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-
-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:
-
- [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
- ^^^^^^^^^^^ <==================== [ HERE ]
- Kernel driver in use: ehci_hcd
- Kernel modules: ehci-hcd
- ...
-
-( If your system does not list a debug port capability then you probably
- wont 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 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 and 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:
-
- 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".
- (If you are using Grub, append it to the 'kernel' line in
- /etc/grub.conf)
-
- 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" 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 debug device connected.
-
-3. Testing that it works fine:
-
- 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.