Based on kernel version 4.2. Page generated on 2015-09-09 12:15 EST.
1 2 Mini-HOWTO for using the earlyprintk=dbgp boot option with a 3 USB2 Debug port key and a debug cable, on x86 systems. 4 5 You need two computers, the 'USB debug key' special gadget and 6 and two USB cables, connected like this: 7 8 [host/target] <-------> [USB debug key] <-------> [client/console] 9 10 1. There are a number of specific hardware requirements: 11 12 a.) Host/target system needs to have USB debug port capability. 13 14 You can check this capability by looking at a 'Debug port' bit in 15 the lspci -vvv output: 16 17 # lspci -vvv 18 ... 19 00:1d.7 USB Controller: Intel Corporation 82801H (ICH8 Family) USB2 EHCI Controller #1 (rev 03) (prog-if 20 [EHCI]) 20 Subsystem: Lenovo ThinkPad T61 21 Control: I/O- Mem+ BusMaster+ SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR+ FastB2B- DisINTx- 22 Status: Cap+ 66MHz- UDF- FastB2B+ ParErr- DEVSEL=medium >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx- 23 Latency: 0 24 Interrupt: pin D routed to IRQ 19 25 Region 0: Memory at fe227000 (32-bit, non-prefetchable) [size=1K] 26 Capabilities:  Power Management version 2 27 Flags: PMEClk- DSI- D1- D2- AuxCurrent=375mA PME(D0+,D1-,D2-,D3hot+,D3cold+) 28 Status: D0 PME-Enable- DSel=0 DScale=0 PME+ 29 Capabilities:  Debug port: BAR=1 offset=00a0 30 ^^^^^^^^^^^ <==================== [ HERE ] 31 Kernel driver in use: ehci_hcd 32 Kernel modules: ehci-hcd 33 ... 34 35 ( If your system does not list a debug port capability then you probably 36 won't be able to use the USB debug key. ) 37 38 b.) You also need a Netchip USB debug cable/key: 39 40 http://www.plxtech.com/products/NET2000/NET20DC/default.asp 41 42 This is a small blue plastic connector with two USB connections, 43 it draws power from its USB connections. 44 45 c.) You need a second client/console system with a high speed USB 2.0 46 port. 47 48 d.) The Netchip device must be plugged directly into the physical 49 debug port on the "host/target" system. You cannot use a USB hub in 50 between the physical debug port and the "host/target" system. 51 52 The EHCI debug controller is bound to a specific physical USB 53 port and the Netchip device will only work as an early printk 54 device in this port. The EHCI host controllers are electrically 55 wired such that the EHCI debug controller is hooked up to the 56 first physical and there is no way to change this via software. 57 You can find the physical port through experimentation by trying 58 each physical port on the system and rebooting. Or you can try 59 and use lsusb or look at the kernel info messages emitted by the 60 usb stack when you plug a usb device into various ports on the 61 "host/target" system. 62 63 Some hardware vendors do not expose the usb debug port with a 64 physical connector and if you find such a device send a complaint 65 to the hardware vendor, because there is no reason not to wire 66 this port into one of the physically accessible ports. 67 68 e.) It is also important to note, that many versions of the Netchip 69 device require the "client/console" system to be plugged into the 70 right and side of the device (with the product logo facing up and 71 readable left to right). The reason being is that the 5 volt 72 power supply is taken from only one side of the device and it 73 must be the side that does not get rebooted. 74 75 2. Software requirements: 76 77 a.) On the host/target system: 78 79 You need to enable the following kernel config option: 80 81 CONFIG_EARLY_PRINTK_DBGP=y 82 83 And you need to add the boot command line: "earlyprintk=dbgp". 84 (If you are using Grub, append it to the 'kernel' line in 85 /etc/grub.conf) 86 87 On systems with more than one EHCI debug controller you must 88 specify the correct EHCI debug controller number. The ordering 89 comes from the PCI bus enumeration of the EHCI controllers. The 90 default with no number argument is "0" the first EHCI debug 91 controller. To use the second EHCI debug controller, you would 92 use the command line: "earlyprintk=dbgp1" 93 94 NOTE: normally earlyprintk console gets turned off once the 95 regular console is alive - use "earlyprintk=dbgp,keep" to keep 96 this channel open beyond early bootup. This can be useful for 97 debugging crashes under Xorg, etc. 98 99 b.) On the client/console system: 100 101 You should enable the following kernel config option: 102 103 CONFIG_USB_SERIAL_DEBUG=y 104 105 On the next bootup with the modified kernel you should 106 get a /dev/ttyUSBx device(s). 107 108 Now this channel of kernel messages is ready to be used: start 109 your favorite terminal emulator (minicom, etc.) and set 110 it up to use /dev/ttyUSB0 - or use a raw 'cat /dev/ttyUSBx' to 111 see the raw output. 112 113 c.) On Nvidia Southbridge based systems: the kernel will try to probe 114 and find out which port has debug device connected. 115 116 3. Testing that it works fine: 117 118 You can test the output by using earlyprintk=dbgp,keep and provoking 119 kernel messages on the host/target system. You can provoke a harmless 120 kernel message by for example doing: 121 122 echo h > /proc/sysrq-trigger 123 124 On the host/target system you should see this help line in "dmesg" output: 125 126 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) 127 128 On the client/console system do: 129 130 cat /dev/ttyUSB0 131 132 And you should see the help line above displayed shortly after you've 133 provoked it on the host system. 134 135 If it does not work then please ask about it on the email@example.com 136 mailing list or contact the x86 maintainers.