linux/Documentation/x86/earlyprintk.txt
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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.
 102