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authorLinus Torvalds <torvalds@linux-foundation.org>2017-05-02 08:00:21 +0200
committerLinus Torvalds <torvalds@linux-foundation.org>2017-05-02 08:00:21 +0200
commit7d6a31c394722c914c61292b457e1999c8bef5ea (patch)
tree3e7e1544d95a7839e651dd5736bb5b325b067c7d /Documentation
parentMerge branch 'x86-cleanups-for-linus' of git://git.kernel.org/pub/scm/linux/k... (diff)
parentusb/doc: Add document for USB3 debug port usage (diff)
downloadlinux-7d6a31c394722c914c61292b457e1999c8bef5ea.tar.xz
linux-7d6a31c394722c914c61292b457e1999c8bef5ea.zip
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
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/admin-guide/kernel-parameters.txt1
-rw-r--r--Documentation/usb/usb3-debug-port.rst100
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 ===============