Merge branch 'x86-debug-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 debug updates from Ingo Molnar:
 "The biggest update is the addition of USB3 debug port based
  early-console.

  Greg was fine with the USB changes and with the routing of these
  patches:

    https://www.spinics.net/lists/linux-usb/msg155093.html"

* 'x86-debug-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  usb/doc: Add document for USB3 debug port usage
  usb/serial: Add DBC debug device support to usb_debug
  x86/earlyprintk: Add support for earlyprintk via USB3 debug port
  usb/early: Add driver for xhci debug capability
  x86/timers: Add simple udelay calibration

2 files changed, 101 insertions, 0 deletions

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index ba294509cd7e..8d676d2a48ac 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -988,6 +988,7 @@
 			earlyprintk=ttySn[,baudrate]
 			earlyprintk=dbgp[debugController#]
 			earlyprintk=pciserial,bus:device.function[,baudrate]
+			earlyprintk=xdbc[xhciController#]
 
 			earlyprintk is useful when the kernel crashes before
 			the normal console is initialized. It is not enabled by
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 ===============