1 2Mini-HOWTO for using the earlyprintk=dbgp boot option with a 3USB2 Debug port key and a debug cable, on x86 systems. 4 5You need two computers, the 'USB debug key' special gadget and 6and two USB cables, connected like this: 7 8 [host/target] <-------> [USB debug key] <-------> [client/console] 9 101. 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: [50] 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: [58] 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 port 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 hand 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. 74 752. 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 85 (If you are using Grub, append it to the 'kernel' line in 86 /etc/grub.conf. If you are using Grub2 on a BIOS firmware system, 87 append it to the 'linux' line in /boot/grub2/grub.cfg. If you are 88 using Grub2 on an EFI firmware system, append it to the 'linux' 89 or 'linuxefi' line in /boot/grub2/grub.cfg or 90 /boot/efi/EFI/<distro>/grub.cfg.) 91 92 On systems with more than one EHCI debug controller you must 93 specify the correct EHCI debug controller number. The ordering 94 comes from the PCI bus enumeration of the EHCI controllers. The 95 default with no number argument is "0" or the first EHCI debug 96 controller. To use the second EHCI debug controller, you would 97 use the command line: "earlyprintk=dbgp1" 98 99 NOTE: normally earlyprintk console gets turned off once the 100 regular console is alive - use "earlyprintk=dbgp,keep" to keep 101 this channel open beyond early bootup. This can be useful for 102 debugging crashes under Xorg, etc. 103 104 b.) On the client/console system: 105 106 You should enable the following kernel config option: 107 108 CONFIG_USB_SERIAL_DEBUG=y 109 110 On the next bootup with the modified kernel you should 111 get a /dev/ttyUSBx device(s). 112 113 Now this channel of kernel messages is ready to be used: start 114 your favorite terminal emulator (minicom, etc.) and set 115 it up to use /dev/ttyUSB0 - or use a raw 'cat /dev/ttyUSBx' to 116 see the raw output. 117 118 c.) On Nvidia Southbridge based systems: the kernel will try to probe 119 and find out which port has a debug device connected. 120 1213. Testing that it works fine: 122 123 You can test the output by using earlyprintk=dbgp,keep and provoking 124 kernel messages on the host/target system. You can provoke a harmless 125 kernel message by for example doing: 126 127 echo h > /proc/sysrq-trigger 128 129 On the host/target system you should see this help line in "dmesg" output: 130 131 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) 132 133 On the client/console system do: 134 135 cat /dev/ttyUSB0 136 137 And you should see the help line above displayed shortly after you've 138 provoked it on the host system. 139 140If it does not work then please ask about it on the linux-kernel@vger.kernel.org 141mailing list or contact the x86 maintainers. 142
