2 Using physical DMA provided by OHCI-1394 FireWire controllers for debugging
3 ---------------------------------------------------------------------------
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.
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.
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.
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.
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.
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).
39 The firewire-ohci driver in drivers/firewire uses filtered physical
40 DMA by default, which is more secure but not suitable for remote debugging.
41 Pass the remote_dma=1 parameter to the driver to get unfiltered physical DMA.
43 Because the firewire-ohci driver depends on the PCI enumeration to be
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.
48 To activate it, enable CONFIG_PROVIDE_OHCI1394_DMA_INIT (Kernel hacking menu:
49 Remote debugging over FireWire early on boot) and pass the parameter
50 "ohci1394_dma=early" to the recompiled kernel on boot.
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.
59 Bernhard Kaindl enhanced firescope to support accessing 64-bit machines
60 from 32-bit firescope and vice versa:
61 - http://v3.sk/~lkundrak/firescope/
63 and he implemented fast system dump (alpha version - read README.txt):
64 - http://halobates.de/firewire/firedump-0.1.tar.bz2
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:
68 - http://halobates.de/firewire/fireproxy-0.33.tar.bz2
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).
76 The OHCI-1394 specification regulates that the OHCI-1394 controller must
77 disable all physical DMA on each bus reset.
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.
85 Step-by-step instructions for using firescope with early OHCI initialization:
87 1) Verify that your hardware is supported:
89 Load the firewire-ohci module and check your kernel logs.
90 You should see a line similar to
92 firewire_ohci 0000:15:00.1: added OHCI v1.0 device as card 2, 4 IR + 4 IT
93 ... contexts, quirks 0x11
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.
102 2) Establish a working FireWire cable connection:
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.
108 If an driver is running on both machines you should see a line like
110 firewire_core 0000:15:00.1: created device fw1: GUID 00061b0020105917, S400
112 on both machines in the kernel log when the cable is plugged in
113 and connects the two machines.
115 3) Test physical DMA using firescope:
117 On the debug host, make sure that /dev/fw* is accessible,
118 then start firescope:
121 Port 0 (/dev/fw1) opened, 2 nodes detected
125 Target : <unspecified>
127 [Ctrl-T] choose target
131 ------> Press Ctrl-T now, the output should be similar to:
133 2 nodes available, local node is: 0
134 0: ffc0, uuid: 00000000 00000000 [LOCAL]
135 1: ffc1, uuid: 00279000 ba4bb801
137 Besides the [LOCAL] node, it must show another node without error message.
139 4) Prepare for debugging with early OHCI-1394 initialization:
141 4.1) Kernel compilation and installation on debug target
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).
147 4.2) Transfer the System.map of the debugged kernel to the debug host
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).
152 5) Retrieving the printk buffer contents:
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.
158 Then, on the debugging host, run firescope, for example by using -A:
160 firescope -A System.map-of-debug-target-kernel
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.
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.
169 Call "firescope -h" to get more information on firescope's options.
173 Documentation and specifications: http://halobates.de/firewire/
175 FireWire is a trademark of Apple Inc. - for more information please refer to:
176 http://en.wikipedia.org/wiki/FireWire