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1 | |
2 | Using physical DMA provided by OHCI-1394 FireWire controllers for debugging | |
3 | --------------------------------------------------------------------------- | |
4 | ||
5 | Introduction | |
6 | ------------ | |
7 | ||
8 | Basically all FireWire controllers which are in use today are compliant | |
9 | to the OHCI-1394 specification which defines the controller to be a PCI | |
10 | bus master which uses DMA to offload data transfers from the CPU and has | |
11 | a "Physical Response Unit" which executes specific requests by employing | |
12 | PCI-Bus master DMA after applying filters defined by the OHCI-1394 driver. | |
13 | ||
14 | Once properly configured, remote machines can send these requests to | |
15 | ask the OHCI-1394 controller to perform read and write requests on | |
16 | physical system memory and, for read requests, send the result of | |
17 | the physical memory read back to the requester. | |
18 | ||
19 | With that, it is possible to debug issues by reading interesting memory | |
20 | locations such as buffers like the printk buffer or the process table. | |
21 | ||
22 | Retrieving a full system memory dump is also possible over the FireWire, | |
23 | using data transfer rates in the order of 10MB/s or more. | |
24 | ||
25 | Memory access is currently limited to the low 4G of physical address | |
26 | space which can be a problem on IA64 machines where memory is located | |
27 | mostly above that limit, but it is rarely a problem on more common | |
28 | hardware such as hardware based on x86, x86-64 and PowerPC. | |
29 | ||
30 | Together with a early initialization of the OHCI-1394 controller for debugging, | |
31 | this facility proved most useful for examining long debugs logs in the printk | |
32 | buffer on to debug early boot problems in areas like ACPI where the system | |
33 | fails to boot and other means for debugging (serial port) are either not | |
34 | available (notebooks) or too slow for extensive debug information (like ACPI). | |
35 | ||
36 | Drivers | |
37 | ------- | |
38 | ||
a9954ce7 | 39 | The firewire-ohci driver in drivers/firewire uses filtered physical |
080de8c2 | 40 | DMA by default, which is more secure but not suitable for remote debugging. |
8bc588e0 | 41 | Pass the remote_dma=1 parameter to the driver to get unfiltered physical DMA. |
09d7328e | 42 | |
a9954ce7 | 43 | Because the firewire-ohci driver depends on the PCI enumeration to be |
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44 | completed, an initialization routine which runs pretty early has been |
45 | implemented for x86. This routine runs long before console_init() can be | |
46 | called, i.e. before the printk buffer appears on the console. | |
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47 | |
48 | To activate it, enable CONFIG_PROVIDE_OHCI1394_DMA_INIT (Kernel hacking menu: | |
080de8c2 SR |
49 | Remote debugging over FireWire early on boot) and pass the parameter |
50 | "ohci1394_dma=early" to the recompiled kernel on boot. | |
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51 | |
52 | Tools | |
53 | ----- | |
54 | ||
55 | firescope - Originally developed by Benjamin Herrenschmidt, Andi Kleen ported | |
56 | it from PowerPC to x86 and x86_64 and added functionality, firescope can now | |
57 | be used to view the printk buffer of a remote machine, even with live update. | |
58 | ||
59 | Bernhard Kaindl enhanced firescope to support accessing 64-bit machines | |
60 | from 32-bit firescope and vice versa: | |
a9954ce7 | 61 | - http://v3.sk/~lkundrak/firescope/ |
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62 | |
63 | and he implemented fast system dump (alpha version - read README.txt): | |
211a6417 | 64 | - http://halobates.de/firewire/firedump-0.1.tar.bz2 |
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65 | |
66 | There is also a gdb proxy for firewire which allows to use gdb to access | |
67 | data which can be referenced from symbols found by gdb in vmlinux: | |
211a6417 | 68 | - http://halobates.de/firewire/fireproxy-0.33.tar.bz2 |
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69 | |
70 | The latest version of this gdb proxy (fireproxy-0.34) can communicate (not | |
71 | yet stable) with kgdb over an memory-based communication module (kgdbom). | |
72 | ||
73 | Getting Started | |
74 | --------------- | |
75 | ||
76 | The OHCI-1394 specification regulates that the OHCI-1394 controller must | |
77 | disable all physical DMA on each bus reset. | |
78 | ||
79 | This means that if you want to debug an issue in a system state where | |
80 | interrupts are disabled and where no polling of the OHCI-1394 controller | |
81 | for bus resets takes place, you have to establish any FireWire cable | |
82 | connections and fully initialize all FireWire hardware __before__ the | |
83 | system enters such state. | |
84 | ||
85 | Step-by-step instructions for using firescope with early OHCI initialization: | |
86 | ||
87 | 1) Verify that your hardware is supported: | |
88 | ||
a9954ce7 | 89 | Load the firewire-ohci module and check your kernel logs. |
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90 | You should see a line similar to |
91 | ||
a9954ce7 LR |
92 | firewire_ohci 0000:15:00.1: added OHCI v1.0 device as card 2, 4 IR + 4 IT |
93 | ... contexts, quirks 0x11 | |
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94 | |
95 | when loading the driver. If you have no supported controller, many PCI, | |
96 | CardBus and even some Express cards which are fully compliant to OHCI-1394 | |
97 | specification are available. If it requires no driver for Windows operating | |
98 | systems, it most likely is. Only specialized shops have cards which are not | |
99 | compliant, they are based on TI PCILynx chips and require drivers for Win- | |
100 | dows operating systems. | |
101 | ||
102 | 2) Establish a working FireWire cable connection: | |
103 | ||
104 | Any FireWire cable, as long at it provides electrically and mechanically | |
105 | stable connection and has matching connectors (there are small 4-pin and | |
106 | large 6-pin FireWire ports) will do. | |
107 | ||
108 | If an driver is running on both machines you should see a line like | |
109 | ||
a9954ce7 | 110 | firewire_core 0000:15:00.1: created device fw1: GUID 00061b0020105917, S400 |
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111 | |
112 | on both machines in the kernel log when the cable is plugged in | |
113 | and connects the two machines. | |
114 | ||
115 | 3) Test physical DMA using firescope: | |
116 | ||
a9954ce7 | 117 | On the debug host, make sure that /dev/fw* is accessible, |
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118 | then start firescope: |
119 | ||
120 | $ firescope | |
a9954ce7 | 121 | Port 0 (/dev/fw1) opened, 2 nodes detected |
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122 | |
123 | FireScope | |
124 | --------- | |
125 | Target : <unspecified> | |
126 | Gen : 1 | |
127 | [Ctrl-T] choose target | |
128 | [Ctrl-H] this menu | |
129 | [Ctrl-Q] quit | |
130 | ||
131 | ------> Press Ctrl-T now, the output should be similar to: | |
132 | ||
133 | 2 nodes available, local node is: 0 | |
134 | 0: ffc0, uuid: 00000000 00000000 [LOCAL] | |
135 | 1: ffc1, uuid: 00279000 ba4bb801 | |
136 | ||
137 | Besides the [LOCAL] node, it must show another node without error message. | |
138 | ||
139 | 4) Prepare for debugging with early OHCI-1394 initialization: | |
140 | ||
141 | 4.1) Kernel compilation and installation on debug target | |
142 | ||
143 | Compile the kernel to be debugged with CONFIG_PROVIDE_OHCI1394_DMA_INIT | |
144 | (Kernel hacking: Provide code for enabling DMA over FireWire early on boot) | |
145 | enabled and install it on the machine to be debugged (debug target). | |
146 | ||
147 | 4.2) Transfer the System.map of the debugged kernel to the debug host | |
148 | ||
149 | Copy the System.map of the kernel be debugged to the debug host (the host | |
150 | which is connected to the debugged machine over the FireWire cable). | |
151 | ||
152 | 5) Retrieving the printk buffer contents: | |
153 | ||
154 | With the FireWire cable connected, the OHCI-1394 driver on the debugging | |
155 | host loaded, reboot the debugged machine, booting the kernel which has | |
156 | CONFIG_PROVIDE_OHCI1394_DMA_INIT enabled, with the option ohci1394_dma=early. | |
157 | ||
158 | Then, on the debugging host, run firescope, for example by using -A: | |
159 | ||
160 | firescope -A System.map-of-debug-target-kernel | |
161 | ||
162 | Note: -A automatically attaches to the first non-local node. It only works | |
163 | reliably if only connected two machines are connected using FireWire. | |
164 | ||
165 | After having attached to the debug target, press Ctrl-D to view the | |
166 | complete printk buffer or Ctrl-U to enter auto update mode and get an | |
167 | updated live view of recent kernel messages logged on the debug target. | |
168 | ||
169 | Call "firescope -h" to get more information on firescope's options. | |
170 | ||
171 | Notes | |
172 | ----- | |
211a6417 | 173 | Documentation and specifications: http://halobates.de/firewire/ |
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174 | |
175 | FireWire is a trademark of Apple Inc. - for more information please refer to: | |
176 | http://en.wikipedia.org/wiki/FireWire |