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  2   Using physical DMA provided by OHCI-1394 FireWire controllers for debugging
  3   ---------------------------------------------------------------------------
  5 Introduction
  6 ------------
  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 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.
 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.
 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).
 40 Drivers
 41 -------
 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.
 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.
 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.
 56 Tools
 57 -----
 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.
 63 Bernhard Kaindl enhanced firescope to support accessing 64-bit machines
 64 from 32-bit firescope and vice versa:
 65 -
 67 and he implemented fast system dump (alpha version - read README.txt):
 68 -
 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 -
 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).
 77 Getting Started
 78 ---------------
 80 The OHCI-1394 specification regulates that the OHCI-1394 controller must
 81 disable all physical DMA on each bus reset.
 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.
 89 Step-by-step instructions for using firescope with early OHCI initialization:
 91 1) Verify that your hardware is supported:
 93    Load the firewire-ohci module and check your kernel logs.
 94    You should see a line similar to
 96    firewire_ohci 0000:15:00.1: added OHCI v1.0 device as card 2, 4 IR + 4 IT
 97    ... contexts, quirks 0x11
 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.
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).
110 2) Establish a working FireWire cable connection:
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.
116    If an driver is running on both machines you should see a line like
118    firewire_core 0000:15:00.1: created device fw1: GUID 00061b0020105917, S400
120    on both machines in the kernel log when the cable is plugged in
121    and connects the two machines.
123 3) Test physical DMA using firescope:
125    On the debug host, make sure that /dev/fw* is accessible,
126    then start firescope:
128         $ firescope
129         Port 0 (/dev/fw1) opened, 2 nodes detected
131         FireScope
132         ---------
133         Target : <unspecified>
134         Gen    : 1
135         [Ctrl-T] choose target
136         [Ctrl-H] this menu
137         [Ctrl-Q] quit
139     ------> Press Ctrl-T now, the output should be similar to:
141         2 nodes available, local node is: 0
142          0: ffc0, uuid: 00000000 00000000 [LOCAL]
143          1: ffc1, uuid: 00279000 ba4bb801
145    Besides the [LOCAL] node, it must show another node without error message.
147 4) Prepare for debugging with early OHCI-1394 initialization:
149    4.1) Kernel compilation and installation on debug target
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).
155    4.2) Transfer the of the debugged kernel to the debug host
157    Copy the of the kernel be debugged to the debug host (the host
158    which is connected to the debugged machine over the FireWire cable).
160 5) Retrieving the printk buffer contents:
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.
166    Then, on the debugging host, run firescope, for example by using -A:
168         firescope -A
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.
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.
177    Call "firescope -h" to get more information on firescope's options.
179 Notes
180 -----
181 Documentation and specifications:
183 FireWire is a trademark of Apple Inc. - for more information please refer to:

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