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diff --git a/Documentation/debugging-via-ohci1394.txt b/Documentation/debugging-via-ohci1394.txt new file mode 100644 index 000000000000..de4804e8b396 --- /dev/null +++ b/Documentation/debugging-via-ohci1394.txt @@ -0,0 +1,179 @@ + + 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 |
