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f212ec4b BK |
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 | ||
39 | The OHCI-1394 drivers in drivers/firewire and drivers/ieee1394 initialize | |
40 | the OHCI-1394 controllers to a working state and can be used to enable | |
41 | physical DMA. By default you only have to load the driver, and physical | |
42 | DMA access will be granted to all remote nodes, but it can be turned off | |
43 | when using the ohci1394 driver. | |
44 | ||
45 | Because these drivers depend on the PCI enumeration to be completed, an | |
46 | initialization routine which can runs pretty early (long before console_init(), | |
47 | which makes the printk buffer appear on the console can be called) was written. | |
48 | ||
49 | To activate it, enable CONFIG_PROVIDE_OHCI1394_DMA_INIT (Kernel hacking menu: | |
50 | Provide code for enabling DMA over FireWire early on boot) and pass the | |
51 | parameter "ohci1394_dma=early" to the recompiled kernel on boot. | |
52 | ||
53 | Tools | |
54 | ----- | |
55 | ||
56 | firescope - Originally developed by Benjamin Herrenschmidt, Andi Kleen ported | |
57 | it from PowerPC to x86 and x86_64 and added functionality, firescope can now | |
58 | be used to view the printk buffer of a remote machine, even with live update. | |
59 | ||
60 | Bernhard Kaindl enhanced firescope to support accessing 64-bit machines | |
61 | from 32-bit firescope and vice versa: | |
62 | - ftp://ftp.suse.de/private/bk/firewire/tools/firescope-0.2.2.tar.bz2 | |
63 | ||
64 | and he implemented fast system dump (alpha version - read README.txt): | |
65 | - ftp://ftp.suse.de/private/bk/firewire/tools/firedump-0.1.tar.bz2 | |
66 | ||
67 | There is also a gdb proxy for firewire which allows to use gdb to access | |
68 | data which can be referenced from symbols found by gdb in vmlinux: | |
69 | - ftp://ftp.suse.de/private/bk/firewire/tools/fireproxy-0.33.tar.bz2 | |
70 | ||
71 | The latest version of this gdb proxy (fireproxy-0.34) can communicate (not | |
72 | yet stable) with kgdb over an memory-based communication module (kgdbom). | |
73 | ||
74 | Getting Started | |
75 | --------------- | |
76 | ||
77 | The OHCI-1394 specification regulates that the OHCI-1394 controller must | |
78 | disable all physical DMA on each bus reset. | |
79 | ||
80 | This means that if you want to debug an issue in a system state where | |
81 | interrupts are disabled and where no polling of the OHCI-1394 controller | |
82 | for bus resets takes place, you have to establish any FireWire cable | |
83 | connections and fully initialize all FireWire hardware __before__ the | |
84 | system enters such state. | |
85 | ||
86 | Step-by-step instructions for using firescope with early OHCI initialization: | |
87 | ||
88 | 1) Verify that your hardware is supported: | |
89 | ||
90 | Load the ohci1394 or the fw-ohci module and check your kernel logs. | |
91 | You should see a line similar to | |
92 | ||
93 | ohci1394: fw-host0: OHCI-1394 1.1 (PCI): IRQ=[18] MMIO=[fe9ff800-fe9fffff] | |
94 | ... Max Packet=[2048] IR/IT contexts=[4/8] | |
95 | ||
96 | when loading the driver. If you have no supported controller, many PCI, | |
97 | CardBus and even some Express cards which are fully compliant to OHCI-1394 | |
98 | specification are available. If it requires no driver for Windows operating | |
99 | systems, it most likely is. Only specialized shops have cards which are not | |
100 | compliant, they are based on TI PCILynx chips and require drivers for Win- | |
101 | dows operating systems. | |
102 | ||
103 | 2) Establish a working FireWire cable connection: | |
104 | ||
105 | Any FireWire cable, as long at it provides electrically and mechanically | |
106 | stable connection and has matching connectors (there are small 4-pin and | |
107 | large 6-pin FireWire ports) will do. | |
108 | ||
109 | If an driver is running on both machines you should see a line like | |
110 | ||
111 | ieee1394: Node added: ID:BUS[0-01:1023] GUID[0090270001b84bba] | |
112 | ||
113 | on both machines in the kernel log when the cable is plugged in | |
114 | and connects the two machines. | |
115 | ||
116 | 3) Test physical DMA using firescope: | |
117 | ||
118 | On the debug host, | |
119 | - load the raw1394 module, | |
120 | - make sure that /dev/raw1394 is accessible, | |
121 | then start firescope: | |
122 | ||
123 | $ firescope | |
124 | Port 0 (ohci1394) opened, 2 nodes detected | |
125 | ||
126 | FireScope | |
127 | --------- | |
128 | Target : <unspecified> | |
129 | Gen : 1 | |
130 | [Ctrl-T] choose target | |
131 | [Ctrl-H] this menu | |
132 | [Ctrl-Q] quit | |
133 | ||
134 | ------> Press Ctrl-T now, the output should be similar to: | |
135 | ||
136 | 2 nodes available, local node is: 0 | |
137 | 0: ffc0, uuid: 00000000 00000000 [LOCAL] | |
138 | 1: ffc1, uuid: 00279000 ba4bb801 | |
139 | ||
140 | Besides the [LOCAL] node, it must show another node without error message. | |
141 | ||
142 | 4) Prepare for debugging with early OHCI-1394 initialization: | |
143 | ||
144 | 4.1) Kernel compilation and installation on debug target | |
145 | ||
146 | Compile the kernel to be debugged with CONFIG_PROVIDE_OHCI1394_DMA_INIT | |
147 | (Kernel hacking: Provide code for enabling DMA over FireWire early on boot) | |
148 | enabled and install it on the machine to be debugged (debug target). | |
149 | ||
150 | 4.2) Transfer the System.map of the debugged kernel to the debug host | |
151 | ||
152 | Copy the System.map of the kernel be debugged to the debug host (the host | |
153 | which is connected to the debugged machine over the FireWire cable). | |
154 | ||
155 | 5) Retrieving the printk buffer contents: | |
156 | ||
157 | With the FireWire cable connected, the OHCI-1394 driver on the debugging | |
158 | host loaded, reboot the debugged machine, booting the kernel which has | |
159 | CONFIG_PROVIDE_OHCI1394_DMA_INIT enabled, with the option ohci1394_dma=early. | |
160 | ||
161 | Then, on the debugging host, run firescope, for example by using -A: | |
162 | ||
163 | firescope -A System.map-of-debug-target-kernel | |
164 | ||
165 | Note: -A automatically attaches to the first non-local node. It only works | |
166 | reliably if only connected two machines are connected using FireWire. | |
167 | ||
168 | After having attached to the debug target, press Ctrl-D to view the | |
169 | complete printk buffer or Ctrl-U to enter auto update mode and get an | |
170 | updated live view of recent kernel messages logged on the debug target. | |
171 | ||
172 | Call "firescope -h" to get more information on firescope's options. | |
173 | ||
174 | Notes | |
175 | ----- | |
176 | Documentation and specifications: ftp://ftp.suse.de/private/bk/firewire/docs | |
177 | ||
178 | FireWire is a trademark of Apple Inc. - for more information please refer to: | |
179 | http://en.wikipedia.org/wiki/FireWire |