Based on kernel version 2.6.25. Page generated on 2008-04-18 21:22 EST.
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, hence is not yet suitable for remote debugging. 45 46 Because ohci1394 depends on the PCI enumeration to be completed, an 47 initialization routine which runs pretty early (long before console_init() 48 which makes the printk buffer appear on the console can be called) was written. 49 50 To activate it, enable CONFIG_PROVIDE_OHCI1394_DMA_INIT (Kernel hacking menu: 51 Provide code for enabling DMA over FireWire early on boot) and pass the 52 parameter "ohci1394_dma=early" to the recompiled kernel on boot. 53 54 Tools 55 ----- 56 57 firescope - Originally developed by Benjamin Herrenschmidt, Andi Kleen ported 58 it from PowerPC to x86 and x86_64 and added functionality, firescope can now 59 be used to view the printk buffer of a remote machine, even with live update. 60 61 Bernhard Kaindl enhanced firescope to support accessing 64-bit machines 62 from 32-bit firescope and vice versa: 63 - ftp://ftp.suse.de/private/bk/firewire/tools/firescope-0.2.2.tar.bz2 64 65 and he implemented fast system dump (alpha version - read README.txt): 66 - ftp://ftp.suse.de/private/bk/firewire/tools/firedump-0.1.tar.bz2 67 68 There is also a gdb proxy for firewire which allows to use gdb to access 69 data which can be referenced from symbols found by gdb in vmlinux: 70 - ftp://ftp.suse.de/private/bk/firewire/tools/fireproxy-0.33.tar.bz2 71 72 The latest version of this gdb proxy (fireproxy-0.34) can communicate (not 73 yet stable) with kgdb over an memory-based communication module (kgdbom). 74 75 Getting Started 76 --------------- 77 78 The OHCI-1394 specification regulates that the OHCI-1394 controller must 79 disable all physical DMA on each bus reset. 80 81 This means that if you want to debug an issue in a system state where 82 interrupts are disabled and where no polling of the OHCI-1394 controller 83 for bus resets takes place, you have to establish any FireWire cable 84 connections and fully initialize all FireWire hardware __before__ the 85 system enters such state. 86 87 Step-by-step instructions for using firescope with early OHCI initialization: 88 89 1) Verify that your hardware is supported: 90 91 Load the ohci1394 or the fw-ohci module and check your kernel logs. 92 You should see a line similar to 93 94 ohci1394: fw-host0: OHCI-1394 1.1 (PCI): IRQ=[18] MMIO=[fe9ff800-fe9fffff] 95 ... Max Packet=[2048] IR/IT contexts=[4/8] 96 97 when loading the driver. If you have no supported controller, many PCI, 98 CardBus and even some Express cards which are fully compliant to OHCI-1394 99 specification are available. If it requires no driver for Windows operating 100 systems, it most likely is. Only specialized shops have cards which are not 101 compliant, they are based on TI PCILynx chips and require drivers for Win- 102 dows operating systems. 103 104 2) Establish a working FireWire cable connection: 105 106 Any FireWire cable, as long at it provides electrically and mechanically 107 stable connection and has matching connectors (there are small 4-pin and 108 large 6-pin FireWire ports) will do. 109 110 If an driver is running on both machines you should see a line like 111 112 ieee1394: Node added: ID:BUS[0-01:1023] GUID[0090270001b84bba] 113 114 on both machines in the kernel log when the cable is plugged in 115 and connects the two machines. 116 117 3) Test physical DMA using firescope: 118 119 On the debug host, 120 - load the raw1394 module, 121 - make sure that /dev/raw1394 is accessible, 122 then start firescope: 123 124 $ firescope 125 Port 0 (ohci1394) opened, 2 nodes detected 126 127 FireScope 128 --------- 129 Target : <unspecified> 130 Gen : 1 131 [Ctrl-T] choose target 132 [Ctrl-H] this menu 133 [Ctrl-Q] quit 134 135 ------> Press Ctrl-T now, the output should be similar to: 136 137 2 nodes available, local node is: 0 138 0: ffc0, uuid: 00000000 00000000 [LOCAL] 139 1: ffc1, uuid: 00279000 ba4bb801 140 141 Besides the [LOCAL] node, it must show another node without error message. 142 143 4) Prepare for debugging with early OHCI-1394 initialization: 144 145 4.1) Kernel compilation and installation on debug target 146 147 Compile the kernel to be debugged with CONFIG_PROVIDE_OHCI1394_DMA_INIT 148 (Kernel hacking: Provide code for enabling DMA over FireWire early on boot) 149 enabled and install it on the machine to be debugged (debug target). 150 151 4.2) Transfer the System.map of the debugged kernel to the debug host 152 153 Copy the System.map of the kernel be debugged to the debug host (the host 154 which is connected to the debugged machine over the FireWire cable). 155 156 5) Retrieving the printk buffer contents: 157 158 With the FireWire cable connected, the OHCI-1394 driver on the debugging 159 host loaded, reboot the debugged machine, booting the kernel which has 160 CONFIG_PROVIDE_OHCI1394_DMA_INIT enabled, with the option ohci1394_dma=early. 161 162 Then, on the debugging host, run firescope, for example by using -A: 163 164 firescope -A System.map-of-debug-target-kernel 165 166 Note: -A automatically attaches to the first non-local node. It only works 167 reliably if only connected two machines are connected using FireWire. 168 169 After having attached to the debug target, press Ctrl-D to view the 170 complete printk buffer or Ctrl-U to enter auto update mode and get an 171 updated live view of recent kernel messages logged on the debug target. 172 173 Call "firescope -h" to get more information on firescope's options. 174 175 Notes 176 ----- 177 Documentation and specifications: ftp://ftp.suse.de/private/bk/firewire/docs 178 179 FireWire is a trademark of Apple Inc. - for more information please refer to: 180 http://en.wikipedia.org/wiki/FireWire
