+.. SPDX-License-Identifier: GPL-2.0
+
+============
+Early Printk
+============
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:
+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- <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
+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
- ...
+ Kernel driver in use: ehci_hcd
+ Kernel modules: ehci-hcd
+ ...
-( If your system does not list a debug port capability then you probably
- won't be able to use the USB debug key. )
+ .. note::
+ If your system does not list a debug port capability then you probably
+ won't be able to use the USB debug key.
- b.) You also need a NetChip USB debug cable/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.) You need a second client/console system with a high speed USB 2.0
- port.
+ c) You need a second client/console system with a high speed USB 2.0 port.
- d.) The NetChip device must be plugged directly into the physical
- debug port on the "host/target" system. You cannot use a USB hub in
+ d) The NetChip device must be plugged directly into the physical
+ debug port on the "host/target" system. You cannot use a USB hub in
between the physical debug port and the "host/target" system.
The EHCI debug controller is bound to a specific physical USB
to the hardware vendor, because there is no reason not to wire
this port into one of the physically accessible ports.
- e.) It is also important to note, that many versions of the NetChip
+ e) It is also important to note, that many versions of the NetChip
device require the "client/console" system to be plugged into the
right hand side of the device (with the product logo facing up and
readable left to right). The reason being is that the 5 volt
power supply is taken from only one side of the device and it
must be the side that does not get rebooted.
-2. Software requirements:
+Software requirements
+=====================
- a.) On the host/target system:
+ a) On the host/target system:
- You need to enable the following kernel config option:
+ You need to enable the following kernel config option::
CONFIG_EARLY_PRINTK_DBGP=y
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. If you are using Grub2 on a BIOS firmware system,
- append it to the 'linux' line in /boot/grub2/grub.cfg. If you are
- using Grub2 on an EFI firmware system, append it to the 'linux'
- or 'linuxefi' line in /boot/grub2/grub.cfg or
- /boot/efi/EFI/<distro>/grub.cfg.)
+ .. note::
+ If you are using Grub, append it to the 'kernel' line in
+ /etc/grub.conf. If you are using Grub2 on a BIOS firmware system,
+ append it to the 'linux' line in /boot/grub2/grub.cfg. If you are
+ using Grub2 on an EFI firmware system, append it to the 'linux'
+ or 'linuxefi' line in /boot/grub2/grub.cfg or
+ /boot/efi/EFI/<distro>/grub.cfg.
On systems with more than one EHCI debug controller you must
specify the correct EHCI debug controller number. The ordering
controller. To use the second EHCI debug controller, you would
use the command line: "earlyprintk=dbgp1"
- 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.
+ .. 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:
+ b) On the client/console system:
- You should enable the following kernel config option:
+ You should enable the following kernel config option::
CONFIG_USB_SERIAL_DEBUG=y
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
+ c) On Nvidia Southbridge based systems: the kernel will try to probe
and find out which port has a debug device connected.
-3. Testing that it works fine:
+Testing
+=======
- 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:
+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:
+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:
+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.
+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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