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1===========================================================================2Using physical DMA provided by OHCI-1394 FireWire controllers for debugging3===========================================================================4 5Introduction6------------7 8Basically all FireWire controllers which are in use today are compliant9to the OHCI-1394 specification which defines the controller to be a PCI10bus master which uses DMA to offload data transfers from the CPU and has11a "Physical Response Unit" which executes specific requests by employing12PCI-Bus master DMA after applying filters defined by the OHCI-1394 driver.13 14Once properly configured, remote machines can send these requests to15ask the OHCI-1394 controller to perform read and write requests on16physical system memory and, for read requests, send the result of17the physical memory read back to the requester.18 19With that, it is possible to debug issues by reading interesting memory20locations such as buffers like the printk buffer or the process table.21 22Retrieving a full system memory dump is also possible over the FireWire,23using data transfer rates in the order of 10MB/s or more.24 25With most FireWire controllers, memory access is limited to the low 4 GB26of physical address space. This can be a problem on machines where memory is27located mostly above that limit, but it is rarely a problem on more common28hardware such as x86, x86-64 and PowerPC.29 30At least LSI FW643e and FW643e2 controllers are known to support access to31physical addresses above 4 GB, but this feature is currently not enabled by32Linux.33 34Together with a early initialization of the OHCI-1394 controller for debugging,35this facility proved most useful for examining long debugs logs in the printk36buffer on to debug early boot problems in areas like ACPI where the system37fails to boot and other means for debugging (serial port) are either not38available (notebooks) or too slow for extensive debug information (like ACPI).39 40Drivers41-------42 43The firewire-ohci driver in drivers/firewire uses filtered physical44DMA by default, which is more secure but not suitable for remote debugging.45Pass the remote_dma=1 parameter to the driver to get unfiltered physical DMA.46 47Because the firewire-ohci driver depends on the PCI enumeration to be48completed, an initialization routine which runs pretty early has been49implemented for x86. This routine runs long before console_init() can be50called, i.e. before the printk buffer appears on the console.51 52To activate it, enable CONFIG_PROVIDE_OHCI1394_DMA_INIT (Kernel hacking menu:53Remote debugging over FireWire early on boot) and pass the parameter54"ohci1394_dma=early" to the recompiled kernel on boot.55 56Tools57-----58 59firescope - Originally developed by Benjamin Herrenschmidt, Andi Kleen ported60it from PowerPC to x86 and x86_64 and added functionality, firescope can now61be used to view the printk buffer of a remote machine, even with live update.62 63Bernhard Kaindl enhanced firescope to support accessing 64-bit machines64from 32-bit firescope and vice versa:65- http://v3.sk/~lkundrak/firescope/66 67and he implemented fast system dump (alpha version - read README.txt):68- http://halobates.de/firewire/firedump-0.1.tar.bz269 70There is also a gdb proxy for firewire which allows to use gdb to access71data which can be referenced from symbols found by gdb in vmlinux:72- http://halobates.de/firewire/fireproxy-0.33.tar.bz273 74The latest version of this gdb proxy (fireproxy-0.34) can communicate (not75yet stable) with kgdb over an memory-based communication module (kgdbom).76 77Getting Started78---------------79 80The OHCI-1394 specification regulates that the OHCI-1394 controller must81disable all physical DMA on each bus reset.82 83This means that if you want to debug an issue in a system state where84interrupts are disabled and where no polling of the OHCI-1394 controller85for bus resets takes place, you have to establish any FireWire cable86connections and fully initialize all FireWire hardware __before__ the87system enters such state.88 89Step-by-step instructions for using firescope with early OHCI initialization:90 911) Verify that your hardware is supported:92 93 Load the firewire-ohci module and check your kernel logs.94 You should see a line similar to::95 96 firewire_ohci 0000:15:00.1: added OHCI v1.0 device as card 2, 4 IR + 4 IT97 ... contexts, quirks 0x1198 99 when loading the driver. If you have no supported controller, many PCI,100 CardBus and even some Express cards which are fully compliant to OHCI-1394101 specification are available. If it requires no driver for Windows operating102 systems, it most likely is. Only specialized shops have cards which are not103 compliant, they are based on TI PCILynx chips and require drivers for Windows104 operating systems.105 106 The mentioned kernel log message contains the string "physUB" if the107 controller implements a writable Physical Upper Bound register. This is108 required for physical DMA above 4 GB (but not utilized by Linux yet).109 1102) Establish a working FireWire cable connection:111 112 Any FireWire cable, as long at it provides electrically and mechanically113 stable connection and has matching connectors (there are small 4-pin and114 large 6-pin FireWire ports) will do.115 116 If an driver is running on both machines you should see a line like::117 118 firewire_core 0000:15:00.1: created device fw1: GUID 00061b0020105917, S400119 120 on both machines in the kernel log when the cable is plugged in121 and connects the two machines.122 1233) Test physical DMA using firescope:124 125 On the debug host, make sure that /dev/fw* is accessible,126 then start firescope::127 128 $ firescope129 Port 0 (/dev/fw1) opened, 2 nodes detected130 131 FireScope132 ---------133 Target : <unspecified>134 Gen : 1135 [Ctrl-T] choose target136 [Ctrl-H] this menu137 [Ctrl-Q] quit138 139 ------> Press Ctrl-T now, the output should be similar to:140 141 2 nodes available, local node is: 0142 0: ffc0, uuid: 00000000 00000000 [LOCAL]143 1: ffc1, uuid: 00279000 ba4bb801144 145 Besides the [LOCAL] node, it must show another node without error message.146 1474) Prepare for debugging with early OHCI-1394 initialization:148 149 4.1) Kernel compilation and installation on debug target150 151 Compile the kernel to be debugged with CONFIG_PROVIDE_OHCI1394_DMA_INIT152 (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 host156 157 Copy the System.map of the kernel be debugged to the debug host (the host158 which is connected to the debugged machine over the FireWire cable).159 1605) Retrieving the printk buffer contents:161 162 With the FireWire cable connected, the OHCI-1394 driver on the debugging163 host loaded, reboot the debugged machine, booting the kernel which has164 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-kernel169 170 Note: -A automatically attaches to the first non-local node. It only works171 reliably if only connected two machines are connected using FireWire.172 173 After having attached to the debug target, press Ctrl-D to view the174 complete printk buffer or Ctrl-U to enter auto update mode and get an175 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 179Notes180-----181 182Documentation and specifications: http://halobates.de/firewire/183 184FireWire is a trademark of Apple Inc. - for more information please refer to:185https://en.wikipedia.org/wiki/FireWire186