Documentation/debugging-via-ohci1394.txt

   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 Win-
 104    dows 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 Documentation and specifications: http://halobates.de/firewire/
 182
 183 FireWire is a trademark of Apple Inc. - for more information please refer to:
 184 https://en.wikipedia.org/wiki/FireWire