1 .. SPDX-License-Identifier: GPL-2.0
2
3 ============
4 Early Printk
5 ============
6
7 Mini-HOWTO for using the earlyprintk=dbgp boot
8 USB2 Debug port key and a debug cable, on x86
9
10 You need two computers, the 'USB debug key' sp
11 two USB cables, connected like this::
12
13 [host/target] <-------> [USB debug key] <---
14
15 Hardware requirements
16 =====================
17
18 a) Host/target system needs to have USB debu
19
20 You can check this capability by looking
21 the lspci -vvv output::
22
23 # lspci -vvv
24 ...
25 00:1d.7 USB Controller: Intel Corporati
26 Subsystem: Lenovo ThinkPad T61
27 Control: I/O- Mem+ BusMaster+ S
28 Status: Cap+ 66MHz- UDF- FastB2
29 Latency: 0
30 Interrupt: pin D routed to IRQ
31 Region 0: Memory at fe227000 (3
32 Capabilities: [50] Power Manage
33 Flags: PMEClk- DSI- D1-
34 Status: D0 PME-Enable-
35 Capabilities: [58] Debug port:
36 ^^^^^^^^^^^ <=====
37 Kernel driver in use: ehci_hcd
38 Kernel modules: ehci-hcd
39 ...
40
41 .. note::
42 If your system does not list a debug po
43 won't be able to use the USB debug key.
44
45 b) You also need a NetChip USB debug cable/k
46
47 http://www.plxtech.com/products/NET200
48
49 This is a small blue plastic connector wi
50 it draws power from its USB connections.
51
52 c) You need a second client/console system w
53
54 d) The NetChip device must be plugged direct
55 debug port on the "host/target" system. Y
56 between the physical debug port and the "
57
58 The EHCI debug controller is bound to a s
59 port and the NetChip device will only wor
60 device in this port. The EHCI host contr
61 wired such that the EHCI debug controller
62 first physical port and there is no way t
63 You can find the physical port through ex
64 each physical port on the system and rebo
65 and use lsusb or look at the kernel info
66 usb stack when you plug a usb device into
67 "host/target" system.
68
69 Some hardware vendors do not expose the u
70 physical connector and if you find such a
71 to the hardware vendor, because there is
72 this port into one of the physically acce
73
74 e) It is also important to note, that many v
75 device require the "client/console" syste
76 right hand side of the device (with the p
77 readable left to right). The reason bein
78 power supply is taken from only one side
79 must be the side that does not get reboot
80
81 Software requirements
82 =====================
83
84 a) On the host/target system:
85
86 You need to enable the following kernel co
87
88 CONFIG_EARLY_PRINTK_DBGP=y
89
90 And you need to add the boot command line:
91
92 .. note::
93 If you are using Grub, append it to the
94 /etc/grub.conf. If you are using Grub2
95 append it to the 'linux' line in /boot/g
96 using Grub2 on an EFI firmware system, a
97 or 'linuxefi' line in /boot/grub2/grub.c
98 /boot/efi/EFI/<distro>/grub.cfg.
99
100 On systems with more than one EHCI debug c
101 specify the correct EHCI debug controller
102 comes from the PCI bus enumeration of the
103 default with no number argument is "0" or
104 controller. To use the second EHCI debug
105 use the command line: "earlyprintk=dbgp1"
106
107 .. note::
108 normally earlyprintk console gets turned
109 regular console is alive - use "earlypri
110 this channel open beyond early bootup. T
111 debugging crashes under Xorg, etc.
112
113 b) On the client/console system:
114
115 You should enable the following kernel con
116
117 CONFIG_USB_SERIAL_DEBUG=y
118
119 On the next bootup with the modified kerne
120 get a /dev/ttyUSBx device(s).
121
122 Now this channel of kernel messages is rea
123 your favorite terminal emulator (minicom,
124 it up to use /dev/ttyUSB0 - or use a raw '
125 see the raw output.
126
127 c) On Nvidia Southbridge based systems: the
128 and find out which port has a debug devic
129
130 Testing
131 =======
132
133 You can test the output by using earlyprintk=d
134 kernel messages on the host/target system. You
135 kernel message by for example doing::
136
137 echo h > /proc/sysrq-trigger
138
139 On the host/target system you should see this
140
141 SysRq : HELP : loglevel(0-9) reBoot Crash
142
143 On the client/console system do::
144
145 cat /dev/ttyUSB0
146
147 And you should see the help line above display
148 provoked it on the host system.
149
150 If it does not work then please ask about it o
151 mailing list or contact the x86 maintainers.
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.