earlyprintk.rst 6.2 KB

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  1. .. SPDX-License-Identifier: GPL-2.0
  2. ============
  3. Early Printk
  4. ============
  5. Mini-HOWTO for using the earlyprintk=dbgp boot option with a
  6. USB2 Debug port key and a debug cable, on x86 systems.
  7. You need two computers, the 'USB debug key' special gadget and
  8. two USB cables, connected like this::
  9. [host/target] <-------> [USB debug key] <-------> [client/console]
  10. Hardware requirements
  11. =====================
  12. a) Host/target system needs to have USB debug port capability.
  13. You can check this capability by looking at a 'Debug port' bit in
  14. the lspci -vvv output::
  15. # lspci -vvv
  16. ...
  17. 00:1d.7 USB Controller: Intel Corporation 82801H (ICH8 Family) USB2 EHCI Controller #1 (rev 03) (prog-if 20 [EHCI])
  18. Subsystem: Lenovo ThinkPad T61
  19. Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx-
  20. Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
  21. Latency: 0
  22. Interrupt: pin D routed to IRQ 19
  23. Region 0: Memory at fe227000 (32-bit, non-prefetchable) [size=1K]
  24. Capabilities: [50] Power Management version 2
  25. Flags: PMEClk- DSI- D1- D2- AuxCurrent=375mA PME(D0+,D1-,D2-,D3hot+,D3cold+)
  26. Status: D0 PME-Enable- DSel=0 DScale=0 PME+
  27. Capabilities: [58] Debug port: BAR=1 offset=00a0
  28. ^^^^^^^^^^^ <==================== [ HERE ]
  29. Kernel driver in use: ehci_hcd
  30. Kernel modules: ehci-hcd
  31. ...
  32. .. note::
  33. If your system does not list a debug port capability then you probably
  34. won't be able to use the USB debug key.
  35. b) You also need a NetChip USB debug cable/key:
  36. http://www.plxtech.com/products/NET2000/NET20DC/default.asp
  37. This is a small blue plastic connector with two USB connections;
  38. it draws power from its USB connections.
  39. c) You need a second client/console system with a high speed USB 2.0 port.
  40. d) The NetChip device must be plugged directly into the physical
  41. debug port on the "host/target" system. You cannot use a USB hub in
  42. between the physical debug port and the "host/target" system.
  43. The EHCI debug controller is bound to a specific physical USB
  44. port and the NetChip device will only work as an early printk
  45. device in this port. The EHCI host controllers are electrically
  46. wired such that the EHCI debug controller is hooked up to the
  47. first physical port and there is no way to change this via software.
  48. You can find the physical port through experimentation by trying
  49. each physical port on the system and rebooting. Or you can try
  50. and use lsusb or look at the kernel info messages emitted by the
  51. usb stack when you plug a usb device into various ports on the
  52. "host/target" system.
  53. Some hardware vendors do not expose the usb debug port with a
  54. physical connector and if you find such a device send a complaint
  55. to the hardware vendor, because there is no reason not to wire
  56. this port into one of the physically accessible ports.
  57. e) It is also important to note, that many versions of the NetChip
  58. device require the "client/console" system to be plugged into the
  59. right hand side of the device (with the product logo facing up and
  60. readable left to right). The reason being is that the 5 volt
  61. power supply is taken from only one side of the device and it
  62. must be the side that does not get rebooted.
  63. Software requirements
  64. =====================
  65. a) On the host/target system:
  66. You need to enable the following kernel config option::
  67. CONFIG_EARLY_PRINTK_DBGP=y
  68. And you need to add the boot command line: "earlyprintk=dbgp".
  69. .. note::
  70. If you are using Grub, append it to the 'kernel' line in
  71. /etc/grub.conf. If you are using Grub2 on a BIOS firmware system,
  72. append it to the 'linux' line in /boot/grub2/grub.cfg. If you are
  73. using Grub2 on an EFI firmware system, append it to the 'linux'
  74. or 'linuxefi' line in /boot/grub2/grub.cfg or
  75. /boot/efi/EFI/<distro>/grub.cfg.
  76. On systems with more than one EHCI debug controller you must
  77. specify the correct EHCI debug controller number. The ordering
  78. comes from the PCI bus enumeration of the EHCI controllers. The
  79. default with no number argument is "0" or the first EHCI debug
  80. controller. To use the second EHCI debug controller, you would
  81. use the command line: "earlyprintk=dbgp1"
  82. .. note::
  83. normally earlyprintk console gets turned off once the
  84. regular console is alive - use "earlyprintk=dbgp,keep" to keep
  85. this channel open beyond early bootup. This can be useful for
  86. debugging crashes under Xorg, etc.
  87. b) On the client/console system:
  88. You should enable the following kernel config option::
  89. CONFIG_USB_SERIAL_DEBUG=y
  90. On the next bootup with the modified kernel you should
  91. get a /dev/ttyUSBx device(s).
  92. Now this channel of kernel messages is ready to be used: start
  93. your favorite terminal emulator (minicom, etc.) and set
  94. it up to use /dev/ttyUSB0 - or use a raw 'cat /dev/ttyUSBx' to
  95. see the raw output.
  96. c) On Nvidia Southbridge based systems: the kernel will try to probe
  97. and find out which port has a debug device connected.
  98. Testing
  99. =======
  100. You can test the output by using earlyprintk=dbgp,keep and provoking
  101. kernel messages on the host/target system. You can provoke a harmless
  102. kernel message by for example doing::
  103. echo h > /proc/sysrq-trigger
  104. On the host/target system you should see this help line in "dmesg" output::
  105. 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)
  106. On the client/console system do::
  107. cat /dev/ttyUSB0
  108. And you should see the help line above displayed shortly after you've
  109. provoked it on the host system.
  110. If it does not work then please ask about it on the linux-kernel@vger.kernel.org
  111. mailing list or contact the x86 maintainers.