linux-next/Documentation/x86/earlyprintk.txt

Mini-HOWTO for using the earlyprintk=dbgp boot option with a
USB2 Debug port key and a debug cable, on x86 systems.

You need two computers, the 'USB debug key' special gadget and
and two USB cables, connected like this:

  [host/target] <-------> [USB debug key] <-------> [client/console]

1. There are a number of specific hardware requirements:

 a.) Host/target system needs to have USB debug port capability.

 You can check this capability by looking at a 'Debug port' bit in
 the lspci -vvv output:

 # lspci -vvv
 ...
 00:1d.7 USB Controller: Intel Corporation 82801H (ICH8 Family) USB2 EHCI Controller #1 (rev 03) (prog-if 20 [EHCI])
         Subsystem: Lenovo ThinkPad T61
         Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx-
         Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
         Latency: 0
         Interrupt: pin D routed to IRQ 19
         Region 0: Memory at fe227000 (32-bit, non-prefetchable) [size=1K]
         Capabilities: [50] Power Management version 2
                 Flags: PMEClk- DSI- D1- D2- AuxCurrent=375mA PME(D0+,D1-,D2-,D3hot+,D3cold+)
                 Status: D0 PME-Enable- DSel=0 DScale=0 PME+
         Capabilities: [58] Debug port: BAR=1 offset=00a0
                            ^^^^^^^^^^^ <==================== [ HERE ]
	 Kernel driver in use: ehci_hcd
         Kernel modules: ehci-hcd
 ...

( If your system does not list a debug port capability then you probably
  won't be able to use the USB debug key. )

 b.) You also need a NetChip USB debug cable/key:

        http://www.plxtech.com/products/NET2000/NET20DC/default.asp

     This is a small blue plastic connector with two USB connections;
     it draws power from its USB connections.

 c.) You need a second client/console system with a high speed USB 2.0
     port.

 d.) The NetChip device must be plugged directly into the physical
     debug port on the "host/target" system.  You cannot use a USB hub in
     between the physical debug port and the "host/target" system.

     The EHCI debug controller is bound to a specific physical USB
     port and the NetChip device will only work as an early printk
     device in this port.  The EHCI host controllers are electrically
     wired such that the EHCI debug controller is hooked up to the
     first physical port and there is no way to change this via software.
     You can find the physical port through experimentation by trying
     each physical port on the system and rebooting.  Or you can try
     and use lsusb or look at the kernel info messages emitted by the
     usb stack when you plug a usb device into various ports on the
     "host/target" system.

     Some hardware vendors do not expose the usb debug port with a
     physical connector and if you find such a device send a complaint
     to the hardware vendor, because there is no reason not to wire
     this port into one of the physically accessible ports.

 e.) It is also important to note, that many versions of the NetChip
     device require the "client/console" system to be plugged into the
     right hand side of the device (with the product logo facing up and
     readable left to right).  The reason being is that the 5 volt
     power supply is taken from only one side of the device and it
     must be the side that does not get rebooted.

2. Software requirements:

 a.) On the host/target system:

    You need to enable the following kernel config option:

      CONFIG_EARLY_PRINTK_DBGP=y

    And you need to add the boot command line: "earlyprintk=dbgp".

    (If you are using Grub, append it to the 'kernel' line in
     /etc/grub.conf.  If you are using Grub2 on a BIOS firmware system,
     append it to the 'linux' line in /boot/grub2/grub.cfg. If you are
     using Grub2 on an EFI firmware system, append it to the 'linux'
     or 'linuxefi' line in /boot/grub2/grub.cfg or
     /boot/efi/EFI/<distro>/grub.cfg.)

    On systems with more than one EHCI debug controller you must
    specify the correct EHCI debug controller number.  The ordering
    comes from the PCI bus enumeration of the EHCI controllers.  The
    default with no number argument is "0" or the first EHCI debug
    controller.  To use the second EHCI debug controller, you would
    use the command line: "earlyprintk=dbgp1"

    NOTE: normally earlyprintk console gets turned off once the
    regular console is alive - use "earlyprintk=dbgp,keep" to keep
    this channel open beyond early bootup. This can be useful for
    debugging crashes under Xorg, etc.

 b.) On the client/console system:

    You should enable the following kernel config option:

      CONFIG_USB_SERIAL_DEBUG=y

    On the next bootup with the modified kernel you should
    get a /dev/ttyUSBx device(s).

    Now this channel of kernel messages is ready to be used: start
    your favorite terminal emulator (minicom, etc.) and set
    it up to use /dev/ttyUSB0 - or use a raw 'cat /dev/ttyUSBx' to
    see the raw output.

 c.) On Nvidia Southbridge based systems: the kernel will try to probe
     and find out which port has a debug device connected.

3. Testing that it works fine:

   You can test the output by using earlyprintk=dbgp,keep and provoking
   kernel messages on the host/target system. You can provoke a harmless
   kernel message by for example doing:

     echo h > /proc/sysrq-trigger

   On the host/target system you should see this help line in "dmesg" output:

     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)

   On the client/console system do:

       cat /dev/ttyUSB0

   And you should see the help line above displayed shortly after you've
   provoked it on the host system.

If it does not work then please ask about it on the linux-kernel@vger.kernel.org
mailing list or contact the x86 maintainers.