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1
2Mini-HOWTO for using the earlyprintk=dbgp boot option with a
3USB2 Debug port key and a debug cable, on x86 systems.
4
5You need two computers, the 'USB debug key' special gadget and
6and two USB cables, connected like this:
7
8 [host/target] <-------> [USB debug key] <-------> [client/console]
9
101. There are three specific hardware requirements:
11
12 a.) Host/target system needs to have USB debug port capability.
13
14 You can check this capability by looking at a 'Debug port' bit in
15 the lspci -vvv output:
16
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 ...
34
35( If your system does not list a debug port capability then you probably
36 wont be able to use the USB debug key. )
37
38 b.) You also need a Netchip USB debug cable/key:
39
40 http://www.plxtech.com/products/NET2000/NET20DC/default.asp
41
42 This is a small blue plastic connector with two USB connections,
43 it draws power from its USB connections.
44
45 c.) Thirdly, you need a second client/console system with a regular USB port.
46
472. Software requirements:
48
49 a.) On the host/target system:
50
51 You need to enable the following kernel config option:
52
53 CONFIG_EARLY_PRINTK_DBGP=y
54
55 And you need to add the boot command line: "earlyprintk=dbgp".
56 (If you are using Grub, append it to the 'kernel' line in
57 /etc/grub.conf)
58
59 NOTE: normally earlyprintk console gets turned off once the
60 regular console is alive - use "earlyprintk=dbgp,keep" to keep
61 this channel open beyond early bootup. This can be useful for
62 debugging crashes under Xorg, etc.
63
64 b.) On the client/console system:
65
66 You should enable the following kernel config option:
67
68 CONFIG_USB_SERIAL_DEBUG=y
69
70 On the next bootup with the modified kernel you should
71 get a /dev/ttyUSBx device(s).
72
73 Now this channel of kernel messages is ready to be used: start
74 your favorite terminal emulator (minicom, etc.) and set
75 it up to use /dev/ttyUSB0 - or use a raw 'cat /dev/ttyUSBx' to
76 see the raw output.
77
78 c.) On Nvidia Southbridge based systems: the kernel will try to probe
79 and find out which port has debug device connected.
80
813. Testing that it works fine:
82
83 You can test the output by using earlyprintk=dbgp,keep and provoking
84 kernel messages on the host/target system. You can provoke a harmless
85 kernel message by for example doing:
86
87 echo h > /proc/sysrq-trigger
88
89 On the host/target system you should see this help line in "dmesg" output:
90
91 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)
92
93 On the client/console system do:
94
95 cat /dev/ttyUSB0
96
97 And you should see the help line above displayed shortly after you've
98 provoked it on the host system.
99
100If it does not work then please ask about it on the linux-kernel@vger.kernel.org
101mailing list or contact the x86 maintainers.