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  s    Include the source file name
220  l    Include line number
221
222For ``print_hex_dump_debug()`` and ``print_hex_dump_bytes()``, only
223the ``p`` flag has meaning, other flags are ignored.
224
225Note the regexp ``^[-+=][fslmpt_]+$`` matches a flags specification.
226To clear all flags at once, use ``=_`` or ``-fslmpt``.
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
234``dyndbg="QUERY"`` or ``module.dyndbg="QUERY"``.  QUERY follows
235the syntax described above, but must not exceed 1023 characters.  Your
236bootloader may impose lower limits.
237
238These ``dyndbg`` params are processed just after the ddebug tables are
239processed, as part of the early_initcall.  Thus you can enable debug
240messages in all code run after this early_initcall via this boot
241parameter.
242
243On an x86 system for example ACPI enablement is a subsys_initcall and::
244
245   dyndbg="file ec.c +p"
246
247will show early Embedded Controller transactions during ACPI setup if
248your machine (typically a laptop) has an Embedded Controller.
249PCI (or other devices) initialization also is a hot candidate for using
250this boot parameter for debugging purposes.
251
252If ``foo`` module is not built-in, ``foo.dyndbg`` will still be processed at
253boot time, without effect, but will be reprocessed when module is
254loaded later. Bare ``dyndbg=`` is only processed at boot.
255
256
257Debug Messages at Module Initialization Time
258============================================
259
260When ``modprobe foo`` is called, modprobe scans ``/proc/cmdline`` for
261``foo.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
267	options foo dyndbg=+pt
268	options foo dyndbg # defaults to +p
269
2702. ``foo.dyndbg`` as given in boot args, ``foo.`` is stripped and passed::
271
272	foo.dyndbg=" func bar +p; func buz +mp"
273
2743. args to modprobe::
275
276	modprobe foo dyndbg==pmf # override previous settings
277
278These ``dyndbg`` queries are applied in order, with last having final say.
279This allows boot args to override or modify those from ``/etc/modprobe.d``
280(sensible, since 1 is system wide, 2 is kernel or boot specific), and
281modprobe args to override both.
282
283In the ``foo.dyndbg="QUERY"`` form, the query must exclude ``module foo``.
284``foo`` is extracted from the param-name, and applied to each query in
285``QUERY``, and only 1 match-spec of each type is allowed.
286
287The ``dyndbg`` option is a "fake" module parameter, which means:
288
289- modules do not need to define it explicitly
290- every module gets it tacitly, whether they use pr_debug or not
291- it doesn't appear in ``/sys/module/$module/parameters/``
292  To see it, grep the control file, or inspect ``/proc/cmdline.``
293
294For ``CONFIG_DYNAMIC_DEBUG`` kernels, any settings given at boot-time (or
295enabled by ``-DDEBUG`` flag during compilation) can be disabled later via
296the debugfs interface if the debug messages are no longer needed::
297
298   echo "module module_name -p" > /proc/dynamic_debug/control
299
300Examples
301========
302
303::
304
305  // enable the message at line 1603 of file svcsock.c
306  :#> ddcmd 'file svcsock.c line 1603 +p'
307
308  // enable all the messages in file svcsock.c
309  :#> ddcmd 'file svcsock.c +p'
310
311  // enable all the messages in the NFS server module
312  :#> ddcmd 'module nfsd +p'
313
314  // enable all 12 messages in the function svc_process()
315  :#> ddcmd 'func svc_process +p'
316
317  // disable all 12 messages in the function svc_process()
318  :#> ddcmd 'func svc_process -p'
319
320  // enable messages for NFS calls READ, READLINK, READDIR and READDIR+.
321  :#> ddcmd 'format "nfsd: READ" +p'
322
323  // enable messages in files of which the paths include string "usb"
324  :#> ddcmd 'file *usb* +p' > /proc/dynamic_debug/control
325
326  // enable all messages
327  :#> ddcmd '+p' > /proc/dynamic_debug/control
328
329  // add module, function to all enabled messages
330  :#> ddcmd '+mf' > /proc/dynamic_debug/control
331
332  // boot-args example, with newlines and comments for readability
333  Kernel command line: ...
334    // see what's going on in dyndbg=value processing
335    dynamic_debug.verbose=3
336    // enable pr_debugs in the btrfs module (can be builtin or loadable)
337    btrfs.dyndbg="+p"
338    // enable pr_debugs in all files under init/
339    // and the function parse_one, #cmt is stripped
340    dyndbg="file init/* +p #cmt ; func parse_one +p"
341    // enable pr_debugs in 2 functions in a module loaded later
342    pc87360.dyndbg="func pc87360_init_device +p; func pc87360_find +p"
343
344Kernel Configuration
345====================
346
347Dynamic Debug is enabled via kernel config items::
348
349  CONFIG_DYNAMIC_DEBUG=y	# build catalog, enables CORE
350  CONFIG_DYNAMIC_DEBUG_CORE=y	# enable mechanics only, skip catalog
351
352If you do not want to enable dynamic debug globally (i.e. in some embedded
353system), you may set ``CONFIG_DYNAMIC_DEBUG_CORE`` as basic support of dynamic
354debug and add ``ccflags := -DDYNAMIC_DEBUG_MODULE`` into the Makefile of any
355modules which you'd like to dynamically debug later.
356
357
358Kernel *prdbg* API
359==================
360
361The following functions are cataloged and controllable when dynamic
362debug is enabled::
363
364  pr_debug()
365  dev_dbg()
366  print_hex_dump_debug()
367  print_hex_dump_bytes()
368
369Otherwise, they are off by default; ``ccflags += -DDEBUG`` or
370``#define DEBUG`` in a source file will enable them appropriately.
371
372If ``CONFIG_DYNAMIC_DEBUG`` is not set, ``print_hex_dump_debug()`` is
373just a shortcut for ``print_hex_dump(KERN_DEBUG)``.
374
375For ``print_hex_dump_debug()``/``print_hex_dump_bytes()``, format string is
376its ``prefix_str`` argument, if it is constant string; or ``hexdump``
377in case ``prefix_str`` is built dynamically.
378