Based on kernel version 4.15. Page generated on 2018-01-29 09:59 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 With most FireWire controllers, memory access is limited to the low 4 GB 26 of physical address space. This can be a problem on IA64 machines where 27 memory is located mostly above that limit, but it is rarely a problem on 28 more common hardware such as x86, x86-64 and PowerPC. 29 30 At least LSI FW643e and FW643e2 controllers are known to support access to 31 physical addresses above 4 GB, but this feature is currently not enabled by 32 Linux. 33 34 Together with a early initialization of the OHCI-1394 controller for debugging, 35 this facility proved most useful for examining long debugs logs in the printk 36 buffer on to debug early boot problems in areas like ACPI where the system 37 fails to boot and other means for debugging (serial port) are either not 38 available (notebooks) or too slow for extensive debug information (like ACPI). 39 40 Drivers 41 ------- 42 43 The firewire-ohci driver in drivers/firewire uses filtered physical 44 DMA by default, which is more secure but not suitable for remote debugging. 45 Pass the remote_dma=1 parameter to the driver to get unfiltered physical DMA. 46 47 Because the firewire-ohci driver depends on the PCI enumeration to be 48 completed, an initialization routine which runs pretty early has been 49 implemented for x86. This routine runs long before console_init() can be 50 called, i.e. before the printk buffer appears on the console. 51 52 To activate it, enable CONFIG_PROVIDE_OHCI1394_DMA_INIT (Kernel hacking menu: 53 Remote debugging over FireWire early on boot) and pass the parameter 54 "ohci1394_dma=early" to the recompiled kernel on boot. 55 56 Tools 57 ----- 58 59 firescope - Originally developed by Benjamin Herrenschmidt, Andi Kleen ported 60 it from PowerPC to x86 and x86_64 and added functionality, firescope can now 61 be used to view the printk buffer of a remote machine, even with live update. 62 63 Bernhard Kaindl enhanced firescope to support accessing 64-bit machines 64 from 32-bit firescope and vice versa: 65 - http://v3.sk/~lkundrak/firescope/ 66 67 and he implemented fast system dump (alpha version - read README.txt): 68 - http://halobates.de/firewire/firedump-0.1.tar.bz2 69 70 There is also a gdb proxy for firewire which allows to use gdb to access 71 data which can be referenced from symbols found by gdb in vmlinux: 72 - http://halobates.de/firewire/fireproxy-0.33.tar.bz2 73 74 The latest version of this gdb proxy (fireproxy-0.34) can communicate (not 75 yet stable) with kgdb over an memory-based communication module (kgdbom). 76 77 Getting Started 78 --------------- 79 80 The OHCI-1394 specification regulates that the OHCI-1394 controller must 81 disable all physical DMA on each bus reset. 82 83 This means that if you want to debug an issue in a system state where 84 interrupts are disabled and where no polling of the OHCI-1394 controller 85 for bus resets takes place, you have to establish any FireWire cable 86 connections and fully initialize all FireWire hardware __before__ the 87 system enters such state. 88 89 Step-by-step instructions for using firescope with early OHCI initialization: 90 91 1) 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 IT 97 ... contexts, quirks 0x11 98 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-1394 101 specification are available. If it requires no driver for Windows operating 102 systems, it most likely is. Only specialized shops have cards which are not 103 compliant, they are based on TI PCILynx chips and require drivers for Windows 104 operating systems. 105 106 The mentioned kernel log message contains the string "physUB" if the 107 controller implements a writable Physical Upper Bound register. This is 108 required for physical DMA above 4 GB (but not utilized by Linux yet). 109 110 2) Establish a working FireWire cable connection: 111 112 Any FireWire cable, as long at it provides electrically and mechanically 113 stable connection and has matching connectors (there are small 4-pin and 114 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, S400 119 120 on both machines in the kernel log when the cable is plugged in 121 and connects the two machines. 122 123 3) Test physical DMA using firescope: 124 125 On the debug host, make sure that /dev/fw* is accessible, 126 then start firescope:: 127 128 $ firescope 129 Port 0 (/dev/fw1) 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 ----- 181 182 Documentation and specifications: http://halobates.de/firewire/ 183 184 FireWire is a trademark of Apple Inc. - for more information please refer to: 185 https://en.wikipedia.org/wiki/FireWire