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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 OHCI-1394 drivers in drivers/firewire and drivers/ieee1394 initialize
+the OHCI-1394 controllers to a working state and can be used to enable
+physical DMA. By default you only have to load the driver, and physical
+DMA access will be granted to all remote nodes, but it can be turned off
+when using the ohci1394 driver.
+
+Because these drivers depend on the PCI enumeration to be completed, an
+initialization routine which can runs pretty early (long before console_init(),
+which makes the printk buffer appear on the console can be called) was written.
+
+To activate it, enable CONFIG_PROVIDE_OHCI1394_DMA_INIT (Kernel hacking menu:
+Provide code for enabling DMA 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:
+- ftp://ftp.suse.de/private/bk/firewire/tools/firescope-0.2.2.tar.bz2
+
+and he implemented fast system dump (alpha version - read README.txt):
+- ftp://ftp.suse.de/private/bk/firewire/tools/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:
+- ftp://ftp.suse.de/private/bk/firewire/tools/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: ftp://ftp.suse.de/private/bk/firewire/docs
+
+FireWire is a trademark of Apple Inc. - for more information please refer to:
+http://en.wikipedia.org/wiki/FireWire