mirror of
https://mirrors.bfsu.edu.cn/git/linux.git
synced 2024-12-05 10:04:12 +08:00
bfbc7e5c92
Drop the doubled word "and". Signed-off-by: Randy Dunlap <rdunlap@infradead.org> Cc: Jonathan Corbet <corbet@lwn.net> Cc: linux-doc@vger.kernel.org Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Ingo Molnar <mingo@redhat.com> Cc: Borislav Petkov <bp@alien8.de> Cc: x86@kernel.org Cc: "H. Peter Anvin" <hpa@zytor.com> Link: https://lore.kernel.org/r/20200703213107.30758-2-rdunlap@infradead.org Signed-off-by: Jonathan Corbet <corbet@lwn.net>
152 lines
6.2 KiB
ReStructuredText
152 lines
6.2 KiB
ReStructuredText
.. 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
|
|
two USB cables, connected like this::
|
|
|
|
[host/target] <-------> [USB debug key] <-------> [client/console]
|
|
|
|
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
|
|
...
|
|
|
|
.. 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:
|
|
|
|
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.
|
|
|
|
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
|
|
port and the NetChip device will only work as an early printk
|
|
device in this port. The EHCI host controllers are electrically
|
|
wired such that the EHCI debug controller is hooked up to the
|
|
first physical port and there is no way to change this via software.
|
|
You can find the physical port through experimentation by trying
|
|
each physical port on the system and rebooting. Or you can try
|
|
and use lsusb or look at the kernel info messages emitted by the
|
|
usb stack when you plug a usb device into various ports on the
|
|
"host/target" system.
|
|
|
|
Some hardware vendors do not expose the usb debug port with a
|
|
physical connector and if you find such a device send a complaint
|
|
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
|
|
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.
|
|
|
|
Software requirements
|
|
=====================
|
|
|
|
a) On the host/target system:
|
|
|
|
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".
|
|
|
|
.. 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
|
|
comes from the PCI bus enumeration of the EHCI controllers. The
|
|
default with no number argument is "0" or the first EHCI debug
|
|
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.
|
|
|
|
b) On the client/console system:
|
|
|
|
You should enable the following kernel config option::
|
|
|
|
CONFIG_USB_SERIAL_DEBUG=y
|
|
|
|
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 a debug device connected.
|
|
|
|
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::
|
|
|
|
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.
|