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diff --git a/Documentation/x86/earlyprintk.rst b/Documentation/x86/earlyprintk.rst new file mode 100644 index 000000000..51ef11e8f --- /dev/null +++ b/Documentation/x86/earlyprintk.rst @@ -0,0 +1,151 @@ +.. 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. |