arch/x86/earlyprintk.rst

1 .. SPDX-License-Identifier: GPL-2.0
7 Mini-HOWTO for using the earlyprintk=dbgp boot option with a
13   [host/target] <-------> [USB debug key] <-------> [client/console]
18   a) Host/target system needs to have USB debug port capability.
21      the lspci -vvv output::
23        # lspci -vvv
25 …00:1d.7 USB Controller: Intel Corporation 82801H (ICH8 Family) USB2 EHCI Controller #1 (rev 03) (p…
27 …Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisIN…
28 …Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- I…
31                Region 0: Memory at fe227000 (32-bit, non-prefetchable) [size=1K]
32                Capabilities: [50] Power Management version 2
33                        Flags: PMEClk- DSI- D1- D2- AuxCurrent=375mA PME(D0+,D1-,D2-,D3hot+,D3cold+)
34                        Status: D0 PME-Enable- DSel=0 DScale=0 PME+
38                Kernel modules: ehci-hcd
42        If your system does not list a debug port capability then you probably
50      it draws power from its USB connections.
52   c) You need a second client/console system with a high speed USB 2.0 port.
55      debug port on the "host/target" system. You cannot use a USB hub in
56      between the physical debug port and the "host/target" system.
58      The EHCI debug controller is bound to a specific physical USB
61      wired such that the EHCI debug controller is hooked up to the
64      each physical port on the system and rebooting.  Or you can try
67      "host/target" system.
75      device require the "client/console" system to be plugged into the
78      power supply is taken from only one side of the device and it
84   a) On the host/target system:
94       /etc/grub.conf.  If you are using Grub2 on a BIOS firmware system,
96       using Grub2 on an EFI firmware system, append it to the 'linux'
100     On systems with more than one EHCI debug controller you must
101     specify the correct EHCI debug controller number.  The ordering
104     controller.  To use the second EHCI debug controller, you would
109       regular console is alive - use "earlyprintk=dbgp,keep" to keep
113   b) On the client/console system:
124     it up to use /dev/ttyUSB0 - or use a raw 'cat /dev/ttyUSBx' to
134 kernel messages on the host/target system. You can provoke a harmless
137      echo h > /proc/sysrq-trigger
139 On the host/target system you should see this help line in "dmesg" output::
141 …-9) reBoot Crashdump terminate-all-tasks(E) memory-full-oom-kill(F) kill-all-tasks(I) saK show-bac…
143 On the client/console system do::
148 provoked it on the host system.
150 If it does not work then please ask about it on the linux-kernel@vger.kernel.org