linux/Documentation/dynamic-debug-howto.txt
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   1
   2Introduction
   3============
   4
   5This document describes how to use the dynamic debug (dyndbg) feature.
   6
   7Dynamic debug is designed to allow you to dynamically enable/disable
   8kernel code to obtain additional kernel information.  Currently, if
   9CONFIG_DYNAMIC_DEBUG is set, then all pr_debug()/dev_dbg() and
  10print_hex_dump_debug()/print_hex_dump_bytes() calls can be dynamically
  11enabled per-callsite.
  12
  13If CONFIG_DYNAMIC_DEBUG is not set, print_hex_dump_debug() is just
  14shortcut for print_hex_dump(KERN_DEBUG).
  15
  16For print_hex_dump_debug()/print_hex_dump_bytes(), format string is
  17its 'prefix_str' argument, if it is constant string; or "hexdump"
  18in case 'prefix_str' is build dynamically.
  19
  20Dynamic debug has even more useful features:
  21
  22 * Simple query language allows turning on and off debugging
  23   statements by matching any combination of 0 or 1 of:
  24
  25   - source filename
  26   - function name
  27   - line number (including ranges of line numbers)
  28   - module name
  29   - format string
  30
  31 * Provides a debugfs control file: <debugfs>/dynamic_debug/control
  32   which can be read to display the complete list of known debug
  33   statements, to help guide you
  34
  35Controlling dynamic debug Behaviour
  36===================================
  37
  38The behaviour of pr_debug()/dev_dbg()s are controlled via writing to a
  39control file in the 'debugfs' filesystem. Thus, you must first mount
  40the debugfs filesystem, in order to make use of this feature.
  41Subsequently, we refer to the control file as:
  42<debugfs>/dynamic_debug/control. For example, if you want to enable
  43printing from source file 'svcsock.c', line 1603 you simply do:
  44
  45nullarbor:~ # echo 'file svcsock.c line 1603 +p' >
  46                                <debugfs>/dynamic_debug/control
  47
  48If you make a mistake with the syntax, the write will fail thus:
  49
  50nullarbor:~ # echo 'file svcsock.c wtf 1 +p' >
  51                                <debugfs>/dynamic_debug/control
  52-bash: echo: write error: Invalid argument
  53
  54Viewing Dynamic Debug Behaviour
  55===========================
  56
  57You can view the currently configured behaviour of all the debug
  58statements via:
  59
  60nullarbor:~ # cat <debugfs>/dynamic_debug/control
  61# filename:lineno [module]function flags format
  62/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"
  63/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:341 [svcxprt_rdma]svc_rdma_init =_ "\011max_inline       : %d\012"
  64/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:340 [svcxprt_rdma]svc_rdma_init =_ "\011sq_depth         : %d\012"
  65/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:338 [svcxprt_rdma]svc_rdma_init =_ "\011max_requests     : %d\012"
  66...
  67
  68
  69You can also apply standard Unix text manipulation filters to this
  70data, e.g.
  71
  72nullarbor:~ # grep -i rdma <debugfs>/dynamic_debug/control  | wc -l
  7362
  74
  75nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l
  7642
  77
  78The third column shows the currently enabled flags for each debug
  79statement callsite (see below for definitions of the flags).  The
  80default value, with no flags enabled, is "=_".  So you can view all
  81the debug statement callsites with any non-default flags:
  82
  83nullarbor:~ # awk '$3 != "=_"' <debugfs>/dynamic_debug/control
  84# filename:lineno [module]function flags format
  85/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"
  86
  87
  88Command Language Reference
  89==========================
  90
  91At the lexical level, a command comprises a sequence of words separated
  92by spaces or tabs.  So these are all equivalent:
  93
  94nullarbor:~ # echo -c 'file svcsock.c line 1603 +p' >
  95                                <debugfs>/dynamic_debug/control
  96nullarbor:~ # echo -c '  file   svcsock.c     line  1603 +p  ' >
  97                                <debugfs>/dynamic_debug/control
  98nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
  99                                <debugfs>/dynamic_debug/control
 100
 101Command submissions are bounded by a write() system call.
 102Multiple commands can be written together, separated by ';' or '\n'.
 103
 104  ~# echo "func pnpacpi_get_resources +p; func pnp_assign_mem +p" \
 105     > <debugfs>/dynamic_debug/control
 106
 107If your query set is big, you can batch them too:
 108
 109  ~# cat query-batch-file > <debugfs>/dynamic_debug/control
 110
 111A another way is to use wildcard. The match rule support '*' (matches
 112zero or more characters) and '?' (matches exactly one character).For
 113example, you can match all usb drivers:
 114
 115  ~# echo "file drivers/usb/* +p" > <debugfs>/dynamic_debug/control
 116
 117At the syntactical level, a command comprises a sequence of match
 118specifications, followed by a flags change specification.
 119
 120command ::= match-spec* flags-spec
 121
 122The match-spec's are used to choose a subset of the known pr_debug()
 123callsites to which to apply the flags-spec.  Think of them as a query
 124with implicit ANDs between each pair.  Note that an empty list of
 125match-specs will select all debug statement callsites.
 126
 127A match specification comprises a keyword, which controls the
 128attribute of the callsite to be compared, and a value to compare
 129against.  Possible keywords are:
 130
 131match-spec ::= 'func' string |
 132               'file' string |
 133               'module' string |
 134               'format' string |
 135               'line' line-range
 136
 137line-range ::= lineno |
 138               '-'lineno |
 139               lineno'-' |
 140               lineno'-'lineno
 141// Note: line-range cannot contain space, e.g.
 142// "1-30" is valid range but "1 - 30" is not.
 143
 144lineno ::= unsigned-int
 145
 146The meanings of each keyword are:
 147
 148func
 149    The given string is compared against the function name
 150    of each callsite.  Example:
 151
 152    func svc_tcp_accept
 153
 154file
 155    The given string is compared against either the full pathname, the
 156    src-root relative pathname, or the basename of the source file of
 157    each callsite.  Examples:
 158
 159    file svcsock.c
 160    file kernel/freezer.c
 161    file /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c
 162
 163module
 164    The given string is compared against the module name
 165    of each callsite.  The module name is the string as
 166    seen in "lsmod", i.e. without the directory or the .ko
 167    suffix and with '-' changed to '_'.  Examples:
 168
 169    module sunrpc
 170    module nfsd
 171
 172format
 173    The given string is searched for in the dynamic debug format
 174    string.  Note that the string does not need to match the
 175    entire format, only some part.  Whitespace and other
 176    special characters can be escaped using C octal character
 177    escape \ooo notation, e.g. the space character is \040.
 178    Alternatively, the string can be enclosed in double quote
 179    characters (") or single quote characters (').
 180    Examples:
 181
 182    format svcrdma:         // many of the NFS/RDMA server pr_debugs
 183    format readahead        // some pr_debugs in the readahead cache
 184    format nfsd:\040SETATTR // one way to match a format with whitespace
 185    format "nfsd: SETATTR"  // a neater way to match a format with whitespace
 186    format 'nfsd: SETATTR'  // yet another way to match a format with whitespace
 187
 188line
 189    The given line number or range of line numbers is compared
 190    against the line number of each pr_debug() callsite.  A single
 191    line number matches the callsite line number exactly.  A
 192    range of line numbers matches any callsite between the first
 193    and last line number inclusive.  An empty first number means
 194    the first line in the file, an empty line number means the
 195    last number in the file.  Examples:
 196
 197    line 1603       // exactly line 1603
 198    line 1600-1605  // the six lines from line 1600 to line 1605
 199    line -1605      // the 1605 lines from line 1 to line 1605
 200    line 1600-      // all lines from line 1600 to the end of the file
 201
 202The flags specification comprises a change operation followed
 203by one or more flag characters.  The change operation is one
 204of the characters:
 205
 206  -    remove the given flags
 207  +    add the given flags
 208  =    set the flags to the given flags
 209
 210The flags are:
 211
 212  p    enables the pr_debug() callsite.
 213  f    Include the function name in the printed message
 214  l    Include line number in the printed message
 215  m    Include module name in the printed message
 216  t    Include thread ID in messages not generated from interrupt context
 217  _    No flags are set. (Or'd with others on input)
 218
 219For print_hex_dump_debug() and print_hex_dump_bytes(), only 'p' flag
 220have meaning, other flags ignored.
 221
 222For display, the flags are preceded by '='
 223(mnemonic: what the flags are currently equal to).
 224
 225Note the regexp ^[-+=][flmpt_]+$ matches a flags specification.
 226To clear all flags at once, use "=_" or "-flmpt".
 227
 228
 229Debug messages during Boot Process
 230==================================
 231
 232To activate debug messages for core code and built-in modules during
 233the boot process, even before userspace and debugfs exists, use
 234dyndbg="QUERY", module.dyndbg="QUERY", or ddebug_query="QUERY"
 235(ddebug_query is obsoleted by dyndbg, and deprecated).  QUERY follows
 236the syntax described above, but must not exceed 1023 characters.  Your
 237bootloader may impose lower limits.
 238
 239These dyndbg params are processed just after the ddebug tables are
 240processed, as part of the arch_initcall.  Thus you can enable debug
 241messages in all code run after this arch_initcall via this boot
 242parameter.
 243
 244On an x86 system for example ACPI enablement is a subsys_initcall and
 245   dyndbg="file ec.c +p"
 246will show early Embedded Controller transactions during ACPI setup if
 247your machine (typically a laptop) has an Embedded Controller.
 248PCI (or other devices) initialization also is a hot candidate for using
 249this boot parameter for debugging purposes.
 250
 251If foo module is not built-in, foo.dyndbg will still be processed at
 252boot time, without effect, but will be reprocessed when module is
 253loaded later.  dyndbg_query= and bare dyndbg= are only processed at
 254boot.
 255
 256
 257Debug Messages at Module Initialization Time
 258============================================
 259
 260When "modprobe foo" is called, modprobe scans /proc/cmdline for
 261foo.params, strips "foo.", and passes them to the kernel along with
 262params given in modprobe args or /etc/modprob.d/*.conf files,
 263in the following order:
 264
 2651. # parameters given via /etc/modprobe.d/*.conf
 266   options foo dyndbg=+pt
 267   options foo dyndbg # defaults to +p
 268
 2692. # foo.dyndbg as given in boot args, "foo." is stripped and passed
 270   foo.dyndbg=" func bar +p; func buz +mp"
 271
 2723. # args to modprobe
 273   modprobe foo dyndbg==pmf # override previous settings
 274
 275These dyndbg queries are applied in order, with last having final say.
 276This allows boot args to override or modify those from /etc/modprobe.d
 277(sensible, since 1 is system wide, 2 is kernel or boot specific), and
 278modprobe args to override both.
 279
 280In the foo.dyndbg="QUERY" form, the query must exclude "module foo".
 281"foo" is extracted from the param-name, and applied to each query in
 282"QUERY", and only 1 match-spec of each type is allowed.
 283
 284The dyndbg option is a "fake" module parameter, which means:
 285
 286- modules do not need to define it explicitly
 287- every module gets it tacitly, whether they use pr_debug or not
 288- it doesn't appear in /sys/module/$module/parameters/
 289  To see it, grep the control file, or inspect /proc/cmdline.
 290
 291For CONFIG_DYNAMIC_DEBUG kernels, any settings given at boot-time (or
 292enabled by -DDEBUG flag during compilation) can be disabled later via
 293the sysfs interface if the debug messages are no longer needed:
 294
 295   echo "module module_name -p" > <debugfs>/dynamic_debug/control
 296
 297Examples
 298========
 299
 300// enable the message at line 1603 of file svcsock.c
 301nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
 302                                <debugfs>/dynamic_debug/control
 303
 304// enable all the messages in file svcsock.c
 305nullarbor:~ # echo -n 'file svcsock.c +p' >
 306                                <debugfs>/dynamic_debug/control
 307
 308// enable all the messages in the NFS server module
 309nullarbor:~ # echo -n 'module nfsd +p' >
 310                                <debugfs>/dynamic_debug/control
 311
 312// enable all 12 messages in the function svc_process()
 313nullarbor:~ # echo -n 'func svc_process +p' >
 314                                <debugfs>/dynamic_debug/control
 315
 316// disable all 12 messages in the function svc_process()
 317nullarbor:~ # echo -n 'func svc_process -p' >
 318                                <debugfs>/dynamic_debug/control
 319
 320// enable messages for NFS calls READ, READLINK, READDIR and READDIR+.
 321nullarbor:~ # echo -n 'format "nfsd: READ" +p' >
 322                                <debugfs>/dynamic_debug/control
 323
 324// enable messages in files of which the paths include string "usb"
 325nullarbor:~ # echo -n '*usb* +p' > <debugfs>/dynamic_debug/control
 326
 327// enable all messages
 328nullarbor:~ # echo -n '+p' > <debugfs>/dynamic_debug/control
 329
 330// add module, function to all enabled messages
 331nullarbor:~ # echo -n '+mf' > <debugfs>/dynamic_debug/control
 332
 333// boot-args example, with newlines and comments for readability
 334Kernel command line: ...
 335  // see whats going on in dyndbg=value processing
 336  dynamic_debug.verbose=1
 337  // enable pr_debugs in 2 builtins, #cmt is stripped
 338  dyndbg="module params +p #cmt ; module sys +p"
 339  // enable pr_debugs in 2 functions in a module loaded later
 340  pc87360.dyndbg="func pc87360_init_device +p; func pc87360_find +p"
 341
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