1Dynamic debug
2+++++++++++++
3
4
5Introduction
6============
7
8Dynamic debug allows you to dynamically enable/disable kernel
9debug-print code to obtain additional kernel information.
10
11If ``/proc/dynamic_debug/control`` exists, your kernel has dynamic
12debug.  You'll need root access (sudo su) to use this.
13
14Dynamic debug provides:
15
16 * a Catalog of all *prdbgs* in your kernel.
17   ``cat /proc/dynamic_debug/control`` to see them.
18
19 * a Simple query/command language to alter *prdbgs* by selecting on
20   any combination of 0 or 1 of:
21
22   - source filename
23   - function name
24   - line number (including ranges of line numbers)
25   - module name
26   - format string
27   - class name (as known/declared by each module)
28
29Viewing Dynamic Debug Behaviour
30===============================
31
32You can view the currently configured behaviour in the *prdbg* catalog::
33
34  :#> head -n7 /proc/dynamic_debug/control
35  # filename:lineno [module]function flags format
36  init/main.c:1179 [main]initcall_blacklist =_ "blacklisting initcall %s\012
37  init/main.c:1218 [main]initcall_blacklisted =_ "initcall %s blacklisted\012"
38  init/main.c:1424 [main]run_init_process =_ "  with arguments:\012"
39  init/main.c:1426 [main]run_init_process =_ "    %s\012"
40  init/main.c:1427 [main]run_init_process =_ "  with environment:\012"
41  init/main.c:1429 [main]run_init_process =_ "    %s\012"
42
43The 3rd space-delimited column shows the current flags, preceded by
44a ``=`` for easy use with grep/cut. ``=p`` shows enabled callsites.
45
46Controlling dynamic debug Behaviour
47===================================
48
49The behaviour of *prdbg* sites are controlled by writing
50query/commands to the control file.  Example::
51
52  # grease the interface
53  :#> alias ddcmd='echo $* > /proc/dynamic_debug/control'
54
55  :#> ddcmd '-p; module main func run* +p'
56  :#> grep =p /proc/dynamic_debug/control
57  init/main.c:1424 [main]run_init_process =p "  with arguments:\012"
58  init/main.c:1426 [main]run_init_process =p "    %s\012"
59  init/main.c:1427 [main]run_init_process =p "  with environment:\012"
60  init/main.c:1429 [main]run_init_process =p "    %s\012"
61
62Error messages go to console/syslog::
63
64  :#> ddcmd mode foo +p
65  dyndbg: unknown keyword "mode"
66  dyndbg: query parse failed
67  bash: echo: write error: Invalid argument
68
69If debugfs is also enabled and mounted, ``dynamic_debug/control`` is
70also under the mount-dir, typically ``/sys/kernel/debug/``.
71
72Command Language Reference
73==========================
74
75At the basic lexical level, a command is a sequence of words separated
76by spaces or tabs.  So these are all equivalent::
77
78  :#> ddcmd file svcsock.c line 1603 +p
79  :#> ddcmd "file svcsock.c line 1603 +p"
80  :#> ddcmd '  file   svcsock.c     line  1603 +p  '
81
82Command submissions are bounded by a write() system call.
83Multiple commands can be written together, separated by ``;`` or ``\n``::
84
85  :#> ddcmd "func pnpacpi_get_resources +p; func pnp_assign_mem +p"
86  :#> ddcmd <<"EOC"
87  func pnpacpi_get_resources +p
88  func pnp_assign_mem +p
89  EOC
90  :#> cat query-batch-file > /proc/dynamic_debug/control
91
92You can also use wildcards in each query term. The match rule supports
93``*`` (matches zero or more characters) and ``?`` (matches exactly one
94character). For example, you can match all usb drivers::
95
96  :#> ddcmd file "drivers/usb/*" +p	# "" to suppress shell expansion
97
98Syntactically, a command is pairs of keyword values, followed by a
99flags change or setting::
100
101  command ::= match-spec* flags-spec
102
103The match-spec's select *prdbgs* from the catalog, upon which to apply
104the flags-spec, all constraints are ANDed together.  An absent keyword
105is the same as keyword "*".
106
107
108A match specification is a keyword, which selects the attribute of
109the callsite to be compared, and a value to compare against.  Possible
110keywords are:::
111
112  match-spec ::= 'func' string |
113		 'file' string |
114		 'module' string |
115		 'format' string |
116		 'class' string |
117		 'line' line-range
118
119  line-range ::= lineno |
120		 '-'lineno |
121		 lineno'-' |
122		 lineno'-'lineno
123
124  lineno ::= unsigned-int
125
126.. note::
127
128  ``line-range`` cannot contain space, e.g.
129  "1-30" is valid range but "1 - 30" is not.
130
131
132The meanings of each keyword are:
133
134func
135    The given string is compared against the function name
136    of each callsite.  Example::
137
138	func svc_tcp_accept
139	func *recv*		# in rfcomm, bluetooth, ping, tcp
140
141file
142    The given string is compared against either the src-root relative
143    pathname, or the basename of the source file of each callsite.
144    Examples::
145
146	file svcsock.c
147	file kernel/freezer.c	# ie column 1 of control file
148	file drivers/usb/*	# all callsites under it
149	file inode.c:start_*	# parse :tail as a func (above)
150	file inode.c:1-100	# parse :tail as a line-range (above)
151
152module
153    The given string is compared against the module name
154    of each callsite.  The module name is the string as
155    seen in ``lsmod``, i.e. without the directory or the ``.ko``
156    suffix and with ``-`` changed to ``_``.  Examples::
157
158	module sunrpc
159	module nfsd
160	module drm*	# both drm, drm_kms_helper
161
162format
163    The given string is searched for in the dynamic debug format
164    string.  Note that the string does not need to match the
165    entire format, only some part.  Whitespace and other
166    special characters can be escaped using C octal character
167    escape ``\ooo`` notation, e.g. the space character is ``\040``.
168    Alternatively, the string can be enclosed in double quote
169    characters (``"``) or single quote characters (``'``).
170    Examples::
171
172	format svcrdma:         // many of the NFS/RDMA server pr_debugs
173	format readahead        // some pr_debugs in the readahead cache
174	format nfsd:\040SETATTR // one way to match a format with whitespace
175	format "nfsd: SETATTR"  // a neater way to match a format with whitespace
176	format 'nfsd: SETATTR'  // yet another way to match a format with whitespace
177
178class
179    The given class_name is validated against each module, which may
180    have declared a list of known class_names.  If the class_name is
181    found for a module, callsite & class matching and adjustment
182    proceeds.  Examples::
183
184	class DRM_UT_KMS	# a DRM.debug category
185	class JUNK		# silent non-match
186	// class TLD_*		# NOTICE: no wildcard in class names
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 last line number means the
195    last line 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  _    enables no flags.
214
215  Decorator flags add to the message-prefix, in order:
216  t    Include thread ID, or <intr>
217  m    Include module name
218  f    Include the function name
219  l    Include line number
220
221For ``print_hex_dump_debug()`` and ``print_hex_dump_bytes()``, only
222the ``p`` flag has meaning, other flags are ignored.
223
224Note the regexp ``^[-+=][flmpt_]+$`` matches a flags specification.
225To clear all flags at once, use ``=_`` or ``-flmpt``.
226
227
228Debug messages during Boot Process
229==================================
230
231To activate debug messages for core code and built-in modules during
232the boot process, even before userspace and debugfs exists, use
233``dyndbg="QUERY"`` or ``module.dyndbg="QUERY"``.  QUERY follows
234the syntax described above, but must not exceed 1023 characters.  Your
235bootloader may impose lower limits.
236
237These ``dyndbg`` params are processed just after the ddebug tables are
238processed, as part of the early_initcall.  Thus you can enable debug
239messages in all code run after this early_initcall via this boot
240parameter.
241
242On an x86 system for example ACPI enablement is a subsys_initcall and::
243
244   dyndbg="file ec.c +p"
245
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. Bare ``dyndbg=`` is only processed at boot.
254
255
256Debug Messages at Module Initialization Time
257============================================
258
259When ``modprobe foo`` is called, modprobe scans ``/proc/cmdline`` for
260``foo.params``, strips ``foo.``, and passes them to the kernel along with
261params given in modprobe args or ``/etc/modprob.d/*.conf`` files,
262in the following order:
263
2641. parameters given via ``/etc/modprobe.d/*.conf``::
265
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
271	foo.dyndbg=" func bar +p; func buz +mp"
272
2733. args to modprobe::
274
275	modprobe foo dyndbg==pmf # override previous settings
276
277These ``dyndbg`` queries are applied in order, with last having final say.
278This allows boot args to override or modify those from ``/etc/modprobe.d``
279(sensible, since 1 is system wide, 2 is kernel or boot specific), and
280modprobe args to override both.
281
282In the ``foo.dyndbg="QUERY"`` form, the query must exclude ``module foo``.
283``foo`` is extracted from the param-name, and applied to each query in
284``QUERY``, and only 1 match-spec of each type is allowed.
285
286The ``dyndbg`` option is a "fake" module parameter, which means:
287
288- modules do not need to define it explicitly
289- every module gets it tacitly, whether they use pr_debug or not
290- it doesn't appear in ``/sys/module/$module/parameters/``
291  To see it, grep the control file, or inspect ``/proc/cmdline.``
292
293For ``CONFIG_DYNAMIC_DEBUG`` kernels, any settings given at boot-time (or
294enabled by ``-DDEBUG`` flag during compilation) can be disabled later via
295the debugfs interface if the debug messages are no longer needed::
296
297   echo "module module_name -p" > /proc/dynamic_debug/control
298
299Examples
300========
301
302::
303
304  // enable the message at line 1603 of file svcsock.c
305  :#> ddcmd 'file svcsock.c line 1603 +p'
306
307  // enable all the messages in file svcsock.c
308  :#> ddcmd 'file svcsock.c +p'
309
310  // enable all the messages in the NFS server module
311  :#> ddcmd 'module nfsd +p'
312
313  // enable all 12 messages in the function svc_process()
314  :#> ddcmd 'func svc_process +p'
315
316  // disable all 12 messages in the function svc_process()
317  :#> ddcmd 'func svc_process -p'
318
319  // enable messages for NFS calls READ, READLINK, READDIR and READDIR+.
320  :#> ddcmd 'format "nfsd: READ" +p'
321
322  // enable messages in files of which the paths include string "usb"
323  :#> ddcmd 'file *usb* +p' > /proc/dynamic_debug/control
324
325  // enable all messages
326  :#> ddcmd '+p' > /proc/dynamic_debug/control
327
328  // add module, function to all enabled messages
329  :#> ddcmd '+mf' > /proc/dynamic_debug/control
330
331  // boot-args example, with newlines and comments for readability
332  Kernel command line: ...
333    // see what's going on in dyndbg=value processing
334    dynamic_debug.verbose=3
335    // enable pr_debugs in the btrfs module (can be builtin or loadable)
336    btrfs.dyndbg="+p"
337    // enable pr_debugs in all files under init/
338    // and the function parse_one, #cmt is stripped
339    dyndbg="file init/* +p #cmt ; func parse_one +p"
340    // enable pr_debugs in 2 functions in a module loaded later
341    pc87360.dyndbg="func pc87360_init_device +p; func pc87360_find +p"
342
343Kernel Configuration
344====================
345
346Dynamic Debug is enabled via kernel config items::
347
348  CONFIG_DYNAMIC_DEBUG=y	# build catalog, enables CORE
349  CONFIG_DYNAMIC_DEBUG_CORE=y	# enable mechanics only, skip catalog
350
351If you do not want to enable dynamic debug globally (i.e. in some embedded
352system), you may set ``CONFIG_DYNAMIC_DEBUG_CORE`` as basic support of dynamic
353debug and add ``ccflags := -DDYNAMIC_DEBUG_MODULE`` into the Makefile of any
354modules which you'd like to dynamically debug later.
355
356
357Kernel *prdbg* API
358==================
359
360The following functions are cataloged and controllable when dynamic
361debug is enabled::
362
363  pr_debug()
364  dev_dbg()
365  print_hex_dump_debug()
366  print_hex_dump_bytes()
367
368Otherwise, they are off by default; ``ccflags += -DDEBUG`` or
369``#define DEBUG`` in a source file will enable them appropriately.
370
371If ``CONFIG_DYNAMIC_DEBUG`` is not set, ``print_hex_dump_debug()`` is
372just a shortcut for ``print_hex_dump(KERN_DEBUG)``.
373
374For ``print_hex_dump_debug()``/``print_hex_dump_bytes()``, format string is
375its ``prefix_str`` argument, if it is constant string; or ``hexdump``
376in case ``prefix_str`` is built dynamically.
377