usb 2-2.1: new SuperSpeed USB device number 3 using xhci_hcd

	[   79.475003] usb 2-2.1: LPM exit latency is zeroed, disabling LPM.

	[   79.475389] usb 2-2.1: New USB device found, idVendor=1d6b, idProduct=0010

	[   79.475390] usb 2-2.1: New USB device strings: Mfr=1, Product=2, SerialNumber=3

	[   79.475391] usb 2-2.1: Product: Linux USB Debug Target

	[   79.475392] usb 2-2.1: Manufacturer: Linux Foundation

	[   79.475393] usb 2-2.1: SerialNumber: 0001



The debug device works now. You can use any communication or debugging

program to talk between the host and the target.