docs-rst: convert kgdb DocBook to ReST

Use pandoc to convert documentation to ReST by calling
Documentation/sphinx/tmplcvt script.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>

1 files changed, 930 insertions, 0 deletions

diff --git a/Documentation/dev-tools/kgdb.rst b/Documentation/dev-tools/kgdb.rst
new file mode 100644
index 000000000000..ea01541806c8
--- /dev/null
+++ b/Documentation/dev-tools/kgdb.rst
@@ -0,0 +1,930 @@
+=================================================
+Using kgdb, kdb and the kernel debugger internals
+=================================================
+
+:Author: Jason Wessel
+
+Introduction
+============
+
+The kernel has two different debugger front ends (kdb and kgdb) which
+interface to the debug core. It is possible to use either of the
+debugger front ends and dynamically transition between them if you
+configure the kernel properly at compile and runtime.
+
+Kdb is simplistic shell-style interface which you can use on a system
+console with a keyboard or serial console. You can use it to inspect
+memory, registers, process lists, dmesg, and even set breakpoints to
+stop in a certain location. Kdb is not a source level debugger, although
+you can set breakpoints and execute some basic kernel run control. Kdb
+is mainly aimed at doing some analysis to aid in development or
+diagnosing kernel problems. You can access some symbols by name in
+kernel built-ins or in kernel modules if the code was built with
+``CONFIG_KALLSYMS``.
+
+Kgdb is intended to be used as a source level debugger for the Linux
+kernel. It is used along with gdb to debug a Linux kernel. The
+expectation is that gdb can be used to "break in" to the kernel to
+inspect memory, variables and look through call stack information
+similar to the way an application developer would use gdb to debug an
+application. It is possible to place breakpoints in kernel code and
+perform some limited execution stepping.
+
+Two machines are required for using kgdb. One of these machines is a
+development machine and the other is the target machine. The kernel to
+be debugged runs on the target machine. The development machine runs an
+instance of gdb against the vmlinux file which contains the symbols (not
+a boot image such as bzImage, zImage, uImage...). In gdb the developer
+specifies the connection parameters and connects to kgdb. The type of
+connection a developer makes with gdb depends on the availability of
+kgdb I/O modules compiled as built-ins or loadable kernel modules in the
+test machine's kernel.
+
+Compiling a kernel
+==================
+
+-  In order to enable compilation of kdb, you must first enable kgdb.
+
+-  The kgdb test compile options are described in the kgdb test suite
+   chapter.
+
+Kernel config options for kgdb
+------------------------------
+
+To enable ``CONFIG_KGDB`` you should look under "Kernel hacking" /
+"Kernel debugging" and select "KGDB: kernel debugger".
+
+While it is not a hard requirement that you have symbols in your vmlinux
+file, gdb tends not to be very useful without the symbolic data, so you
+will want to turn on ``CONFIG_DEBUG_INFO`` which is called "Compile the
+kernel with debug info" in the config menu.
+
+It is advised, but not required, that you turn on the
+``CONFIG_FRAME_POINTER`` kernel option which is called "Compile the
+kernel with frame pointers" in the config menu. This option inserts code
+to into the compiled executable which saves the frame information in
+registers or on the stack at different points which allows a debugger
+such as gdb to more accurately construct stack back traces while
+debugging the kernel.
+
+If the architecture that you are using supports the kernel option
+CONFIG_STRICT_KERNEL_RWX, you should consider turning it off. This
+option will prevent the use of software breakpoints because it marks
+certain regions of the kernel's memory space as read-only. If kgdb
+supports it for the architecture you are using, you can use hardware
+breakpoints if you desire to run with the CONFIG_STRICT_KERNEL_RWX
+option turned on, else you need to turn off this option.
+
+Next you should choose one of more I/O drivers to interconnect debugging
+host and debugged target. Early boot debugging requires a KGDB I/O
+driver that supports early debugging and the driver must be built into
+the kernel directly. Kgdb I/O driver configuration takes place via
+kernel or module parameters which you can learn more about in the in the
+section that describes the parameter "kgdboc".
+
+Here is an example set of .config symbols to enable or disable for kgdb:
+
+-  # CONFIG_STRICT_KERNEL_RWX is not set
+
+-  CONFIG_FRAME_POINTER=y
+
+-  CONFIG_KGDB=y
+
+-  CONFIG_KGDB_SERIAL_CONSOLE=y
+
+Kernel config options for kdb
+-----------------------------
+
+Kdb is quite a bit more complex than the simple gdbstub sitting on top
+of the kernel's debug core. Kdb must implement a shell, and also adds
+some helper functions in other parts of the kernel, responsible for
+printing out interesting data such as what you would see if you ran
+"lsmod", or "ps". In order to build kdb into the kernel you follow the
+same steps as you would for kgdb.
+
+The main config option for kdb is ``CONFIG_KGDB_KDB`` which is called
+"KGDB_KDB: include kdb frontend for kgdb" in the config menu. In theory
+you would have already also selected an I/O driver such as the
+CONFIG_KGDB_SERIAL_CONSOLE interface if you plan on using kdb on a
+serial port, when you were configuring kgdb.
+
+If you want to use a PS/2-style keyboard with kdb, you would select
+CONFIG_KDB_KEYBOARD which is called "KGDB_KDB: keyboard as input
+device" in the config menu. The CONFIG_KDB_KEYBOARD option is not used
+for anything in the gdb interface to kgdb. The CONFIG_KDB_KEYBOARD
+option only works with kdb.
+
+Here is an example set of .config symbols to enable/disable kdb:
+
+-  # CONFIG_STRICT_KERNEL_RWX is not set
+
+-  CONFIG_FRAME_POINTER=y
+
+-  CONFIG_KGDB=y
+
+-  CONFIG_KGDB_SERIAL_CONSOLE=y
+
+-  CONFIG_KGDB_KDB=y
+
+-  CONFIG_KDB_KEYBOARD=y
+
+Kernel Debugger Boot Arguments
+==============================
+
+This section describes the various runtime kernel parameters that affect
+the configuration of the kernel debugger. The following chapter covers
+using kdb and kgdb as well as providing some examples of the
+configuration parameters.
+
+Kernel parameter: kgdboc
+------------------------
+
+The kgdboc driver was originally an abbreviation meant to stand for
+"kgdb over console". Today it is the primary mechanism to configure how
+to communicate from gdb to kgdb as well as the devices you want to use
+to interact with the kdb shell.
+
+For kgdb/gdb, kgdboc is designed to work with a single serial port. It
+is intended to cover the circumstance where you want to use a serial
+console as your primary console as well as using it to perform kernel
+debugging. It is also possible to use kgdb on a serial port which is not
+designated as a system console. Kgdboc may be configured as a kernel
+built-in or a kernel loadable module. You can only make use of
+``kgdbwait`` and early debugging if you build kgdboc into the kernel as
+a built-in.
+
+Optionally you can elect to activate kms (Kernel Mode Setting)
+integration. When you use kms with kgdboc and you have a video driver
+that has atomic mode setting hooks, it is possible to enter the debugger
+on the graphics console. When the kernel execution is resumed, the
+previous graphics mode will be restored. This integration can serve as a
+useful tool to aid in diagnosing crashes or doing analysis of memory
+with kdb while allowing the full graphics console applications to run.
+
+kgdboc arguments
+~~~~~~~~~~~~~~~~
+
+Usage: ``kgdboc=[kms][[,]kbd][[,]serial_device][,baud]``
+
+The order listed above must be observed if you use any of the optional
+configurations together.
+
+Abbreviations:
+
+-  kms = Kernel Mode Setting
+
+-  kbd = Keyboard
+
+You can configure kgdboc to use the keyboard, and/or a serial device
+depending on if you are using kdb and/or kgdb, in one of the following
+scenarios. The order listed above must be observed if you use any of the
+optional configurations together. Using kms + only gdb is generally not
+a useful combination.
+
+Using loadable module or built-in
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+1. As a kernel built-in:
+
+   Use the kernel boot argument: ``kgdboc=<tty-device>,[baud]``
+
+2. As a kernel loadable module:
+
+   Use the command: ``modprobe kgdboc kgdboc=<tty-device>,[baud]``
+
+   Here are two examples of how you might format the kgdboc string. The
+   first is for an x86 target using the first serial port. The second
+   example is for the ARM Versatile AB using the second serial port.
+
+   1. ``kgdboc=ttyS0,115200``
+
+   2. ``kgdboc=ttyAMA1,115200``
+
+Configure kgdboc at runtime with sysfs
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+At run time you can enable or disable kgdboc by echoing a parameters
+into the sysfs. Here are two examples:
+
+1. Enable kgdboc on ttyS0
+
+   ``echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc``
+
+2. Disable kgdboc
+
+   ``echo "" > /sys/module/kgdboc/parameters/kgdboc``
+
+NOTE: You do not need to specify the baud if you are configuring the
+console on tty which is already configured or open.
+
+More examples
+^^^^^^^^^^^^^
+
+You can configure kgdboc to use the keyboard, and/or a serial device
+depending on if you are using kdb and/or kgdb, in one of the following
+scenarios.
+
+1. kdb and kgdb over only a serial port
+
+   ``kgdboc=<serial_device>[,baud]``
+
+   Example: ``kgdboc=ttyS0,115200``
+
+2. kdb and kgdb with keyboard and a serial port
+
+   ``kgdboc=kbd,<serial_device>[,baud]``
+
+   Example: ``kgdboc=kbd,ttyS0,115200``
+
+3. kdb with a keyboard
+
+   ``kgdboc=kbd``
+
+4. kdb with kernel mode setting
+
+   ``kgdboc=kms,kbd``
+
+5. kdb with kernel mode setting and kgdb over a serial port
+
+   ``kgdboc=kms,kbd,ttyS0,115200``
+
+NOTE: Kgdboc does not support interrupting the target via the gdb remote
+protocol. You must manually send a sysrq-g unless you have a proxy that
+splits console output to a terminal program. A console proxy has a
+separate TCP port for the debugger and a separate TCP port for the
+"human" console. The proxy can take care of sending the sysrq-g for you.
+
+When using kgdboc with no debugger proxy, you can end up connecting the
+debugger at one of two entry points. If an exception occurs after you
+have loaded kgdboc, a message should print on the console stating it is
+waiting for the debugger. In this case you disconnect your terminal
+program and then connect the debugger in its place. If you want to
+interrupt the target system and forcibly enter a debug session you have
+to issue a Sysrq sequence and then type the letter ``g``. Then you
+disconnect the terminal session and connect gdb. Your options if you
+don't like this are to hack gdb to send the sysrq-g for you as well as
+on the initial connect, or to use a debugger proxy that allows an
+unmodified gdb to do the debugging.
+
+Kernel parameter: kgdbwait
+--------------------------
+
+The Kernel command line option ``kgdbwait`` makes kgdb wait for a
+debugger connection during booting of a kernel. You can only use this
+option if you compiled a kgdb I/O driver into the kernel and you
+specified the I/O driver configuration as a kernel command line option.
+The kgdbwait parameter should always follow the configuration parameter
+for the kgdb I/O driver in the kernel command line else the I/O driver
+will not be configured prior to asking the kernel to use it to wait.
+
+The kernel will stop and wait as early as the I/O driver and
+architecture allows when you use this option. If you build the kgdb I/O
+driver as a loadable kernel module kgdbwait will not do anything.
+
+Kernel parameter: kgdbcon
+-------------------------
+
+The kgdbcon feature allows you to see printk() messages inside gdb while
+gdb is connected to the kernel. Kdb does not make use of the kgdbcon
+feature.
+
+Kgdb supports using the gdb serial protocol to send console messages to
+the debugger when the debugger is connected and running. There are two
+ways to activate this feature.
+
+1. Activate with the kernel command line option:
+
+   ``kgdbcon``
+
+2. Use sysfs before configuring an I/O driver
+
+   ``echo 1 > /sys/module/kgdb/parameters/kgdb_use_con``
+
+   NOTE: If you do this after you configure the kgdb I/O driver, the
+   setting will not take effect until the next point the I/O is
+   reconfigured.
+
+IMPORTANT NOTE: You cannot use kgdboc + kgdbcon on a tty that is an
+active system console. An example of incorrect usage is
+``console=ttyS0,115200 kgdboc=ttyS0 kgdbcon``
+
+It is possible to use this option with kgdboc on a tty that is not a
+system console.
+
+Run time parameter: kgdbreboot
+------------------------------
+
+The kgdbreboot feature allows you to change how the debugger deals with
+the reboot notification. You have 3 choices for the behavior. The
+default behavior is always set to 0.
+
+1. echo -1 > /sys/module/debug_core/parameters/kgdbreboot
+
+   Ignore the reboot notification entirely.
+
+2. echo 0 > /sys/module/debug_core/parameters/kgdbreboot
+
+   Send the detach message to any attached debugger client.
+
+3. echo 1 > /sys/module/debug_core/parameters/kgdbreboot
+
+   Enter the debugger on reboot notify.
+
+Using kdb
+=========
+
+Quick start for kdb on a serial port
+------------------------------------
+
+This is a quick example of how to use kdb.
+
+1. Configure kgdboc at boot using kernel parameters:
+
+   -  ``console=ttyS0,115200 kgdboc=ttyS0,115200``
+
+   OR
+
+   Configure kgdboc after the kernel has booted; assuming you are using
+   a serial port console:
+
+   -  ``echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc``
+
+2. Enter the kernel debugger manually or by waiting for an oops or
+   fault. There are several ways you can enter the kernel debugger
+   manually; all involve using the sysrq-g, which means you must have
+   enabled CONFIG_MAGIC_SYSRQ=y in your kernel config.
+
+   -  When logged in as root or with a super user session you can run:
+
+      ``echo g > /proc/sysrq-trigger``
+
+   -  Example using minicom 2.2
+
+      Press: ``Control-a``
+
+      Press: ``f``
+
+      Press: ``g``
+
+   -  When you have telneted to a terminal server that supports sending
+      a remote break
+
+      Press: ``Control-]``
+
+      Type in:\ ``send break``
+
+      Press: ``Enter``
+
+      Press: ``g``
+
+3. From the kdb prompt you can run the "help" command to see a complete
+   list of the commands that are available.
+
+   Some useful commands in kdb include:
+
+   -  lsmod -- Shows where kernel modules are loaded
+
+   -  ps -- Displays only the active processes
+
+   -  ps A -- Shows all the processes
+
+   -  summary -- Shows kernel version info and memory usage
+
+   -  bt -- Get a backtrace of the current process using dump_stack()
+
+   -  dmesg -- View the kernel syslog buffer
+
+   -  go -- Continue the system
+
+4. When you are done using kdb you need to consider rebooting the system
+   or using the "go" command to resuming normal kernel execution. If you
+   have paused the kernel for a lengthy period of time, applications
+   that rely on timely networking or anything to do with real wall clock
+   time could be adversely affected, so you should take this into
+   consideration when using the kernel debugger.
+
+Quick start for kdb using a keyboard connected console
+------------------------------------------------------
+
+This is a quick example of how to use kdb with a keyboard.
+
+1. Configure kgdboc at boot using kernel parameters:
+
+   -  ``kgdboc=kbd``
+
+   OR
+
+   Configure kgdboc after the kernel has booted:
+
+   -  ``echo kbd > /sys/module/kgdboc/parameters/kgdboc``
+
+2. Enter the kernel debugger manually or by waiting for an oops or
+   fault. There are several ways you can enter the kernel debugger
+   manually; all involve using the sysrq-g, which means you must have
+   enabled CONFIG_MAGIC_SYSRQ=y in your kernel config.
+
+   -  When logged in as root or with a super user session you can run:
+
+      ``echo g > /proc/sysrq-trigger``
+
+   -  Example using a laptop keyboard
+
+      Press and hold down: ``Alt``
+
+      Press and hold down: ``Fn``
+
+      Press and release the key with the label: ``SysRq``
+
+      Release: ``Fn``
+
+      Press and release: ``g``
+
+      Release: ``Alt``
+
+   -  Example using a PS/2 101-key keyboard
+
+      Press and hold down: ``Alt``
+
+      Press and release the key with the label: ``SysRq``
+
+      Press and release: ``g``
+
+      Release: ``Alt``
+
+3. Now type in a kdb command such as "help", "dmesg", "bt" or "go" to
+   continue kernel execution.
+
+Using kgdb / gdb
+================
+
+In order to use kgdb you must activate it by passing configuration
+information to one of the kgdb I/O drivers. If you do not pass any
+configuration information kgdb will not do anything at all. Kgdb will
+only actively hook up to the kernel trap hooks if a kgdb I/O driver is
+loaded and configured. If you unconfigure a kgdb I/O driver, kgdb will
+unregister all the kernel hook points.
+
+All kgdb I/O drivers can be reconfigured at run time, if
+``CONFIG_SYSFS`` and ``CONFIG_MODULES`` are enabled, by echo'ing a new
+config string to ``/sys/module/<driver>/parameter/<option>``. The driver
+can be unconfigured by passing an empty string. You cannot change the
+configuration while the debugger is attached. Make sure to detach the
+debugger with the ``detach`` command prior to trying to unconfigure a
+kgdb I/O driver.
+
+Connecting with gdb to a serial port
+------------------------------------
+
+1. Configure kgdboc
+
+   Configure kgdboc at boot using kernel parameters:
+
+   -  ``kgdboc=ttyS0,115200``
+
+   OR
+
+   Configure kgdboc after the kernel has booted:
+
+   -  ``echo ttyS0 > /sys/module/kgdboc/parameters/kgdboc``
+
+2. Stop kernel execution (break into the debugger)
+
+   In order to connect to gdb via kgdboc, the kernel must first be
+   stopped. There are several ways to stop the kernel which include
+   using kgdbwait as a boot argument, via a sysrq-g, or running the
+   kernel until it takes an exception where it waits for the debugger to
+   attach.
+
+   -  When logged in as root or with a super user session you can run:
+
+      ``echo g > /proc/sysrq-trigger``
+
+   -  Example using minicom 2.2
+
+      Press: ``Control-a``
+
+      Press: ``f``
+
+      Press: ``g``
+
+   -  When you have telneted to a terminal server that supports sending
+      a remote break
+
+      Press: ``Control-]``
+
+      Type in:\ ``send break``
+
+      Press: ``Enter``
+
+      Press: ``g``
+
+3. Connect from gdb
+
+   Example (using a directly connected port):
+
+   ::
+
+           % gdb ./vmlinux
+           (gdb) set remotebaud 115200
+           (gdb) target remote /dev/ttyS0
+
+
+   Example (kgdb to a terminal server on TCP port 2012):
+
+   ::
+
+           % gdb ./vmlinux
+           (gdb) target remote 192.168.2.2:2012
+
+
+   Once connected, you can debug a kernel the way you would debug an
+   application program.
+
+   If you are having problems connecting or something is going seriously
+   wrong while debugging, it will most often be the case that you want
+   to enable gdb to be verbose about its target communications. You do
+   this prior to issuing the ``target
+       remote`` command by typing in: ``set debug remote 1``
+
+Remember if you continue in gdb, and need to "break in" again, you need
+to issue an other sysrq-g. It is easy to create a simple entry point by
+putting a breakpoint at ``sys_sync`` and then you can run "sync" from a
+shell or script to break into the debugger.
+
+kgdb and kdb interoperability
+=============================
+
+It is possible to transition between kdb and kgdb dynamically. The debug
+core will remember which you used the last time and automatically start
+in the same mode.
+
+Switching between kdb and kgdb
+------------------------------
+
+Switching from kgdb to kdb
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There are two ways to switch from kgdb to kdb: you can use gdb to issue
+a maintenance packet, or you can blindly type the command $3#33.
+Whenever the kernel debugger stops in kgdb mode it will print the
+message ``KGDB or $3#33 for KDB``. It is important to note that you have
+to type the sequence correctly in one pass. You cannot type a backspace
+or delete because kgdb will interpret that as part of the debug stream.
+
+1. Change from kgdb to kdb by blindly typing:
+
+   ``$3#33``
+
+2. Change from kgdb to kdb with gdb
+
+   ``maintenance packet 3``
+
+   NOTE: Now you must kill gdb. Typically you press control-z and issue
+   the command: kill -9 %
+
+Change from kdb to kgdb
+~~~~~~~~~~~~~~~~~~~~~~~
+
+There are two ways you can change from kdb to kgdb. You can manually
+enter kgdb mode by issuing the kgdb command from the kdb shell prompt,
+or you can connect gdb while the kdb shell prompt is active. The kdb
+shell looks for the typical first commands that gdb would issue with the
+gdb remote protocol and if it sees one of those commands it
+automatically changes into kgdb mode.
+
+1. From kdb issue the command:
+
+   ``kgdb``
+
+   Now disconnect your terminal program and connect gdb in its place
+
+2. At the kdb prompt, disconnect the terminal program and connect gdb in
+   its place.
+
+Running kdb commands from gdb
+-----------------------------
+
+It is possible to run a limited set of kdb commands from gdb, using the
+gdb monitor command. You don't want to execute any of the run control or
+breakpoint operations, because it can disrupt the state of the kernel
+debugger. You should be using gdb for breakpoints and run control
+operations if you have gdb connected. The more useful commands to run
+are things like lsmod, dmesg, ps or possibly some of the memory
+information commands. To see all the kdb commands you can run
+``monitor help``.
+
+Example:
+
+.. raw:: html
+
+   <div class="informalexample">
+
+::
+
+    (gdb) monitor ps
+    1 idle process (state I) and
+    27 sleeping system daemon (state M) processes suppressed,
+    use 'ps A' to see all.
+    Task Addr       Pid   Parent [*] cpu State Thread     Command
+
+    0xc78291d0        1        0  0    0   S  0xc7829404  init
+    0xc7954150      942        1  0    0   S  0xc7954384  dropbear
+    0xc78789c0      944        1  0    0   S  0xc7878bf4  sh
+    (gdb)
+
+
+.. raw:: html
+
+   </div>
+
+kgdb Test Suite
+===============
+
+When kgdb is enabled in the kernel config you can also elect to enable
+the config parameter KGDB_TESTS. Turning this on will enable a special
+kgdb I/O module which is designed to test the kgdb internal functions.
+
+The kgdb tests are mainly intended for developers to test the kgdb
+internals as well as a tool for developing a new kgdb architecture
+specific implementation. These tests are not really for end users of the
+Linux kernel. The primary source of documentation would be to look in
+the drivers/misc/kgdbts.c file.
+
+The kgdb test suite can also be configured at compile time to run the
+core set of tests by setting the kernel config parameter
+KGDB_TESTS_ON_BOOT. This particular option is aimed at automated
+regression testing and does not require modifying the kernel boot config
+arguments. If this is turned on, the kgdb test suite can be disabled by
+specifying "kgdbts=" as a kernel boot argument.
+
+Kernel Debugger Internals
+=========================
+
+Architecture Specifics
+----------------------
+
+The kernel debugger is organized into a number of components:
+
+1. The debug core
+
+   The debug core is found in kernel/debugger/debug_core.c. It
+   contains:
+
+   -  A generic OS exception handler which includes sync'ing the
+      processors into a stopped state on an multi-CPU system.
+
+   -  The API to talk to the kgdb I/O drivers
+
+   -  The API to make calls to the arch-specific kgdb implementation
+
+   -  The logic to perform safe memory reads and writes to memory while
+      using the debugger
+
+   -  A full implementation for software breakpoints unless overridden
+      by the arch
+
+   -  The API to invoke either the kdb or kgdb frontend to the debug
+      core.
+
+   -  The structures and callback API for atomic kernel mode setting.
+
+      NOTE: kgdboc is where the kms callbacks are invoked.
+
+2. kgdb arch-specific implementation
+
+   This implementation is generally found in arch/\*/kernel/kgdb.c. As
+   an example, arch/x86/kernel/kgdb.c contains the specifics to
+   implement HW breakpoint as well as the initialization to dynamically
+   register and unregister for the trap handlers on this architecture.
+   The arch-specific portion implements:
+
+   -  contains an arch-specific trap catcher which invokes
+      kgdb_handle_exception() to start kgdb about doing its work
+
+   -  translation to and from gdb specific packet format to pt_regs
+
+   -  Registration and unregistration of architecture specific trap
+      hooks
+
+   -  Any special exception handling and cleanup
+
+   -  NMI exception handling and cleanup
+
+   -  (optional) HW breakpoints
+
+3. gdbstub frontend (aka kgdb)
+
+   The gdbstub is located in kernel/debug/gdbstub.c. It contains:
+
+   -  All the logic to implement the gdb serial protocol
+
+4. kdb frontend
+
+   The kdb debugger shell is broken down into a number of components.
+   The kdb core is located in kernel/debug/kdb. There are a number of
+   helper functions in some of the other kernel components to make it
+   possible for kdb to examine and report information about the kernel
+   without taking locks that could cause a kernel deadlock. The kdb core
+   contains implements the following functionality.
+
+   -  A simple shell
+
+   -  The kdb core command set
+
+   -  A registration API to register additional kdb shell commands.
+
+      -  A good example of a self-contained kdb module is the "ftdump"
+         command for dumping the ftrace buffer. See:
+         kernel/trace/trace_kdb.c
+
+      -  For an example of how to dynamically register a new kdb command
+         you can build the kdb_hello.ko kernel module from
+         samples/kdb/kdb_hello.c. To build this example you can set
+         CONFIG_SAMPLES=y and CONFIG_SAMPLE_KDB=m in your kernel
+         config. Later run "modprobe kdb_hello" and the next time you
+         enter the kdb shell, you can run the "hello" command.
+
+   -  The implementation for kdb_printf() which emits messages directly
+      to I/O drivers, bypassing the kernel log.
+
+   -  SW / HW breakpoint management for the kdb shell
+
+5. kgdb I/O driver
+
+   Each kgdb I/O driver has to provide an implementation for the
+   following:
+
+   -  configuration via built-in or module
+
+   -  dynamic configuration and kgdb hook registration calls
+
+   -  read and write character interface
+
+   -  A cleanup handler for unconfiguring from the kgdb core
+
+   -  (optional) Early debug methodology
+
+   Any given kgdb I/O driver has to operate very closely with the
+   hardware and must do it in such a way that does not enable interrupts
+   or change other parts of the system context without completely
+   restoring them. The kgdb core will repeatedly "poll" a kgdb I/O
+   driver for characters when it needs input. The I/O driver is expected
+   to return immediately if there is no data available. Doing so allows
+   for the future possibility to touch watchdog hardware in such a way
+   as to have a target system not reset when these are enabled.
+
+If you are intent on adding kgdb architecture specific support for a new
+architecture, the architecture should define ``HAVE_ARCH_KGDB`` in the
+architecture specific Kconfig file. This will enable kgdb for the
+architecture, and at that point you must create an architecture specific
+kgdb implementation.
+
+There are a few flags which must be set on every architecture in their
+<asm/kgdb.h> file. These are:
+
+-  NUMREGBYTES: The size in bytes of all of the registers, so that we
+   can ensure they will all fit into a packet.
+
+-  BUFMAX: The size in bytes of the buffer GDB will read into. This must
+   be larger than NUMREGBYTES.
+
+-  CACHE_FLUSH_IS_SAFE: Set to 1 if it is always safe to call
+   flush_cache_range or flush_icache_range. On some architectures,
+   these functions may not be safe to call on SMP since we keep other
+   CPUs in a holding pattern.
+
+There are also the following functions for the common backend, found in
+kernel/kgdb.c, that must be supplied by the architecture-specific
+backend unless marked as (optional), in which case a default function
+maybe used if the architecture does not need to provide a specific
+implementation.
+
+.. kernel-doc:: include/linux/kgdb.h
+   :internal:
+
+kgdboc internals
+----------------
+
+kgdboc and uarts
+~~~~~~~~~~~~~~~~
+
+The kgdboc driver is actually a very thin driver that relies on the
+underlying low level to the hardware driver having "polling hooks" to
+which the tty driver is attached. In the initial implementation of
+kgdboc the serial_core was changed to expose a low level UART hook for
+doing polled mode reading and writing of a single character while in an
+atomic context. When kgdb makes an I/O request to the debugger, kgdboc
+invokes a callback in the serial core which in turn uses the callback in
+the UART driver.
+
+When using kgdboc with a UART, the UART driver must implement two
+callbacks in the ``struct uart_ops``. Example from drivers/8250.c:
+
+::
+
+    #ifdef CONFIG_CONSOLE_POLL
+        .poll_get_char = serial8250_get_poll_char,
+        .poll_put_char = serial8250_put_poll_char,
+    #endif
+
+
+Any implementation specifics around creating a polling driver use the
+``#ifdef CONFIG_CONSOLE_POLL``, as shown above. Keep in mind that
+polling hooks have to be implemented in such a way that they can be
+called from an atomic context and have to restore the state of the UART
+chip on return such that the system can return to normal when the
+debugger detaches. You need to be very careful with any kind of lock you
+consider, because failing here is most likely going to mean pressing the
+reset button.
+
+kgdboc and keyboards
+~~~~~~~~~~~~~~~~~~~~
+
+The kgdboc driver contains logic to configure communications with an
+attached keyboard. The keyboard infrastructure is only compiled into the
+kernel when CONFIG_KDB_KEYBOARD=y is set in the kernel configuration.
+
+The core polled keyboard driver driver for PS/2 type keyboards is in
+drivers/char/kdb_keyboard.c. This driver is hooked into the debug core
+when kgdboc populates the callback in the array called
+``kdb_poll_funcs[]``. The kdb_get_kbd_char() is the top-level
+function which polls hardware for single character input.
+
+kgdboc and kms
+~~~~~~~~~~~~~~
+
+The kgdboc driver contains logic to request the graphics display to
+switch to a text context when you are using "kgdboc=kms,kbd", provided
+that you have a video driver which has a frame buffer console and atomic
+kernel mode setting support.
+
+Every time the kernel debugger is entered it calls
+kgdboc_pre_exp_handler() which in turn calls con_debug_enter() in
+the virtual console layer. On resuming kernel execution, the kernel
+debugger calls kgdboc_post_exp_handler() which in turn calls
+con_debug_leave().
+
+Any video driver that wants to be compatible with the kernel debugger
+and the atomic kms callbacks must implement the mode_set_base_atomic,
+fb_debug_enter and fb_debug_leave operations. For the
+fb_debug_enter and fb_debug_leave the option exists to use the
+generic drm fb helper functions or implement something custom for the
+hardware. The following example shows the initialization of the
+.mode_set_base_atomic operation in
+drivers/gpu/drm/i915/intel_display.c:
+
+.. raw:: html
+
+   <div class="informalexample">
+
+::
+
+    static const struct drm_crtc_helper_funcs intel_helper_funcs = {
+    [...]
+            .mode_set_base_atomic = intel_pipe_set_base_atomic,
+    [...]
+    };
+
+
+.. raw:: html
+
+   </div>
+
+Here is an example of how the i915 driver initializes the
+fb_debug_enter and fb_debug_leave functions to use the generic drm
+helpers in drivers/gpu/drm/i915/intel_fb.c:
+
+.. raw:: html
+
+   <div class="informalexample">
+
+::
+
+    static struct fb_ops intelfb_ops = {
+    [...]
+           .fb_debug_enter = drm_fb_helper_debug_enter,
+           .fb_debug_leave = drm_fb_helper_debug_leave,
+    [...]
+    };
+
+
+.. raw:: html
+
+   </div>
+
+Credits
+=======
+
+The following people have contributed to this document:
+
+1. Amit Kale\ amitkale@linsyssoft.com
+
+2. Tom Rini\ trini@kernel.crashing.org
+
+In March 2008 this document was completely rewritten by:
+
+-  Jason Wessel\ jason.wessel@windriver.com
+
+In Jan 2010 this document was updated to include kdb.
+
+-  Jason Wessel\ jason.wessel@windriver.com