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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 EST.

2	Mini-HOWTO for using the earlyprintk=dbgp boot option with a
3	USB2 Debug port key and a debug cable, on x86 systems.
5	You need two computers, the 'USB debug key' special gadget and
6	and two USB cables, connected like this:
8	  [host/target] <-------> [USB debug key] <-------> [client/console]
10	1. There are a number of specific hardware requirements:
12	 a.) Host/target system needs to have USB debug port capability.
14	 You can check this capability by looking at a 'Debug port' bit in
15	 the lspci -vvv output:
17	 # lspci -vvv
18	 ...
19	 00:1d.7 USB Controller: Intel Corporation 82801H (ICH8 Family) USB2 EHCI Controller #1 (rev 03) (prog-if 20 [EHCI])
20	         Subsystem: Lenovo ThinkPad T61
21	         Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx-
22	         Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
23	         Latency: 0
24	         Interrupt: pin D routed to IRQ 19
25	         Region 0: Memory at fe227000 (32-bit, non-prefetchable) [size=1K]
26	         Capabilities: [50] Power Management version 2
27	                 Flags: PMEClk- DSI- D1- D2- AuxCurrent=375mA PME(D0+,D1-,D2-,D3hot+,D3cold+)
28	                 Status: D0 PME-Enable- DSel=0 DScale=0 PME+
29	         Capabilities: [58] Debug port: BAR=1 offset=00a0
30	                            ^^^^^^^^^^^ <==================== [ HERE ]
31		 Kernel driver in use: ehci_hcd
32	         Kernel modules: ehci-hcd
33	 ...
35	( If your system does not list a debug port capability then you probably
36	  won't be able to use the USB debug key. )
38	 b.) You also need a Netchip USB debug cable/key:
40	        http://www.plxtech.com/products/NET2000/NET20DC/default.asp
42	     This is a small blue plastic connector with two USB connections,
43	     it draws power from its USB connections.
45	 c.) You need a second client/console system with a high speed USB 2.0
46	     port.
48	 d.) The Netchip device must be plugged directly into the physical
49	     debug port on the "host/target" system.  You cannot use a USB hub in
50	     between the physical debug port and the "host/target" system.
52	     The EHCI debug controller is bound to a specific physical USB
53	     port and the Netchip device will only work as an early printk
54	     device in this port.  The EHCI host controllers are electrically
55	     wired such that the EHCI debug controller is hooked up to the
56	     first physical and there is no way to change this via software.
57	     You can find the physical port through experimentation by trying
58	     each physical port on the system and rebooting.  Or you can try
59	     and use lsusb or look at the kernel info messages emitted by the
60	     usb stack when you plug a usb device into various ports on the
61	     "host/target" system.
63	     Some hardware vendors do not expose the usb debug port with a
64	     physical connector and if you find such a device send a complaint
65	     to the hardware vendor, because there is no reason not to wire
66	     this port into one of the physically accessible ports.
68	 e.) It is also important to note, that many versions of the Netchip
69	     device require the "client/console" system to be plugged into the
70	     right and side of the device (with the product logo facing up and
71	     readable left to right).  The reason being is that the 5 volt
72	     power supply is taken from only one side of the device and it
73	     must be the side that does not get rebooted.
75	2. Software requirements:
77	 a.) On the host/target system:
79	    You need to enable the following kernel config option:
83	    And you need to add the boot command line: "earlyprintk=dbgp".
84	    (If you are using Grub, append it to the 'kernel' line in
85	     /etc/grub.conf)
87	    On systems with more than one EHCI debug controller you must
88	    specify the correct EHCI debug controller number.  The ordering
89	    comes from the PCI bus enumeration of the EHCI controllers.  The
90	    default with no number argument is "0" the first EHCI debug
91	    controller.  To use the second EHCI debug controller, you would
92	    use the command line: "earlyprintk=dbgp1"
94	    NOTE: normally earlyprintk console gets turned off once the
95	    regular console is alive - use "earlyprintk=dbgp,keep" to keep
96	    this channel open beyond early bootup. This can be useful for
97	    debugging crashes under Xorg, etc.
99	 b.) On the client/console system:
101	    You should enable the following kernel config option:
105	    On the next bootup with the modified kernel you should
106	    get a /dev/ttyUSBx device(s).
108	    Now this channel of kernel messages is ready to be used: start
109	    your favorite terminal emulator (minicom, etc.) and set
110	    it up to use /dev/ttyUSB0 - or use a raw 'cat /dev/ttyUSBx' to
111	    see the raw output.
113	 c.) On Nvidia Southbridge based systems: the kernel will try to probe
114	     and find out which port has debug device connected.
116	3. Testing that it works fine:
118	   You can test the output by using earlyprintk=dbgp,keep and provoking
119	   kernel messages on the host/target system. You can provoke a harmless
120	   kernel message by for example doing:
122	     echo h > /proc/sysrq-trigger
124	   On the host/target system you should see this help line in "dmesg" output:
126	     SysRq : HELP : loglevel(0-9) reBoot Crashdump terminate-all-tasks(E) memory-full-oom-kill(F) kill-all-tasks(I) saK show-backtrace-all-active-cpus(L) show-memory-usage(M) nice-all-RT-tasks(N) powerOff show-registers(P) show-all-timers(Q) unRaw Sync show-task-states(T) Unmount show-blocked-tasks(W) dump-ftrace-buffer(Z)
128	   On the client/console system do:
130	       cat /dev/ttyUSB0
132	   And you should see the help line above displayed shortly after you've
133	   provoked it on the host system.
135	If it does not work then please ask about it on the linux-kernel@vger.kernel.org
136	mailing list or contact the x86 maintainers.
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