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diff --git a/Documentation/usb/usb3-debug-port.rst b/Documentation/usb/usb3-debug-port.rst new file mode 100644 index 000000000000..feb1a36a65b7 --- /dev/null +++ b/Documentation/usb/usb3-debug-port.rst @@ -0,0 +1,100 @@ +=============== +USB3 debug port +=============== + +:Author: Lu Baolu <baolu.lu@linux.intel.com> +:Date: March 2017 + +GENERAL +======= + +This is a HOWTO for using the USB3 debug port on x86 systems. + +Before using any kernel debugging functionality based on USB3 +debug port, you need to:: + + 1) check whether any USB3 debug port is available in + your system; + 2) check which port is used for debugging purposes; + 3) have a USB 3.0 super-speed A-to-A debugging cable. + +INTRODUCTION +============ + +The xHCI debug capability (DbC) is an optional but standalone +functionality provided by the xHCI host controller. The xHCI +specification describes DbC in the section 7.6. + +When DbC is initialized and enabled, it will present a debug +device through the debug port (normally the first USB3 +super-speed port). The debug device is fully compliant with +the USB framework and provides the equivalent of a very high +performance full-duplex serial link between the debug target +(the system under debugging) and a debug host. + +EARLY PRINTK +============ + +DbC has been designed to log early printk messages. One use for +this feature is kernel debugging. For example, when your machine +crashes very early before the regular console code is initialized. +Other uses include simpler, lockless logging instead of a full- +blown printk console driver and klogd. + +On the debug target system, you need to customize a debugging +kernel with CONFIG_EARLY_PRINTK_USB_XDBC enabled. And, add below +kernel boot parameter:: + + "earlyprintk=xdbc" + +If there are multiple xHCI controllers in your system, you can +append a host contoller index to this kernel parameter. This +index starts from 0. + +Current design doesn't support DbC runtime suspend/resume. As +the result, you'd better disable runtime power management for +USB subsystem by adding below kernel boot parameter:: + + "usbcore.autosuspend=-1" + +Before starting the debug target, you should connect the debug +port to a USB port (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 the debug target, DbC will be detected and +initialized. After initialization, the debug host should be able +to enumerate the debug device in debug target. The debug host +will then bind the debug device with the usb_debug driver module +and create the /dev/ttyUSB device. + +If the debug device enumeration goes smoothly, you should be able +to see below kernel messages on the 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 use any communication program, for example minicom, to +read and view the messages. Below simple bash scripts can help +you to check the sanity of the setup. + +.. code-block:: sh + + ===== start of bash scripts ============= + #!/bin/bash + + while true ; do + while [ ! -d /sys/class/tty/ttyUSB0 ] ; do + : + done + cat /dev/ttyUSB0 + done + ===== end of bash scripts =============== |