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- SERR- /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.