diff options
author | Mauro Carvalho Chehab <mchehab+huawei@kernel.org> | 2020-05-01 18:37:50 +0300 |
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committer | Jonathan Corbet <corbet@lwn.net> | 2020-05-15 20:59:17 +0300 |
commit | a74e2a226452ea75d26b1f83860bff91a11da1ac (patch) | |
tree | a3936b2095e66b4861e329fc1d1c129c7d0795be /Documentation/debugging-via-ohci1394.txt | |
parent | 283b69bf7948c2470b5b7085bf291f834f3b9d95 (diff) |
docs: debugging-via-ohci1394.txt: add it to the core-api book
There is an special chapter inside the core-api book about
some debug infrastructure like tracepoints and debug objects.
It sounded to me that this is the best place to add a chapter
explaining how to use a FireWire controller to do remote
kernel debugging, as explained on this document.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/9b489d36d08ad89d3ad5aefef1f52a0715b29716.1588345503.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Diffstat (limited to 'Documentation/debugging-via-ohci1394.txt')
-rw-r--r-- | Documentation/debugging-via-ohci1394.txt | 185 |
1 files changed, 0 insertions, 185 deletions
diff --git a/Documentation/debugging-via-ohci1394.txt b/Documentation/debugging-via-ohci1394.txt deleted file mode 100644 index 981ad4f89fd3..000000000000 --- a/Documentation/debugging-via-ohci1394.txt +++ /dev/null @@ -1,185 +0,0 @@ -=========================================================================== -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. - -With most FireWire controllers, memory access is limited to the low 4 GB -of physical address space. This 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 x86, x86-64 and PowerPC. - -At least LSI FW643e and FW643e2 controllers are known to support access to -physical addresses above 4 GB, but this feature is currently not enabled by -Linux. - -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 firewire-ohci driver in drivers/firewire uses filtered physical -DMA by default, which is more secure but not suitable for remote debugging. -Pass the remote_dma=1 parameter to the driver to get unfiltered physical DMA. - -Because the firewire-ohci driver depends 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://v3.sk/~lkundrak/firescope/ - -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 firewire-ohci module and check your kernel logs. - You should see a line similar to:: - - firewire_ohci 0000:15:00.1: added OHCI v1.0 device as card 2, 4 IR + 4 IT - ... contexts, quirks 0x11 - - 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 Windows - operating systems. - - The mentioned kernel log message contains the string "physUB" if the - controller implements a writable Physical Upper Bound register. This is - required for physical DMA above 4 GB (but not utilized by Linux yet). - -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:: - - firewire_core 0000:15:00.1: created device fw1: GUID 00061b0020105917, S400 - - 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, make sure that /dev/fw* is accessible, - then start firescope:: - - $ firescope - Port 0 (/dev/fw1) 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: -https://en.wikipedia.org/wiki/FireWire |