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>

2 files changed, 186 insertions, 0 deletions

diff --git a/Documentation/core-api/debugging-via-ohci1394.rst b/Documentation/core-api/debugging-via-ohci1394.rst
new file mode 100644
index 000000000000..981ad4f89fd3
--- /dev/null
+++ b/Documentation/core-api/debugging-via-ohci1394.rst
@@ -0,0 +1,185 @@
+===========================================================================
+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
diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index c00aef843341..2cfd07a34173 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -98,6 +98,7 @@ Interfaces for kernel debugging
 
    debug-objects
    tracepoint
+   debugging-via-ohci1394
 
 Everything else
 ===============