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+
+ Using physical DMA provided by OHCI-1394 FireWire controllers for debugging
+ ---------------------------------------------------------------------------
+
+Introduction
+------------
+
+Basically all FireWire controllers which are in use today are compliant
+to the OHCI-1394 specification which defines the controller to be a PCI
+bus master which uses DMA to offload data transfers from the CPU and has
+a "Physical Response Unit" which executes specific requests by employing
+PCI-Bus master DMA after applying filters defined by the OHCI-1394 driver.
+
+Once properly configured, remote machines can send these requests to
+ask the OHCI-1394 controller to perform read and write requests on
+physical system memory and, for read requests, send the result of
+the physical memory read back to the requester.
+
+With that, it is possible to debug issues by reading interesting memory
+locations such as buffers like the printk buffer or the process table.
+
+Retrieving a full system memory dump is also possible over the FireWire,
+using data transfer rates in the order of 10MB/s or more.
+
+Memory access is currently limited to the low 4G of physical address
+space which can be a problem on IA64 machines where memory is located
+mostly above that limit, but it is rarely a problem on more common
+hardware such as hardware based on x86, x86-64 and PowerPC.
+
+Together with a early initialization of the OHCI-1394 controller for debugging,
+this facility proved most useful for examining long debugs logs in the printk
+buffer on to debug early boot problems in areas like ACPI where the system
+fails to boot and other means for debugging (serial port) are either not
+available (notebooks) or too slow for extensive debug information (like ACPI).
+
+Drivers
+-------
+
+The ohci1394 driver in drivers/ieee1394 initializes the OHCI-1394 controllers
+to a working state and enables physical DMA by default for all remote nodes.
+This can be turned off by ohci1394's module parameter phys_dma=0.
+
+The alternative firewire-ohci driver in drivers/firewire uses filtered physical
+DMA by default, which is more secure but not suitable for remote debugging.
+Compile the driver with CONFIG_FIREWIRE_OHCI_REMOTE_DMA (Kernel hacking menu:
+Remote debugging over FireWire with firewire-ohci) to get unfiltered physical
+DMA.
+
+Because ohci1394 and firewire-ohci depend on the PCI enumeration to be
+completed, an initialization routine which runs pretty early has been
+implemented for x86. This routine runs long before console_init() can be
+called, i.e. before the printk buffer appears on the console.
+
+To activate it, enable CONFIG_PROVIDE_OHCI1394_DMA_INIT (Kernel hacking menu:
+Remote debugging over FireWire early on boot) and pass the parameter
+"ohci1394_dma=early" to the recompiled kernel on boot.
+
+Tools
+-----
+
+firescope - Originally developed by Benjamin Herrenschmidt, Andi Kleen ported
+it from PowerPC to x86 and x86_64 and added functionality, firescope can now
+be used to view the printk buffer of a remote machine, even with live update.
+
+Bernhard Kaindl enhanced firescope to support accessing 64-bit machines
+from 32-bit firescope and vice versa:
+- http://halobates.de/firewire/firescope-0.2.2.tar.bz2
+
+and he implemented fast system dump (alpha version - read README.txt):
+- http://halobates.de/firewire/firedump-0.1.tar.bz2
+
+There is also a gdb proxy for firewire which allows to use gdb to access
+data which can be referenced from symbols found by gdb in vmlinux:
+- http://halobates.de/firewire/fireproxy-0.33.tar.bz2
+
+The latest version of this gdb proxy (fireproxy-0.34) can communicate (not
+yet stable) with kgdb over an memory-based communication module (kgdbom).
+
+Getting Started
+---------------
+
+The OHCI-1394 specification regulates that the OHCI-1394 controller must
+disable all physical DMA on each bus reset.
+
+This means that if you want to debug an issue in a system state where
+interrupts are disabled and where no polling of the OHCI-1394 controller
+for bus resets takes place, you have to establish any FireWire cable
+connections and fully initialize all FireWire hardware __before__ the
+system enters such state.
+
+Step-by-step instructions for using firescope with early OHCI initialization:
+
+1) Verify that your hardware is supported:
+
+ Load the ohci1394 or the fw-ohci module and check your kernel logs.
+ You should see a line similar to
+
+ ohci1394: fw-host0: OHCI-1394 1.1 (PCI): IRQ=[18] MMIO=[fe9ff800-fe9fffff]
+ ... Max Packet=[2048] IR/IT contexts=[4/8]
+
+ when loading the driver. If you have no supported controller, many PCI,
+ CardBus and even some Express cards which are fully compliant to OHCI-1394
+ specification are available. If it requires no driver for Windows operating
+ systems, it most likely is. Only specialized shops have cards which are not
+ compliant, they are based on TI PCILynx chips and require drivers for Win-
+ dows operating systems.
+
+2) Establish a working FireWire cable connection:
+
+ Any FireWire cable, as long at it provides electrically and mechanically
+ stable connection and has matching connectors (there are small 4-pin and
+ large 6-pin FireWire ports) will do.
+
+ If an driver is running on both machines you should see a line like
+
+ ieee1394: Node added: ID:BUS[0-01:1023] GUID[0090270001b84bba]
+
+ on both machines in the kernel log when the cable is plugged in
+ and connects the two machines.
+
+3) Test physical DMA using firescope:
+
+ On the debug host,
+ - load the raw1394 module,
+ - make sure that /dev/raw1394 is accessible,
+ then start firescope:
+
+ $ firescope
+ Port 0 (ohci1394) opened, 2 nodes detected
+
+ FireScope
+ ---------
+ Target : <unspecified>
+ Gen : 1
+ [Ctrl-T] choose target
+ [Ctrl-H] this menu
+ [Ctrl-Q] quit
+
+ ------> Press Ctrl-T now, the output should be similar to:
+
+ 2 nodes available, local node is: 0
+ 0: ffc0, uuid: 00000000 00000000 [LOCAL]
+ 1: ffc1, uuid: 00279000 ba4bb801
+
+ Besides the [LOCAL] node, it must show another node without error message.
+
+4) Prepare for debugging with early OHCI-1394 initialization:
+
+ 4.1) Kernel compilation and installation on debug target
+
+ Compile the kernel to be debugged with CONFIG_PROVIDE_OHCI1394_DMA_INIT
+ (Kernel hacking: Provide code for enabling DMA over FireWire early on boot)
+ enabled and install it on the machine to be debugged (debug target).
+
+ 4.2) Transfer the System.map of the debugged kernel to the debug host
+
+ Copy the System.map of the kernel be debugged to the debug host (the host
+ which is connected to the debugged machine over the FireWire cable).
+
+5) Retrieving the printk buffer contents:
+
+ With the FireWire cable connected, the OHCI-1394 driver on the debugging
+ host loaded, reboot the debugged machine, booting the kernel which has
+ CONFIG_PROVIDE_OHCI1394_DMA_INIT enabled, with the option ohci1394_dma=early.
+
+ Then, on the debugging host, run firescope, for example by using -A:
+
+ firescope -A System.map-of-debug-target-kernel
+
+ Note: -A automatically attaches to the first non-local node. It only works
+ reliably if only connected two machines are connected using FireWire.
+
+ After having attached to the debug target, press Ctrl-D to view the
+ complete printk buffer or Ctrl-U to enter auto update mode and get an
+ updated live view of recent kernel messages logged on the debug target.
+
+ Call "firescope -h" to get more information on firescope's options.
+
+Notes
+-----
+Documentation and specifications: http://halobates.de/firewire/
+
+FireWire is a trademark of Apple Inc. - for more information please refer to:
+http://en.wikipedia.org/wiki/FireWire