Introduction
  ============
  
  This document describes how to use the dynamic debug (ddebug) 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() calls can be

  dynamically enabled per-callsite.
  
  Dynamic debug has even more useful features:
  
   * Simple query language allows turning on and off debugging statements by
     matching any combination 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()s 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
  
  Note in particular that the third column shows the enabled behaviour
  flags for each debug statement callsite (see below for definitions of the
  flags).  The default value, no extra behaviour 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 whitespace characters.  Note that newlines are treated as word
  separators and do *not* end a command or allow multiple commands to
  be done together.  So these are all equivalent:
  
  nullarbor:~ # echo -c 'file svcsock.c line 1603 +p' >
  				<debugfs>/dynamic_debug/control
  nullarbor:~ # echo -c '  file   svcsock.c     line  1603 +p  ' >
  				<debugfs>/dynamic_debug/control
  nullarbor:~ # echo -c 'file svcsock.c
  line 1603 +p' >
  				<debugfs>/dynamic_debug/control
  nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
  				<debugfs>/dynamic_debug/control
  
  Commands are bounded by a write() system call.  If you want to do
  multiple commands you need to do a separate "echo" for each, like:
  
  nullarbor:~ # echo 'file svcsock.c line 1603 +p' > /proc/dprintk ;\
  > echo 'file svcsock.c line 1563 +p' > /proc/dprintk
  
  or even like:
  
  nullarbor:~ # (
  > echo 'file svcsock.c line 1603 +p' ;\
  > echo 'file svcsock.c line 1563 +p' ;\
  > ) > /proc/dprintk
  
  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 dprintk()
  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 is possible, but is not very useful because it will not
  match any 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
  // Note: line-range cannot contain space, e.g.
  // "1-30" is valid range but "1 - 30" is not.
  
  lineno ::= unsigned-int
  
  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 or the basename of the source file of each
      callsite.  Examples:
  
      file svcsock.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 dprintks
      format readahead	    // some dprintks 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 dprintk() 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:

  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

  p
      Causes a printk() message to be emitted to dmesg

  t
      Include thread ID in messages not generated from interrupt context

  Note the regexp ^[-+=][flmpt]+$ matches a flags specification.

  Note also that there is no convenient syntax to remove all

  the flags at once, you need to use "-flmpt".

  
  Debug messages during boot process
  ==================================
  
  To be able to activate debug messages during the boot process,
  even before userspace and debugfs exists, use the boot parameter:
  ddebug_query="QUERY"
  
  QUERY follows the syntax described above, but must not exceed 1023
  characters. The enablement of debug messages is done as an arch_initcall.
  Thus you can enable debug messages in all code processed after this
  arch_initcall via this boot parameter.
  On an x86 system for example ACPI enablement is a subsys_initcall and
  ddebug_query="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.

  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