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151 lines
6.2 KiB
151 lines
6.2 KiB
.. SPDX-License-Identifier: GPL-2.0 |
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============ |
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Early Printk |
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============ |
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Mini-HOWTO for using the earlyprintk=dbgp boot option with a |
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USB2 Debug port key and a debug cable, on x86 systems. |
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You need two computers, the 'USB debug key' special gadget and |
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two USB cables, connected like this:: |
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[host/target] <-------> [USB debug key] <-------> [client/console] |
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Hardware requirements |
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===================== |
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a) Host/target system needs to have USB debug port capability. |
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You can check this capability by looking at a 'Debug port' bit in |
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the lspci -vvv output:: |
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# lspci -vvv |
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... |
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00:1d.7 USB Controller: Intel Corporation 82801H (ICH8 Family) USB2 EHCI Controller #1 (rev 03) (prog-if 20 [EHCI]) |
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Subsystem: Lenovo ThinkPad T61 |
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Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx- |
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Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx- |
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Latency: 0 |
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Interrupt: pin D routed to IRQ 19 |
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Region 0: Memory at fe227000 (32-bit, non-prefetchable) [size=1K] |
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Capabilities: [50] Power Management version 2 |
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Flags: PMEClk- DSI- D1- D2- AuxCurrent=375mA PME(D0+,D1-,D2-,D3hot+,D3cold+) |
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Status: D0 PME-Enable- DSel=0 DScale=0 PME+ |
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Capabilities: [58] Debug port: BAR=1 offset=00a0 |
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^^^^^^^^^^^ <==================== [ HERE ] |
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Kernel driver in use: ehci_hcd |
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Kernel modules: ehci-hcd |
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... |
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.. note:: |
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If your system does not list a debug port capability then you probably |
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won't be able to use the USB debug key. |
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b) You also need a NetChip USB debug cable/key: |
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http://www.plxtech.com/products/NET2000/NET20DC/default.asp |
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This is a small blue plastic connector with two USB connections; |
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it draws power from its USB connections. |
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c) You need a second client/console system with a high speed USB 2.0 port. |
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d) The NetChip device must be plugged directly into the physical |
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debug port on the "host/target" system. You cannot use a USB hub in |
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between the physical debug port and the "host/target" system. |
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The EHCI debug controller is bound to a specific physical USB |
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port and the NetChip device will only work as an early printk |
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device in this port. The EHCI host controllers are electrically |
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wired such that the EHCI debug controller is hooked up to the |
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first physical port and there is no way to change this via software. |
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You can find the physical port through experimentation by trying |
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each physical port on the system and rebooting. Or you can try |
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and use lsusb or look at the kernel info messages emitted by the |
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usb stack when you plug a usb device into various ports on the |
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"host/target" system. |
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Some hardware vendors do not expose the usb debug port with a |
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physical connector and if you find such a device send a complaint |
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to the hardware vendor, because there is no reason not to wire |
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this port into one of the physically accessible ports. |
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e) It is also important to note, that many versions of the NetChip |
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device require the "client/console" system to be plugged into the |
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right hand side of the device (with the product logo facing up and |
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readable left to right). The reason being is that the 5 volt |
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power supply is taken from only one side of the device and it |
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must be the side that does not get rebooted. |
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Software requirements |
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===================== |
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a) On the host/target system: |
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You need to enable the following kernel config option:: |
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CONFIG_EARLY_PRINTK_DBGP=y |
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And you need to add the boot command line: "earlyprintk=dbgp". |
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.. note:: |
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If you are using Grub, append it to the 'kernel' line in |
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/etc/grub.conf. If you are using Grub2 on a BIOS firmware system, |
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append it to the 'linux' line in /boot/grub2/grub.cfg. If you are |
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using Grub2 on an EFI firmware system, append it to the 'linux' |
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or 'linuxefi' line in /boot/grub2/grub.cfg or |
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/boot/efi/EFI/<distro>/grub.cfg. |
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On systems with more than one EHCI debug controller you must |
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specify the correct EHCI debug controller number. The ordering |
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comes from the PCI bus enumeration of the EHCI controllers. The |
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default with no number argument is "0" or the first EHCI debug |
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controller. To use the second EHCI debug controller, you would |
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use the command line: "earlyprintk=dbgp1" |
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.. note:: |
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normally earlyprintk console gets turned off once the |
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regular console is alive - use "earlyprintk=dbgp,keep" to keep |
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this channel open beyond early bootup. This can be useful for |
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debugging crashes under Xorg, etc. |
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b) On the client/console system: |
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You should enable the following kernel config option:: |
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CONFIG_USB_SERIAL_DEBUG=y |
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On the next bootup with the modified kernel you should |
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get a /dev/ttyUSBx device(s). |
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Now this channel of kernel messages is ready to be used: start |
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your favorite terminal emulator (minicom, etc.) and set |
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it up to use /dev/ttyUSB0 - or use a raw 'cat /dev/ttyUSBx' to |
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see the raw output. |
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c) On Nvidia Southbridge based systems: the kernel will try to probe |
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and find out which port has a debug device connected. |
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Testing |
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======= |
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You can test the output by using earlyprintk=dbgp,keep and provoking |
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kernel messages on the host/target system. You can provoke a harmless |
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kernel message by for example doing:: |
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echo h > /proc/sysrq-trigger |
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On the host/target system you should see this help line in "dmesg" output:: |
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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) |
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On the client/console system do:: |
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cat /dev/ttyUSB0 |
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And you should see the help line above displayed shortly after you've |
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provoked it on the host system. |
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If it does not work then please ask about it on the [email protected] |
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mailing list or contact the x86 maintainers.
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