1Dynamic debug
2+++++++++++++
3
4
5Introduction
6============
7
8This document describes how to use the dynamic debug (dyndbg) feature.
9
10Dynamic debug is designed to allow you to dynamically enable/disable
11kernel code to obtain additional kernel information.  Currently, if
12``CONFIG_DYNAMIC_DEBUG`` is set, then all ``pr_debug()``/``dev_dbg()`` and
13``print_hex_dump_debug()``/``print_hex_dump_bytes()`` calls can be dynamically
14enabled per-callsite.
15
16If you do not want to enable dynamic debug globally (i.e. in some embedded
17system), you may set ``CONFIG_DYNAMIC_DEBUG_CORE`` as basic support of dynamic
18debug and add ``ccflags := -DDYNAMIC_DEBUG_MODULE`` into the Makefile of any
19modules which you'd like to dynamically debug later.
20
21If ``CONFIG_DYNAMIC_DEBUG`` is not set, ``print_hex_dump_debug()`` is just
22shortcut for ``print_hex_dump(KERN_DEBUG)``.
23
24For ``print_hex_dump_debug()``/``print_hex_dump_bytes()``, format string is
25its ``prefix_str`` argument, if it is constant string; or ``hexdump``
26in case ``prefix_str`` is built dynamically.
27
28Dynamic debug has even more useful features:
29
30 * Simple query language allows turning on and off debugging
31   statements by matching any combination of 0 or 1 of:
32
33   - source filename
34   - function name
35   - line number (including ranges of line numbers)
36   - module name
37   - format string
38
39 * Provides a debugfs control file: ``<debugfs>/dynamic_debug/control``
40   which can be read to display the complete list of known debug
41   statements, to help guide you
42
43Controlling dynamic debug Behaviour
44===================================
45
46The behaviour of ``pr_debug()``/``dev_dbg()`` are controlled via writing to a
47control file in the 'debugfs' filesystem. Thus, you must first mount
48the debugfs filesystem, in order to make use of this feature.
49Subsequently, we refer to the control file as:
50``<debugfs>/dynamic_debug/control``. For example, if you want to enable
51printing from source file ``svcsock.c``, line 1603 you simply do::
52
53  nullarbor:~ # echo 'file svcsock.c line 1603 +p' >
54				<debugfs>/dynamic_debug/control
55
56If you make a mistake with the syntax, the write will fail thus::
57
58  nullarbor:~ # echo 'file svcsock.c wtf 1 +p' >
59				<debugfs>/dynamic_debug/control
60  -bash: echo: write error: Invalid argument
61
62Note, for systems without 'debugfs' enabled, the control file can be
63found in ``/proc/dynamic_debug/control``.
64
65Viewing Dynamic Debug Behaviour
66===============================
67
68You can view the currently configured behaviour of all the debug
69statements via::
70
71  nullarbor:~ # cat <debugfs>/dynamic_debug/control
72  # filename:lineno [module]function flags format
73  net/sunrpc/svc_rdma.c:323 [svcxprt_rdma]svc_rdma_cleanup =_ "SVCRDMA Module Removed, deregister RPC RDMA transport\012"
74  net/sunrpc/svc_rdma.c:341 [svcxprt_rdma]svc_rdma_init =_ "\011max_inline       : %d\012"
75  net/sunrpc/svc_rdma.c:340 [svcxprt_rdma]svc_rdma_init =_ "\011sq_depth         : %d\012"
76  net/sunrpc/svc_rdma.c:338 [svcxprt_rdma]svc_rdma_init =_ "\011max_requests     : %d\012"
77  ...
78
79
80You can also apply standard Unix text manipulation filters to this
81data, e.g.::
82
83  nullarbor:~ # grep -i rdma <debugfs>/dynamic_debug/control  | wc -l
84  62
85
86  nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l
87  42
88
89The third column shows the currently enabled flags for each debug
90statement callsite (see below for definitions of the flags).  The
91default value, with no flags enabled, is ``=_``.  So you can view all
92the debug statement callsites with any non-default flags::
93
94  nullarbor:~ # awk '$3 != "=_"' <debugfs>/dynamic_debug/control
95  # filename:lineno [module]function flags format
96  net/sunrpc/svcsock.c:1603 [sunrpc]svc_send p "svc_process: st_sendto returned %d\012"
97
98Command Language Reference
99==========================
100
101At the lexical level, a command comprises a sequence of words separated
102by spaces or tabs.  So these are all equivalent::
103
104  nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
105				<debugfs>/dynamic_debug/control
106  nullarbor:~ # echo -n '  file   svcsock.c     line  1603 +p  ' >
107				<debugfs>/dynamic_debug/control
108  nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
109				<debugfs>/dynamic_debug/control
110
111Command submissions are bounded by a write() system call.
112Multiple commands can be written together, separated by ``;`` or ``\n``::
113
114  ~# echo "func pnpacpi_get_resources +p; func pnp_assign_mem +p" \
115     > <debugfs>/dynamic_debug/control
116
117If your query set is big, you can batch them too::
118
119  ~# cat query-batch-file > <debugfs>/dynamic_debug/control
120
121Another way is to use wildcards. The match rule supports ``*`` (matches
122zero or more characters) and ``?`` (matches exactly one character). For
123example, you can match all usb drivers::
124
125  ~# echo "file drivers/usb/* +p" > <debugfs>/dynamic_debug/control
126
127At the syntactical level, a command comprises a sequence of match
128specifications, followed by a flags change specification::
129
130  command ::= match-spec* flags-spec
131
132The match-spec's are used to choose a subset of the known pr_debug()
133callsites to which to apply the flags-spec.  Think of them as a query
134with implicit ANDs between each pair.  Note that an empty list of
135match-specs will select all debug statement callsites.
136
137A match specification comprises a keyword, which controls the
138attribute of the callsite to be compared, and a value to compare
139against.  Possible keywords are:::
140
141  match-spec ::= 'func' string |
142		 'file' string |
143		 'module' string |
144		 'format' string |
145		 'line' line-range
146
147  line-range ::= lineno |
148		 '-'lineno |
149		 lineno'-' |
150		 lineno'-'lineno
151
152  lineno ::= unsigned-int
153
154.. note::
155
156  ``line-range`` cannot contain space, e.g.
157  "1-30" is valid range but "1 - 30" is not.
158
159
160The meanings of each keyword are:
161
162func
163    The given string is compared against the function name
164    of each callsite.  Example::
165
166	func svc_tcp_accept
167	func *recv*		# in rfcomm, bluetooth, ping, tcp
168
169file
170    The given string is compared against either the src-root relative
171    pathname, or the basename of the source file of each callsite.
172    Examples::
173
174	file svcsock.c
175	file kernel/freezer.c	# ie column 1 of control file
176	file drivers/usb/*	# all callsites under it
177	file inode.c:start_*	# parse :tail as a func (above)
178	file inode.c:1-100	# parse :tail as a line-range (above)
179
180module
181    The given string is compared against the module name
182    of each callsite.  The module name is the string as
183    seen in ``lsmod``, i.e. without the directory or the ``.ko``
184    suffix and with ``-`` changed to ``_``.  Examples::
185
186	module sunrpc
187	module nfsd
188	module drm*	# both drm, drm_kms_helper
189
190format
191    The given string is searched for in the dynamic debug format
192    string.  Note that the string does not need to match the
193    entire format, only some part.  Whitespace and other
194    special characters can be escaped using C octal character
195    escape ``\ooo`` notation, e.g. the space character is ``\040``.
196    Alternatively, the string can be enclosed in double quote
197    characters (``"``) or single quote characters (``'``).
198    Examples::
199
200	format svcrdma:         // many of the NFS/RDMA server pr_debugs
201	format readahead        // some pr_debugs in the readahead cache
202	format nfsd:\040SETATTR // one way to match a format with whitespace
203	format "nfsd: SETATTR"  // a neater way to match a format with whitespace
204	format 'nfsd: SETATTR'  // yet another way to match a format with whitespace
205
206line
207    The given line number or range of line numbers is compared
208    against the line number of each ``pr_debug()`` callsite.  A single
209    line number matches the callsite line number exactly.  A
210    range of line numbers matches any callsite between the first
211    and last line number inclusive.  An empty first number means
212    the first line in the file, an empty last line number means the
213    last line number in the file.  Examples::
214
215	line 1603           // exactly line 1603
216	line 1600-1605      // the six lines from line 1600 to line 1605
217	line -1605          // the 1605 lines from line 1 to line 1605
218	line 1600-          // all lines from line 1600 to the end of the file
219
220The flags specification comprises a change operation followed
221by one or more flag characters.  The change operation is one
222of the characters::
223
224  -    remove the given flags
225  +    add the given flags
226  =    set the flags to the given flags
227
228The flags are::
229
230  p    enables the pr_debug() callsite.
231  f    Include the function name in the printed message
232  l    Include line number in the printed message
233  m    Include module name in the printed message
234  t    Include thread ID in messages not generated from interrupt context
235  _    No flags are set. (Or'd with others on input)
236
237For ``print_hex_dump_debug()`` and ``print_hex_dump_bytes()``, only ``p`` flag
238have meaning, other flags ignored.
239
240For display, the flags are preceded by ``=``
241(mnemonic: what the flags are currently equal to).
242
243Note the regexp ``^[-+=][flmpt_]+$`` matches a flags specification.
244To clear all flags at once, use ``=_`` or ``-flmpt``.
245
246
247Debug messages during Boot Process
248==================================
249
250To activate debug messages for core code and built-in modules during
251the boot process, even before userspace and debugfs exists, use
252``dyndbg="QUERY"`` or ``module.dyndbg="QUERY"``.  QUERY follows
253the syntax described above, but must not exceed 1023 characters.  Your
254bootloader may impose lower limits.
255
256These ``dyndbg`` params are processed just after the ddebug tables are
257processed, as part of the early_initcall.  Thus you can enable debug
258messages in all code run after this early_initcall via this boot
259parameter.
260
261On an x86 system for example ACPI enablement is a subsys_initcall and::
262
263   dyndbg="file ec.c +p"
264
265will show early Embedded Controller transactions during ACPI setup if
266your machine (typically a laptop) has an Embedded Controller.
267PCI (or other devices) initialization also is a hot candidate for using
268this boot parameter for debugging purposes.
269
270If ``foo`` module is not built-in, ``foo.dyndbg`` will still be processed at
271boot time, without effect, but will be reprocessed when module is
272loaded later. Bare ``dyndbg=`` is only processed at boot.
273
274
275Debug Messages at Module Initialization Time
276============================================
277
278When ``modprobe foo`` is called, modprobe scans ``/proc/cmdline`` for
279``foo.params``, strips ``foo.``, and passes them to the kernel along with
280params given in modprobe args or ``/etc/modprob.d/*.conf`` files,
281in the following order:
282
2831. parameters given via ``/etc/modprobe.d/*.conf``::
284
285	options foo dyndbg=+pt
286	options foo dyndbg # defaults to +p
287
2882. ``foo.dyndbg`` as given in boot args, ``foo.`` is stripped and passed::
289
290	foo.dyndbg=" func bar +p; func buz +mp"
291
2923. args to modprobe::
293
294	modprobe foo dyndbg==pmf # override previous settings
295
296These ``dyndbg`` queries are applied in order, with last having final say.
297This allows boot args to override or modify those from ``/etc/modprobe.d``
298(sensible, since 1 is system wide, 2 is kernel or boot specific), and
299modprobe args to override both.
300
301In the ``foo.dyndbg="QUERY"`` form, the query must exclude ``module foo``.
302``foo`` is extracted from the param-name, and applied to each query in
303``QUERY``, and only 1 match-spec of each type is allowed.
304
305The ``dyndbg`` option is a "fake" module parameter, which means:
306
307- modules do not need to define it explicitly
308- every module gets it tacitly, whether they use pr_debug or not
309- it doesn't appear in ``/sys/module/$module/parameters/``
310  To see it, grep the control file, or inspect ``/proc/cmdline.``
311
312For ``CONFIG_DYNAMIC_DEBUG`` kernels, any settings given at boot-time (or
313enabled by ``-DDEBUG`` flag during compilation) can be disabled later via
314the debugfs interface if the debug messages are no longer needed::
315
316   echo "module module_name -p" > <debugfs>/dynamic_debug/control
317
318Examples
319========
320
321::
322
323  // enable the message at line 1603 of file svcsock.c
324  nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
325				<debugfs>/dynamic_debug/control
326
327  // enable all the messages in file svcsock.c
328  nullarbor:~ # echo -n 'file svcsock.c +p' >
329				<debugfs>/dynamic_debug/control
330
331  // enable all the messages in the NFS server module
332  nullarbor:~ # echo -n 'module nfsd +p' >
333				<debugfs>/dynamic_debug/control
334
335  // enable all 12 messages in the function svc_process()
336  nullarbor:~ # echo -n 'func svc_process +p' >
337				<debugfs>/dynamic_debug/control
338
339  // disable all 12 messages in the function svc_process()
340  nullarbor:~ # echo -n 'func svc_process -p' >
341				<debugfs>/dynamic_debug/control
342
343  // enable messages for NFS calls READ, READLINK, READDIR and READDIR+.
344  nullarbor:~ # echo -n 'format "nfsd: READ" +p' >
345				<debugfs>/dynamic_debug/control
346
347  // enable messages in files of which the paths include string "usb"
348  nullarbor:~ # echo -n 'file *usb* +p' > <debugfs>/dynamic_debug/control
349
350  // enable all messages
351  nullarbor:~ # echo -n '+p' > <debugfs>/dynamic_debug/control
352
353  // add module, function to all enabled messages
354  nullarbor:~ # echo -n '+mf' > <debugfs>/dynamic_debug/control
355
356  // boot-args example, with newlines and comments for readability
357  Kernel command line: ...
358    // see whats going on in dyndbg=value processing
359    dynamic_debug.verbose=3
360    // enable pr_debugs in the btrfs module (can be builtin or loadable)
361    btrfs.dyndbg="+p"
362    // enable pr_debugs in all files under init/
363    // and the function parse_one, #cmt is stripped
364    dyndbg="file init/* +p #cmt ; func parse_one +p"
365    // enable pr_debugs in 2 functions in a module loaded later
366    pc87360.dyndbg="func pc87360_init_device +p; func pc87360_find +p"
367