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Source at commit ec7cab4cbb721bff91ec924ec691efd8daf36579 created 12 years 8 months ago. By Maarten ter Huurne, MIPS: JZ4740: A320: Updated quickstart documentation. | |
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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 | |
39 | The ohci1394 driver in drivers/ieee1394 initializes the OHCI-1394 controllers |
40 | to a working state and enables physical DMA by default for all remote nodes. |
41 | This can be turned off by ohci1394's module parameter phys_dma=0. |
42 | |
43 | The alternative firewire-ohci driver in drivers/firewire uses filtered physical |
44 | DMA by default, which is more secure but not suitable for remote debugging. |
45 | Compile the driver with CONFIG_FIREWIRE_OHCI_REMOTE_DMA (Kernel hacking menu: |
46 | Remote debugging over FireWire with firewire-ohci) to get unfiltered physical |
47 | DMA. |
48 | |
49 | Because ohci1394 and firewire-ohci depend on the PCI enumeration to be |
50 | completed, an initialization routine which runs pretty early has been |
51 | implemented for x86. This routine runs long before console_init() can be |
52 | called, i.e. before the printk buffer appears on the console. |
53 | |
54 | To activate it, enable CONFIG_PROVIDE_OHCI1394_DMA_INIT (Kernel hacking menu: |
55 | Remote debugging over FireWire early on boot) and pass the parameter |
56 | "ohci1394_dma=early" to the recompiled kernel on boot. |
57 | |
58 | Tools |
59 | ----- |
60 | |
61 | firescope - Originally developed by Benjamin Herrenschmidt, Andi Kleen ported |
62 | it from PowerPC to x86 and x86_64 and added functionality, firescope can now |
63 | be used to view the printk buffer of a remote machine, even with live update. |
64 | |
65 | Bernhard Kaindl enhanced firescope to support accessing 64-bit machines |
66 | from 32-bit firescope and vice versa: |
67 | - http://halobates.de/firewire/firescope-0.2.2.tar.bz2 |
68 | |
69 | and he implemented fast system dump (alpha version - read README.txt): |
70 | - http://halobates.de/firewire/firedump-0.1.tar.bz2 |
71 | |
72 | There is also a gdb proxy for firewire which allows to use gdb to access |
73 | data which can be referenced from symbols found by gdb in vmlinux: |
74 | - http://halobates.de/firewire/fireproxy-0.33.tar.bz2 |
75 | |
76 | The latest version of this gdb proxy (fireproxy-0.34) can communicate (not |
77 | yet stable) with kgdb over an memory-based communication module (kgdbom). |
78 | |
79 | Getting Started |
80 | --------------- |
81 | |
82 | The OHCI-1394 specification regulates that the OHCI-1394 controller must |
83 | disable all physical DMA on each bus reset. |
84 | |
85 | This means that if you want to debug an issue in a system state where |
86 | interrupts are disabled and where no polling of the OHCI-1394 controller |
87 | for bus resets takes place, you have to establish any FireWire cable |
88 | connections and fully initialize all FireWire hardware __before__ the |
89 | system enters such state. |
90 | |
91 | Step-by-step instructions for using firescope with early OHCI initialization: |
92 | |
93 | 1) Verify that your hardware is supported: |
94 | |
95 | Load the ohci1394 or the fw-ohci module and check your kernel logs. |
96 | You should see a line similar to |
97 | |
98 | ohci1394: fw-host0: OHCI-1394 1.1 (PCI): IRQ=[18] MMIO=[fe9ff800-fe9fffff] |
99 | ... Max Packet=[2048] IR/IT contexts=[4/8] |
100 | |
101 | when loading the driver. If you have no supported controller, many PCI, |
102 | CardBus and even some Express cards which are fully compliant to OHCI-1394 |
103 | specification are available. If it requires no driver for Windows operating |
104 | systems, it most likely is. Only specialized shops have cards which are not |
105 | compliant, they are based on TI PCILynx chips and require drivers for Win- |
106 | dows operating systems. |
107 | |
108 | 2) Establish a working FireWire cable connection: |
109 | |
110 | Any FireWire cable, as long at it provides electrically and mechanically |
111 | stable connection and has matching connectors (there are small 4-pin and |
112 | large 6-pin FireWire ports) will do. |
113 | |
114 | If an driver is running on both machines you should see a line like |
115 | |
116 | ieee1394: Node added: ID:BUS[0-01:1023] GUID[0090270001b84bba] |
117 | |
118 | on both machines in the kernel log when the cable is plugged in |
119 | and connects the two machines. |
120 | |
121 | 3) Test physical DMA using firescope: |
122 | |
123 | On the debug host, |
124 | - load the raw1394 module, |
125 | - make sure that /dev/raw1394 is accessible, |
126 | then start firescope: |
127 | |
128 | $ firescope |
129 | Port 0 (ohci1394) opened, 2 nodes detected |
130 | |
131 | FireScope |
132 | --------- |
133 | Target : <unspecified> |
134 | Gen : 1 |
135 | [Ctrl-T] choose target |
136 | [Ctrl-H] this menu |
137 | [Ctrl-Q] quit |
138 | |
139 | ------> Press Ctrl-T now, the output should be similar to: |
140 | |
141 | 2 nodes available, local node is: 0 |
142 | 0: ffc0, uuid: 00000000 00000000 [LOCAL] |
143 | 1: ffc1, uuid: 00279000 ba4bb801 |
144 | |
145 | Besides the [LOCAL] node, it must show another node without error message. |
146 | |
147 | 4) Prepare for debugging with early OHCI-1394 initialization: |
148 | |
149 | 4.1) Kernel compilation and installation on debug target |
150 | |
151 | Compile the kernel to be debugged with CONFIG_PROVIDE_OHCI1394_DMA_INIT |
152 | (Kernel hacking: Provide code for enabling DMA over FireWire early on boot) |
153 | enabled and install it on the machine to be debugged (debug target). |
154 | |
155 | 4.2) Transfer the System.map of the debugged kernel to the debug host |
156 | |
157 | Copy the System.map of the kernel be debugged to the debug host (the host |
158 | which is connected to the debugged machine over the FireWire cable). |
159 | |
160 | 5) Retrieving the printk buffer contents: |
161 | |
162 | With the FireWire cable connected, the OHCI-1394 driver on the debugging |
163 | host loaded, reboot the debugged machine, booting the kernel which has |
164 | CONFIG_PROVIDE_OHCI1394_DMA_INIT enabled, with the option ohci1394_dma=early. |
165 | |
166 | Then, on the debugging host, run firescope, for example by using -A: |
167 | |
168 | firescope -A System.map-of-debug-target-kernel |
169 | |
170 | Note: -A automatically attaches to the first non-local node. It only works |
171 | reliably if only connected two machines are connected using FireWire. |
172 | |
173 | After having attached to the debug target, press Ctrl-D to view the |
174 | complete printk buffer or Ctrl-U to enter auto update mode and get an |
175 | updated live view of recent kernel messages logged on the debug target. |
176 | |
177 | Call "firescope -h" to get more information on firescope's options. |
178 | |
179 | Notes |
180 | ----- |
181 | Documentation and specifications: http://halobates.de/firewire/ |
182 | |
183 | FireWire is a trademark of Apple Inc. - for more information please refer to: |
184 | http://en.wikipedia.org/wiki/FireWire |
185 |
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