8 This document describes how to use the dynamic debug (dyndbg) feature.
10 Dynamic debug is designed to allow you to dynamically enable/disable
11 kernel 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
16 If you do not want to enable dynamic debug globally (i.e. in some embedded
17 system), you may set ``CONFIG_DYNAMIC_DEBUG_CORE`` as basic support of dynamic
18 debug and add ``ccflags := -DDYNAMIC_DEBUG_MODULE`` into the Makefile of any
19 modules which you'd like to dynamically debug later.
21 If ``CONFIG_DYNAMIC_DEBUG`` is not set, ``print_hex_dump_debug()`` is just
22 shortcut for ``print_hex_dump(KERN_DEBUG)``.
24 For ``print_hex_dump_debug()``/``print_hex_dump_bytes()``, format string is
25 its ``prefix_str`` argument, if it is constant string; or ``hexdump``
26 in case ``prefix_str`` is built dynamically.
28 Dynamic debug has even more useful features:
30 * Simple query language allows turning on and off debugging
31 statements by matching any combination of 0 or 1 of:
35 - line number (including ranges of line numbers)
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
43 Controlling dynamic debug Behaviour
44 ===================================
46 The behaviour of ``pr_debug()``/``dev_dbg()`` are controlled via writing to a
47 control file in the 'debugfs' filesystem. Thus, you must first mount
48 the debugfs filesystem, in order to make use of this feature.
49 Subsequently, we refer to the control file as:
50 ``<debugfs>/dynamic_debug/control``. For example, if you want to enable
51 printing from source file ``svcsock.c``, line 1603 you simply do::
53 nullarbor:~ # echo 'file svcsock.c line 1603 +p' >
54 <debugfs>/dynamic_debug/control
56 If you make a mistake with the syntax, the write will fail thus::
58 nullarbor:~ # echo 'file svcsock.c wtf 1 +p' >
59 <debugfs>/dynamic_debug/control
60 -bash: echo: write error: Invalid argument
62 Note, for systems without 'debugfs' enabled, the control file can be
63 found in ``/proc/dynamic_debug/control``.
65 Viewing Dynamic Debug Behaviour
66 ===============================
68 You can view the currently configured behaviour of all the debug
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"
80 You can also apply standard Unix text manipulation filters to this
83 nullarbor:~ # grep -i rdma <debugfs>/dynamic_debug/control | wc -l
86 nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l
89 The third column shows the currently enabled flags for each debug
90 statement callsite (see below for definitions of the flags). The
91 default value, with no flags enabled, is ``=_``. So you can view all
92 the debug statement callsites with any non-default flags::
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"
98 Command Language Reference
99 ==========================
101 At the lexical level, a command comprises a sequence of words separated
102 by spaces or tabs. So these are all equivalent::
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
111 Command submissions are bounded by a write() system call.
112 Multiple commands can be written together, separated by ``;`` or ``\n``::
114 ~# echo "func pnpacpi_get_resources +p; func pnp_assign_mem +p" \
115 > <debugfs>/dynamic_debug/control
117 If your query set is big, you can batch them too::
119 ~# cat query-batch-file > <debugfs>/dynamic_debug/control
121 Another way is to use wildcards. The match rule supports ``*`` (matches
122 zero or more characters) and ``?`` (matches exactly one character). For
123 example, you can match all usb drivers::
125 ~# echo "file drivers/usb/* +p" > <debugfs>/dynamic_debug/control
127 At the syntactical level, a command comprises a sequence of match
128 specifications, followed by a flags change specification::
130 command ::= match-spec* flags-spec
132 The match-spec's are used to choose a subset of the known pr_debug()
133 callsites to which to apply the flags-spec. Think of them as a query
134 with implicit ANDs between each pair. Note that an empty list of
135 match-specs will select all debug statement callsites.
137 A match specification comprises a keyword, which controls the
138 attribute of the callsite to be compared, and a value to compare
139 against. Possible keywords are:::
141 match-spec ::= 'func' string |
147 line-range ::= lineno |
152 lineno ::= unsigned-int
156 ``line-range`` cannot contain space, e.g.
157 "1-30" is valid range but "1 - 30" is not.
159 ``module=foo`` combined keyword=value form is interchangably accepted
161 The meanings of each keyword are:
164 The given string is compared against the function name
165 of each callsite. Example::
168 func *recv* # in rfcomm, bluetooth, ping, tcp
171 The given string is compared against either the src-root relative
172 pathname, or the basename of the source file of each callsite.
176 file kernel/freezer.c # ie column 1 of control file
177 file drivers/usb/* # all callsites under it
178 file inode.c:start_* # parse :tail as a func (above)
179 file inode.c:1-100 # parse :tail as a line-range (above)
182 The given string is compared against the module name
183 of each callsite. The module name is the string as
184 seen in ``lsmod``, i.e. without the directory or the ``.ko``
185 suffix and with ``-`` changed to ``_``. Examples::
189 module drm* # both drm, drm_kms_helper
192 The given string is searched for in the dynamic debug format
193 string. Note that the string does not need to match the
194 entire format, only some part. Whitespace and other
195 special characters can be escaped using C octal character
196 escape ``\ooo`` notation, e.g. the space character is ``\040``.
197 Alternatively, the string can be enclosed in double quote
198 characters (``"``) or single quote characters (``'``).
201 format svcrdma: // many of the NFS/RDMA server pr_debugs
202 format readahead // some pr_debugs in the readahead cache
203 format nfsd:\040SETATTR // one way to match a format with whitespace
204 format "nfsd: SETATTR" // a neater way to match a format with whitespace
205 format 'nfsd: SETATTR' // yet another way to match a format with whitespace
208 The given line number or range of line numbers is compared
209 against the line number of each ``pr_debug()`` callsite. A single
210 line number matches the callsite line number exactly. A
211 range of line numbers matches any callsite between the first
212 and last line number inclusive. An empty first number means
213 the first line in the file, an empty last line number means the
214 last line number in the file. Examples::
216 line 1603 // exactly line 1603
217 line 1600-1605 // the six lines from line 1600 to line 1605
218 line -1605 // the 1605 lines from line 1 to line 1605
219 line 1600- // all lines from line 1600 to the end of the file
221 The flags specification comprises a change operation followed
222 by one or more flag characters. The change operation is one
225 - remove the given flags
226 + add the given flags
227 = set the flags to the given flags
231 p enables the pr_debug() callsite.
232 f Include the function name in the printed message
233 l Include line number in the printed message
234 m Include module name in the printed message
235 t Include thread ID in messages not generated from interrupt context
236 _ No flags are set. (Or'd with others on input)
238 For ``print_hex_dump_debug()`` and ``print_hex_dump_bytes()``, only ``p`` flag
239 have meaning, other flags ignored.
241 For display, the flags are preceded by ``=``
242 (mnemonic: what the flags are currently equal to).
244 Note the regexp ``^[-+=][flmpt_]+$`` matches a flags specification.
245 To clear all flags at once, use ``=_`` or ``-flmpt``.
248 Debug messages during Boot Process
249 ==================================
251 To activate debug messages for core code and built-in modules during
252 the boot process, even before userspace and debugfs exists, use
253 ``dyndbg="QUERY"``, ``module.dyndbg="QUERY"``, or ``ddebug_query="QUERY"``
254 (``ddebug_query`` is obsoleted by ``dyndbg``, and deprecated). QUERY follows
255 the syntax described above, but must not exceed 1023 characters. Your
256 bootloader may impose lower limits.
258 These ``dyndbg`` params are processed just after the ddebug tables are
259 processed, as part of the early_initcall. Thus you can enable debug
260 messages in all code run after this early_initcall via this boot
263 On an x86 system for example ACPI enablement is a subsys_initcall and::
265 dyndbg="file ec.c +p"
267 will show early Embedded Controller transactions during ACPI setup if
268 your machine (typically a laptop) has an Embedded Controller.
269 PCI (or other devices) initialization also is a hot candidate for using
270 this boot parameter for debugging purposes.
272 If ``foo`` module is not built-in, ``foo.dyndbg`` will still be processed at
273 boot time, without effect, but will be reprocessed when module is
274 loaded later. ``ddebug_query=`` and bare ``dyndbg=`` are only processed at
278 Debug Messages at Module Initialization Time
279 ============================================
281 When ``modprobe foo`` is called, modprobe scans ``/proc/cmdline`` for
282 ``foo.params``, strips ``foo.``, and passes them to the kernel along with
283 params given in modprobe args or ``/etc/modprob.d/*.conf`` files,
284 in the following order:
286 1. parameters given via ``/etc/modprobe.d/*.conf``::
288 options foo dyndbg=+pt
289 options foo dyndbg # defaults to +p
291 2. ``foo.dyndbg`` as given in boot args, ``foo.`` is stripped and passed::
293 foo.dyndbg=" func bar +p; func buz +mp"
295 3. args to modprobe::
297 modprobe foo dyndbg==pmf # override previous settings
299 These ``dyndbg`` queries are applied in order, with last having final say.
300 This allows boot args to override or modify those from ``/etc/modprobe.d``
301 (sensible, since 1 is system wide, 2 is kernel or boot specific), and
302 modprobe args to override both.
304 In the ``foo.dyndbg="QUERY"`` form, the query must exclude ``module foo``.
305 ``foo`` is extracted from the param-name, and applied to each query in
306 ``QUERY``, and only 1 match-spec of each type is allowed.
308 The ``dyndbg`` option is a "fake" module parameter, which means:
310 - modules do not need to define it explicitly
311 - every module gets it tacitly, whether they use pr_debug or not
312 - it doesn't appear in ``/sys/module/$module/parameters/``
313 To see it, grep the control file, or inspect ``/proc/cmdline.``
315 For ``CONFIG_DYNAMIC_DEBUG`` kernels, any settings given at boot-time (or
316 enabled by ``-DDEBUG`` flag during compilation) can be disabled later via
317 the debugfs interface if the debug messages are no longer needed::
319 echo "module module_name -p" > <debugfs>/dynamic_debug/control
326 // enable the message at line 1603 of file svcsock.c
327 nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
328 <debugfs>/dynamic_debug/control
330 // enable all the messages in file svcsock.c
331 nullarbor:~ # echo -n 'file svcsock.c +p' >
332 <debugfs>/dynamic_debug/control
334 // enable all the messages in the NFS server module
335 nullarbor:~ # echo -n 'module nfsd +p' >
336 <debugfs>/dynamic_debug/control
338 // enable all 12 messages in the function svc_process()
339 nullarbor:~ # echo -n 'func svc_process +p' >
340 <debugfs>/dynamic_debug/control
342 // disable all 12 messages in the function svc_process()
343 nullarbor:~ # echo -n 'func svc_process -p' >
344 <debugfs>/dynamic_debug/control
346 // enable messages for NFS calls READ, READLINK, READDIR and READDIR+.
347 nullarbor:~ # echo -n 'format "nfsd: READ" +p' >
348 <debugfs>/dynamic_debug/control
350 // enable messages in files of which the paths include string "usb"
351 nullarbor:~ # echo -n '*usb* +p' > <debugfs>/dynamic_debug/control
353 // enable all messages
354 nullarbor:~ # echo -n '+p' > <debugfs>/dynamic_debug/control
356 // add module, function to all enabled messages
357 nullarbor:~ # echo -n '+mf' > <debugfs>/dynamic_debug/control
359 // boot-args example, with newlines and comments for readability
360 Kernel command line: ...
361 // see whats going on in dyndbg=value processing
362 dynamic_debug.verbose=1
363 // enable pr_debugs in 2 builtins, #cmt is stripped
364 dyndbg="module params +p #cmt ; module sys +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"