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 a number of 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.) You need a second client/console system with a high speed USB 2.0
  46     port.
  47
  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.
  51
  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.
  62
  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.
  67
  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.
  74
  752. Software requirements:
  76
  77 a.) On the host/target system:
  78
  79    You need to enable the following kernel config option:
  80
  81      CONFIG_EARLY_PRINTK_DBGP=y
  82
  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)
  86
  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"
  93
  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.
  98
  99 b.) On the client/console system:
 100
 101    You should enable the following kernel config option:
 102
 103      CONFIG_USB_SERIAL_DEBUG=y
 104
 105    On the next bootup with the modified kernel you should
 106    get a /dev/ttyUSBx device(s).
 107
 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.
 112
 113 c.) On Nvidia Southbridge based systems: the kernel will try to probe
 114     and find out which port has debug device connected.
 115
 1163. Testing that it works fine:
 117
 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:
 121
 122     echo h > /proc/sysrq-trigger
 123
 124   On the host/target system you should see this help line in "dmesg" output:
 125
 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)
 127
 128   On the client/console system do:
 129
 130       cat /dev/ttyUSB0
 131
 132   And you should see the help line above displayed shortly after you've
 133   provoked it on the host system.
 134
 135If it does not work then please ask about it on the linux-kernel@vger.kernel.org
 136mailing list or contact the x86 maintainers.
 137
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