path: root/Documentation/x86/earlyprintk.txt
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authorYinghai Lu <yinghai@kernel.org>2009-03-04 16:11:35 -0800
committerIngo Molnar <mingo@elte.hu>2009-03-05 10:57:52 +0100
commita1aade478862fca8710904532cbc6efed97fd05a (patch)
treee857999367c0c911b609aa295f77564f9f3f5eb3 /Documentation/x86/earlyprintk.txt
parente56d0cfe7790fd3218ae4f6aae1335547fea8763 (diff)
x86/doc: mini-howto for using earlyprintk=dbgp
[ mingo: small edits and extensions. ] Signed-off-by: Yinghai Lu <yinghai@kernel.org> Cc: "Eric W. Biederman" <ebiederm@xmission.com> Cc: Greg KH <gregkh@suse.de> Cc: Randy Dunlap <randy.dunlap@oracle.com> Cc: Alan Stern <stern@rowland.harvard.edu> Cc: Sarah Sharp <sarah.a.sharp@intel.com> Cc: Andrew Morton <akpm@linux-foundation.org> LKML-Reference: <49AF18B7.4050305@kernel.org> Signed-off-by: Ingo Molnar <mingo@elte.hu>
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+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 three 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
+ wont 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.) Thirdly, you need a second client/console system with a regular USB port.
+2. Software requirements:
+ a.) On the host/target system:
+ You need to enable the following kernel config option:
+ 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)
+ 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:
+ 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 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.

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