arch/x86/earlyprintk.rst

1 .. SPDX-License-Identifier: GPL-2.0
7 Mini-HOWTO for using the earlyprintk=dbgp boot option with a
8 USB2 Debug port key and a debug cable, on x86 systems.
13   [host/target] <-------> [USB debug key] <-------> [client/console]
18   a) Host/target system needs to have USB debug port capability.
20      You can check this capability by looking at a 'Debug port' bit in
21      the lspci -vvv output::
23        # lspci -vvv
25 …roller: Intel Corporation 82801H (ICH8 Family) USB2 EHCI Controller #1 (rev 03) (prog-if 20 [EHCI])
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]
33                        Flags: PMEClk- DSI- D1- D2- AuxCurrent=375mA PME(D0+,D1-,D2-,D3hot+,D3cold+)
34                        Status: D0 PME-Enable- DSel=0 DScale=0 PME+
35                Capabilities: [58] Debug port: BAR=1 offset=00a0
38                Kernel modules: ehci-hcd
42        If your system does not list a debug port capability then you probably
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.
59      port and the NetChip device will only work as an early printk
60      device in this port.  The EHCI host controllers are electrically
62      first physical port and there is no way to change this via software.
63      You can find the physical port through experimentation by trying
64      each physical port on the system and rebooting.  Or you can try
67      "host/target" system.
69      Some hardware vendors do not expose the usb debug port with a
72      this port into one of the physically accessible ports.
84   a) On the host/target system:
109       regular console is alive - use "earlyprintk=dbgp,keep" to keep
124     it up to use /dev/ttyUSB0 - or use a raw 'cat /dev/ttyUSBx' to
128      and find out which port has a debug device connected.
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…
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