earlyprintk.rst

  1.. SPDX-License-Identifier: GPL-2.0
  2
  3============
  4Early Printk
  5============
  6
  7Mini-HOWTO for using the earlyprintk=dbgp boot option with a
  8USB2 Debug port key and a debug cable, on x86 systems.
  9
 10You need two computers, the 'USB debug key' special gadget and
 11two USB cables, connected like this::
 12
 13  [host/target] <-------> [USB debug key] <-------> [client/console]
 14
 15Hardware requirements
 16=====================
 17
 18  a) Host/target system needs to have USB debug port capability.
 19
 20     You can check this capability by looking at a 'Debug port' bit in
 21     the lspci -vvv output::
 22
 23       # lspci -vvv
 24       ...
 25       00:1d.7 USB Controller: Intel Corporation 82801H (ICH8 Family) USB2 EHCI Controller #1 (rev 03) (prog-if 20 [EHCI])
 26               Subsystem: Lenovo ThinkPad T61
 27               Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx-
 28               Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
 29               Latency: 0
 30               Interrupt: pin D routed to IRQ 19
 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+
 35               Capabilities: [58] Debug port: BAR=1 offset=00a0
 36                            ^^^^^^^^^^^ <==================== [ HERE ]
 37               Kernel driver in use: ehci_hcd
 38               Kernel modules: ehci-hcd
 39       ...
 40
 41     .. note::
 42       If your system does not list a debug port capability then you probably
 43       won't be able to use the USB debug key.
 44
 45  b) You also need a NetChip USB debug cable/key:
 46
 47        http://www.plxtech.com/products/NET2000/NET20DC/default.asp
 48
 49     This is a small blue plastic connector with two USB connections;
 50     it draws power from its USB connections.
 51
 52  c) You need a second client/console system with a high speed USB 2.0 port.
 53
 54  d) The NetChip device must be plugged directly into the physical
 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.
 57
 58     The EHCI debug controller is bound to a specific physical USB
 59     port and the NetChip device will only work as an early printk
 60     device in this port.  The EHCI host controllers are electrically
 61     wired such that the EHCI debug controller is hooked up to the
 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
 65     and use lsusb or look at the kernel info messages emitted by the
 66     usb stack when you plug a usb device into various ports on the
 67     "host/target" system.
 68
 69     Some hardware vendors do not expose the usb debug port with a
 70     physical connector and if you find such a device send a complaint
 71     to the hardware vendor, because there is no reason not to wire
 72     this port into one of the physically accessible ports.
 73
 74  e) It is also important to note, that many versions of the NetChip
 75     device require the "client/console" system to be plugged into the
 76     right hand side of the device (with the product logo facing up and
 77     readable left to right).  The reason being is that the 5 volt
 78     power supply is taken from only one side of the device and it
 79     must be the side that does not get rebooted.
 80
 81Software requirements
 82=====================
 83
 84  a) On the host/target system:
 85
 86    You need to enable the following kernel config option::
 87
 88      CONFIG_EARLY_PRINTK_DBGP=y
 89
 90    And you need to add the boot command line: "earlyprintk=dbgp".
 91
 92    .. note::
 93      If you are using Grub, append it to the 'kernel' line in
 94      /etc/grub.conf.  If you are using Grub2 on a BIOS firmware system,
 95      append it to the 'linux' line in /boot/grub2/grub.cfg. If you are
 96      using Grub2 on an EFI firmware system, append it to the 'linux'
 97      or 'linuxefi' line in /boot/grub2/grub.cfg or
 98      /boot/efi/EFI/<distro>/grub.cfg.
 99
100    On systems with more than one EHCI debug controller you must
101    specify the correct EHCI debug controller number.  The ordering
102    comes from the PCI bus enumeration of the EHCI controllers.  The
103    default with no number argument is "0" or the first EHCI debug
104    controller.  To use the second EHCI debug controller, you would
105    use the command line: "earlyprintk=dbgp1"
106
107    .. note::
108      normally earlyprintk console gets turned off once the
109      regular console is alive - use "earlyprintk=dbgp,keep" to keep
110      this channel open beyond early bootup. This can be useful for
111      debugging crashes under Xorg, etc.
112
113  b) On the client/console system:
114
115    You should enable the following kernel config option::
116
117      CONFIG_USB_SERIAL_DEBUG=y
118
119    On the next bootup with the modified kernel you should
120    get a /dev/ttyUSBx device(s).
121
122    Now this channel of kernel messages is ready to be used: start
123    your favorite terminal emulator (minicom, etc.) and set
124    it up to use /dev/ttyUSB0 - or use a raw 'cat /dev/ttyUSBx' to
125    see the raw output.
126
127  c) On Nvidia Southbridge based systems: the kernel will try to probe
128     and find out which port has a debug device connected.
129
130Testing
131=======
132
133You can test the output by using earlyprintk=dbgp,keep and provoking
134kernel messages on the host/target system. You can provoke a harmless
135kernel message by for example doing::
136
137     echo h > /proc/sysrq-trigger
138
139On the host/target system you should see this help line in "dmesg" output::
140
141     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)
142
143On the client/console system do::
144
145       cat /dev/ttyUSB0
146
147And you should see the help line above displayed shortly after you've
148provoked it on the host system.
149
150If it does not work then please ask about it on the linux-kernel@vger.kernel.org
151mailing list or contact the x86 maintainers.