Dynamic debug
  +++++++++++++

  
  Introduction
  ============

  This document describes how to use the dynamic debug (dyndbg) feature.

  Dynamic debug is designed to allow you to dynamically enable/disable
  kernel code to obtain additional kernel information.  Currently, if

  ``CONFIG_DYNAMIC_DEBUG`` is set, then all ``pr_debug()``/``dev_dbg()`` and
  ``print_hex_dump_debug()``/``print_hex_dump_bytes()`` calls can be dynamically

  enabled per-callsite.

  If ``CONFIG_DYNAMIC_DEBUG`` is not set, ``print_hex_dump_debug()`` is just
  shortcut for ``print_hex_dump(KERN_DEBUG)``.

  For ``print_hex_dump_debug()``/``print_hex_dump_bytes()``, format string is
  its ``prefix_str`` argument, if it is constant string; or ``hexdump``
  in case ``prefix_str`` is build dynamically.

  
  Dynamic debug has even more useful features:

   * Simple query language allows turning on and off debugging
     statements by matching any combination of 0 or 1 of:

  
     - source filename
     - function name
     - line number (including ranges of line numbers)
     - module name
     - format string

   * Provides a debugfs control file: ``<debugfs>/dynamic_debug/control``

     which can be read to display the complete list of known debug
     statements, to help guide you

  
  Controlling dynamic debug Behaviour

  ===================================

  The behaviour of ``pr_debug()``/``dev_dbg()`` are controlled via writing to a

  control file in the 'debugfs' filesystem. Thus, you must first mount
  the debugfs filesystem, in order to make use of this feature.
  Subsequently, we refer to the control file as:

  ``<debugfs>/dynamic_debug/control``. For example, if you want to enable
  printing from source file ``svcsock.c``, line 1603 you simply do::

    nullarbor:~ # echo 'file svcsock.c line 1603 +p' >

  				<debugfs>/dynamic_debug/control

  If you make a mistake with the syntax, the write will fail thus::

    nullarbor:~ # echo 'file svcsock.c wtf 1 +p' >

  				<debugfs>/dynamic_debug/control

    -bash: echo: write error: Invalid argument

  
  Viewing Dynamic Debug Behaviour

  ===============================

  You can view the currently configured behaviour of all the debug

  statements via::

    nullarbor:~ # cat <debugfs>/dynamic_debug/control
    # filename:lineno [module]function flags format
    /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:323 [svcxprt_rdma]svc_rdma_cleanup =_ "SVCRDMA Module Removed, deregister RPC RDMA transport\012"
    /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:341 [svcxprt_rdma]svc_rdma_init =_ "\011max_inline       : %d\012"
    /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:340 [svcxprt_rdma]svc_rdma_init =_ "\011sq_depth         : %d\012"
    /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:338 [svcxprt_rdma]svc_rdma_init =_ "\011max_requests     : %d\012"
    ...

  
  
  You can also apply standard Unix text manipulation filters to this

  data, e.g.::

    nullarbor:~ # grep -i rdma <debugfs>/dynamic_debug/control  | wc -l
    62

    nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l
    42

  The third column shows the currently enabled flags for each debug
  statement callsite (see below for definitions of the flags).  The

  default value, with no flags enabled, is ``=_``.  So you can view all
  the debug statement callsites with any non-default flags::

    nullarbor:~ # awk '$3 != "=_"' <debugfs>/dynamic_debug/control
    # filename:lineno [module]function flags format
    /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c:1603 [sunrpc]svc_send p "svc_process: st_sendto returned %d\012"

  
  Command Language Reference
  ==========================
  
  At the lexical level, a command comprises a sequence of words separated

  by spaces or tabs.  So these are all equivalent::

    nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >

  				<debugfs>/dynamic_debug/control

    nullarbor:~ # echo -n '  file   svcsock.c     line  1603 +p  ' >

  				<debugfs>/dynamic_debug/control

    nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >

  				<debugfs>/dynamic_debug/control

  Command submissions are bounded by a write() system call.

  Multiple commands can be written together, separated by ``;`` or ``
  ``::

    ~# echo "func pnpacpi_get_resources +p; func pnp_assign_mem +p" \
       > <debugfs>/dynamic_debug/control

  If your query set is big, you can batch them too::

    ~# cat query-batch-file > <debugfs>/dynamic_debug/control

  A another way is to use wildcard. The match rule support ``*`` (matches
  zero or more characters) and ``?`` (matches exactly one character).For
  example, you can match all usb drivers::

  
    ~# echo "file drivers/usb/* +p" > <debugfs>/dynamic_debug/control

  At the syntactical level, a command comprises a sequence of match

  specifications, followed by a flags change specification::

    command ::= match-spec* flags-spec

  The match-spec's are used to choose a subset of the known pr_debug()

  callsites to which to apply the flags-spec.  Think of them as a query
  with implicit ANDs between each pair.  Note that an empty list of

  match-specs will select all debug statement callsites.

  A match specification comprises a keyword, which controls the
  attribute of the callsite to be compared, and a value to compare

  against.  Possible keywords are:::
  
    match-spec ::= 'func' string |
  		 'file' string |
  		 'module' string |
  		 'format' string |
  		 'line' line-range

    line-range ::= lineno |
  		 '-'lineno |
  		 lineno'-' |
  		 lineno'-'lineno

    lineno ::= unsigned-int
  
  .. note::
  
    ``line-range`` cannot contain space, e.g.
    "1-30" is valid range but "1 - 30" is not.

  
  The meanings of each keyword are:
  
  func
      The given string is compared against the function name

      of each callsite.  Example::

  	func svc_tcp_accept

  
  file

      The given string is compared against either the full pathname, the
      src-root relative pathname, or the basename of the source file of

      each callsite.  Examples::

  	file svcsock.c
  	file kernel/freezer.c
  	file /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c

  
  module
      The given string is compared against the module name
      of each callsite.  The module name is the string as

      seen in ``lsmod``, i.e. without the directory or the ``.ko``
      suffix and with ``-`` changed to ``_``.  Examples::

  	module sunrpc
  	module nfsd

  
  format
      The given string is searched for in the dynamic debug format
      string.  Note that the string does not need to match the
      entire format, only some part.  Whitespace and other
      special characters can be escaped using C octal character

      escape ``\ooo`` notation, e.g. the space character is ``\040``.

      Alternatively, the string can be enclosed in double quote

      characters (``"``) or single quote characters (``'``).
      Examples::

  	format svcrdma:         // many of the NFS/RDMA server pr_debugs
  	format readahead        // some pr_debugs in the readahead cache
  	format nfsd:\040SETATTR // one way to match a format with whitespace
  	format "nfsd: SETATTR"  // a neater way to match a format with whitespace
  	format 'nfsd: SETATTR'  // yet another way to match a format with whitespace

  
  line
      The given line number or range of line numbers is compared

      against the line number of each ``pr_debug()`` callsite.  A single

      line number matches the callsite line number exactly.  A
      range of line numbers matches any callsite between the first
      and last line number inclusive.  An empty first number means
      the first line in the file, an empty line number means the

      last number in the file.  Examples::

  	line 1603           // exactly line 1603
  	line 1600-1605      // the six lines from line 1600 to line 1605
  	line -1605          // the 1605 lines from line 1 to line 1605
  	line 1600-          // all lines from line 1600 to the end of the file

  
  The flags specification comprises a change operation followed
  by one or more flag characters.  The change operation is one

  of the characters::

    -    remove the given flags
    +    add the given flags
    =    set the flags to the given flags

  The flags are::

    p    enables the pr_debug() callsite.
    f    Include the function name in the printed message
    l    Include line number in the printed message
    m    Include module name in the printed message
    t    Include thread ID in messages not generated from interrupt context
    _    No flags are set. (Or'd with others on input)

  For ``print_hex_dump_debug()`` and ``print_hex_dump_bytes()``, only ``p`` flag

  have meaning, other flags ignored.

  For display, the flags are preceded by ``=``

  (mnemonic: what the flags are currently equal to).

  Note the regexp ``^[-+=][flmpt_]+$`` matches a flags specification.
  To clear all flags at once, use ``=_`` or ``-flmpt``.

  Debug messages during Boot Process

  ==================================

  To activate debug messages for core code and built-in modules during
  the boot process, even before userspace and debugfs exists, use

  ``dyndbg="QUERY"``, ``module.dyndbg="QUERY"``, or ``ddebug_query="QUERY"``
  (``ddebug_query`` is obsoleted by ``dyndbg``, and deprecated).  QUERY follows

  the syntax described above, but must not exceed 1023 characters.  Your
  bootloader may impose lower limits.

  These ``dyndbg`` params are processed just after the ddebug tables are

  processed, as part of the arch_initcall.  Thus you can enable debug
  messages in all code run after this arch_initcall via this boot
  parameter.

  On an x86 system for example ACPI enablement is a subsys_initcall and::

     dyndbg="file ec.c +p"

  will show early Embedded Controller transactions during ACPI setup if
  your machine (typically a laptop) has an Embedded Controller.
  PCI (or other devices) initialization also is a hot candidate for using
  this boot parameter for debugging purposes.

  If ``foo`` module is not built-in, ``foo.dyndbg`` will still be processed at

  boot time, without effect, but will be reprocessed when module is

  loaded later. ``dyndbg_query=`` and bare ``dyndbg=`` are only processed at

  boot.
  
  
  Debug Messages at Module Initialization Time
  ============================================

  When ``modprobe foo`` is called, modprobe scans ``/proc/cmdline`` for
  ``foo.params``, strips ``foo.``, and passes them to the kernel along with
  params given in modprobe args or ``/etc/modprob.d/*.conf`` files,

  in the following order:

  1. parameters given via ``/etc/modprobe.d/*.conf``::
  
  	options foo dyndbg=+pt
  	options foo dyndbg # defaults to +p
  
  2. ``foo.dyndbg`` as given in boot args, ``foo.`` is stripped and passed::

  	foo.dyndbg=" func bar +p; func buz +mp"

  3. args to modprobe::

  	modprobe foo dyndbg==pmf # override previous settings
  
  These ``dyndbg`` queries are applied in order, with last having final say.
  This allows boot args to override or modify those from ``/etc/modprobe.d``

  (sensible, since 1 is system wide, 2 is kernel or boot specific), and
  modprobe args to override both.

  In the ``foo.dyndbg="QUERY"`` form, the query must exclude ``module foo``.
  ``foo`` is extracted from the param-name, and applied to each query in
  ``QUERY``, and only 1 match-spec of each type is allowed.

  The ``dyndbg`` option is a "fake" module parameter, which means:

  
  - modules do not need to define it explicitly
  - every module gets it tacitly, whether they use pr_debug or not

  - it doesn't appear in ``/sys/module/$module/parameters/``
    To see it, grep the control file, or inspect ``/proc/cmdline.``

  For ``CONFIG_DYNAMIC_DEBUG`` kernels, any settings given at boot-time (or
  enabled by ``-DDEBUG`` flag during compilation) can be disabled later via
  the sysfs interface if the debug messages are no longer needed::

  
     echo "module module_name -p" > <debugfs>/dynamic_debug/control

  Examples
  ========

  ::
  
    // enable the message at line 1603 of file svcsock.c
    nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >

  				<debugfs>/dynamic_debug/control

    // enable all the messages in file svcsock.c
    nullarbor:~ # echo -n 'file svcsock.c +p' >

  				<debugfs>/dynamic_debug/control

    // enable all the messages in the NFS server module
    nullarbor:~ # echo -n 'module nfsd +p' >

  				<debugfs>/dynamic_debug/control

    // enable all 12 messages in the function svc_process()
    nullarbor:~ # echo -n 'func svc_process +p' >

  				<debugfs>/dynamic_debug/control

    // disable all 12 messages in the function svc_process()
    nullarbor:~ # echo -n 'func svc_process -p' >

  				<debugfs>/dynamic_debug/control

    // enable messages for NFS calls READ, READLINK, READDIR and READDIR+.
    nullarbor:~ # echo -n 'format "nfsd: READ" +p' >

  				<debugfs>/dynamic_debug/control

    // enable messages in files of which the paths include string "usb"
    nullarbor:~ # echo -n '*usb* +p' > <debugfs>/dynamic_debug/control

    // enable all messages
    nullarbor:~ # echo -n '+p' > <debugfs>/dynamic_debug/control

    // add module, function to all enabled messages
    nullarbor:~ # echo -n '+mf' > <debugfs>/dynamic_debug/control

    // boot-args example, with newlines and comments for readability
    Kernel command line: ...
      // see whats going on in dyndbg=value processing
      dynamic_debug.verbose=1
      // enable pr_debugs in 2 builtins, #cmt is stripped
      dyndbg="module params +p #cmt ; module sys +p"
      // enable pr_debugs in 2 functions in a module loaded later
      pc87360.dyndbg="func pc87360_init_device +p; func pc87360_find +p"