dynamic debug: update docs

updates the documentation for 'dynamic debug' feature.
Signed-off-by: NGreg Banks <gnb@sgi.com>
Signed-off-by: NJason Baron <jbaron@redhat.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>

Documentation/dynamic-debug-howto.txt

+
+Introduction
+============
+
+This document describes how to use the dynamic debug (ddebug) feature.
+
+Dynamic debug is designed to allow you to dynamically enable/disable kernel
+code to obtain additional kernel information. Currently, if
+CONFIG_DYNAMIC_DEBUG is set, then all pr_debug()/dev_debug() calls can be
+dynamically enabled per-callsite.
+
+Dynamic debug has even more useful features:
+
+ * Simple query language allows turning on and off debugging statements by
+   matching any combination of:
+
+   - source filename
+   - function name
+   - line number (including ranges of line numbers)
+   - module name
+   - format string
+
+ * Provides a debugfs control file: <debugfs>/dynamic_debug/control which can be
+   read to display the complete list of known debug statements, to help guide you
+
+Controlling dynamic debug Behaviour
+===============================
+
+The behaviour of pr_debug()/dev_debug()s are controlled via writing to a
+control file in the 'debugfs' filesystem. Thus, you must first mount the debugfs
+filesystem, in order to make use of this feature. Subsequently, we refer to the
+control file as: <debugfs>/dynamic_debug/control. For example, if you want to
+enable printing from source file 'svcsock.c', line 1603 you simply do:
+
+nullarbor:~ # echo 'file svcsock.c line 1603 +p' >
+				<debugfs>/dynamic_debug/control
+
+If you make a mistake with the syntax, the write will fail thus:
+
+nullarbor:~ # echo 'file svcsock.c wtf 1 +p' >
+				<debugfs>/dynamic_debug/control
+-bash: echo: write error: Invalid argument
+
+Viewing Dynamic Debug Behaviour
+===========================
+
+You can view the currently configured behaviour of all the debug statements
+via:
+
+nullarbor:~ # cat <debugfs>/dynamic_debug/control
+# filename:lineno [module]function flags format
+/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:323 [svcxprt_rdma]svc_rdma_cleanup - "SVCRDMA\040Module\040Removed,\040deregister\040RPC\040RDMA\040transport\012"
+/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:341 [svcxprt_rdma]svc_rdma_init - "\011max_inline\040\040\040\040\040\040\040:\040%d\012"
+/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:340 [svcxprt_rdma]svc_rdma_init - "\011sq_depth\040\040\040\040\040\040\040\040\040:\040%d\012"
+/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:338 [svcxprt_rdma]svc_rdma_init - "\011max_requests\040\040\040\040\040:\040%d\012"
+...
+
+
+You can also apply standard Unix text manipulation filters to this
+data, e.g.
+
+nullarbor:~ # grep -i rdma <debugfs>/dynamic_debug/control  | wc -l
+62
+
+nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l
+42
+
+Note in particular that the third column shows the enabled behaviour
+flags for each debug statement callsite (see below for definitions of the
+flags).  The default value, no extra behaviour enabled, is "-".  So
+you can view all the debug statement callsites with any non-default flags:
+
+nullarbor:~ # awk '$3 != "-"' <debugfs>/dynamic_debug/control
+# filename:lineno [module]function flags format
+/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c:1603 [sunrpc]svc_send p "svc_process:\040st_sendto\040returned\040%d\012"
+
+
+Command Language Reference
+==========================
+
+At the lexical level, a command comprises a sequence of words separated
+by whitespace characters.  Note that newlines are treated as word
+separators and do *not* end a command or allow multiple commands to
+be done together.  So these are all equivalent:
+
+nullarbor:~ # echo -c 'file svcsock.c line 1603 +p' >
+				<debugfs>/dynamic_debug/control
+nullarbor:~ # echo -c '  file   svcsock.c     line  1603 +p  ' >
+				<debugfs>/dynamic_debug/control
+nullarbor:~ # echo -c 'file svcsock.c\nline 1603 +p' >
+				<debugfs>/dynamic_debug/control
+nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
+				<debugfs>/dynamic_debug/control
+
+Commands are bounded by a write() system call.  If you want to do
+multiple commands you need to do a separate "echo" for each, like:
+
+nullarbor:~ # echo 'file svcsock.c line 1603 +p' > /proc/dprintk ;\
+> echo 'file svcsock.c line 1563 +p' > /proc/dprintk
+
+or even like:
+
+nullarbor:~ # (
+> echo 'file svcsock.c line 1603 +p' ;\
+> echo 'file svcsock.c line 1563 +p' ;\
+> ) > /proc/dprintk
+
+At the syntactical level, a command comprises a sequence of match
+specifications, followed by a flags change specification.
+
+command ::= match-spec* flags-spec
+
+The match-spec's are used to choose a subset of the known dprintk()
+callsites to which to apply the flags-spec.  Think of them as a query
+with implicit ANDs between each pair.  Note that an empty list of
+match-specs is possible, but is not very useful because it will not
+match any debug statement callsites.
+
+A match specification comprises a keyword, which controls the attribute
+of the callsite to be compared, and a value to compare against.  Possible
+keywords are:
+
+match-spec ::= 'func' string |
+	       'file' string |
+	       'module' string |
+	       'format' string |
+	       'line' line-range
+
+line-range ::= lineno |
+	       '-'lineno |
+	       lineno'-' |
+	       lineno'-'lineno
+// Note: line-range cannot contain space, e.g.
+// "1-30" is valid range but "1 - 30" is not.
+
+lineno ::= unsigned-int
+
+The meanings of each keyword are:
+
+func
+    The given string is compared against the function name
+    of each callsite.  Example:
+
+    func svc_tcp_accept
+
+file
+    The given string is compared against either the full
+    pathname or the basename of the source file of each
+    callsite.  Examples:
+
+    file svcsock.c
+    file /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c
+
+module
+    The given string is compared against the module name
+    of each callsite.  The module name is the string as
+    seen in "lsmod", i.e. without the directory or the .ko
+    suffix and with '-' changed to '_'.  Examples:
+
+    module sunrpc
+    module nfsd
+
+format
+    The given string is searched for in the dynamic debug format
+    string.  Note that the string does not need to match the
+    entire format, only some part.  Whitespace and other
+    special characters can be escaped using C octal character
+    escape \ooo notation, e.g. the space character is \040.
+    Examples:
+
+    format svcrdma:	    // many of the NFS/RDMA server dprintks
+    format readahead	    // some dprintks in the readahead cache
+    format nfsd:\040SETATTR // how to match a format with whitespace
+
+line
+    The given line number or range of line numbers is compared
+    against the line number of each dprintk() callsite.  A single
+    line number matches the callsite line number exactly.  A
+    range of line numbers matches any callsite between the first
+    and last line number inclusive.  An empty first number means
+    the first line in the file, an empty line number means the
+    last number in the file.  Examples:
+
+    line 1603	    // exactly line 1603
+    line 1600-1605  // the six lines from line 1600 to line 1605
+    line -1605	    // the 1605 lines from line 1 to line 1605
+    line 1600-	    // all lines from line 1600 to the end of the file
+
+The flags specification comprises a change operation followed
+by one or more flag characters.  The change operation is one
+of the characters:
+
+-
+    remove the given flags
+
++
+    add the given flags
+
+=
+    set the flags to the given flags
+
+The flags are:
+
+p
+    Causes a printk() message to be emitted to dmesg
+
+Note the regexp ^[-+=][scp]+$ matches a flags specification.
+Note also that there is no convenient syntax to remove all
+the flags at once, you need to use "-psc".
+
+Examples
+========
+
+// enable the message at line 1603 of file svcsock.c
+nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
+				<debugfs>/dynamic_debug/control
+
+// enable all the messages in file svcsock.c
+nullarbor:~ # echo -n 'file svcsock.c +p' >
+				<debugfs>/dynamic_debug/control
+
+// enable all the messages in the NFS server module
+nullarbor:~ # echo -n 'module nfsd +p' >
+				<debugfs>/dynamic_debug/control
+
+// enable all 12 messages in the function svc_process()
+nullarbor:~ # echo -n 'func svc_process +p' >
+				<debugfs>/dynamic_debug/control
+
+// disable all 12 messages in the function svc_process()
+nullarbor:~ # echo -n 'func svc_process -p' >
+				<debugfs>/dynamic_debug/control

lib/Kconfig.debug

@@ -848,59 +848,69 @@ config BUILD_DOCSRC
 	  Say N if you are unsure.
 
 config DYNAMIC_DEBUG
-	bool "Enable dynamic printk() call support"
+	bool "Enable dynamic printk() support"
 	default n
 	depends on PRINTK
+	depends on DEBUG_FS
 	select PRINTK_DEBUG
 	help
 
 	  Compiles debug level messages into the kernel, which would not
 	  otherwise be available at runtime. These messages can then be
-	  enabled/disabled on a per module basis. This mechanism implicitly
-	  enables all pr_debug() and dev_dbg() calls. The impact of this
-	  compile option is a larger kernel text size of about 2%.
+	  enabled/disabled based on various levels of scope - per source file,
+	  function, module, format string, and line number. This mechanism
+	  implicitly enables all pr_debug() and dev_dbg() calls. The impact of
+	  this compile option is a larger kernel text size of about 2%.
 
 	  Usage:
 
-	  Dynamic debugging is controlled by the debugfs file,
-	  dynamic_printk/modules. This file contains a list of the modules that
-	  can be enabled. The format of the file is the module name, followed
-	  by a set of flags that can be enabled. The first flag is always the
-	  'enabled' flag. For example:
+	  Dynamic debugging is controlled via the 'dynamic_debug/ddebug' file,
+	  which is contained in the 'debugfs' filesystem. Thus, the debugfs
+	  filesystem must first be mounted before making use of this feature.
+	  We refer the control file as: <debugfs>/dynamic_debug/ddebug. This
+	  file contains a list of the debug statements that can be enabled. The
+	  format for each line of the file is:
 
-		<module_name> <enabled=0/1>
-				.
-				.
-				.
+		filename:lineno [module]function flags format
 
-	  <module_name> : Name of the module in which the debug call resides
-	  <enabled=0/1> : whether the messages are enabled or not
+	  filename : source file of the debug statement
+	  lineno : line number of the debug statement
+	  module : module that contains the debug statement
+	  function : function that contains the debug statement
+          flags : 'p' means the line is turned 'on' for printing
+          format : the format used for the debug statement
 
 	  From a live system:
 
-		snd_hda_intel enabled=0
-		fixup enabled=0
-		driver enabled=0
+		nullarbor:~ # cat <debugfs>/dynamic_debug/ddebug
+		# filename:lineno [module]function flags format
+		fs/aio.c:222 [aio]__put_ioctx - "__put_ioctx:\040freeing\040%p\012"
+		fs/aio.c:248 [aio]ioctx_alloc - "ENOMEM:\040nr_events\040too\040high\012"
+		fs/aio.c:1770 [aio]sys_io_cancel - "calling\040cancel\012"
 
-	  Enable a module:
+	  Example usage:
 
-	  	$echo "set enabled=1 <module_name>" > dynamic_printk/modules
+		// enable the message at line 1603 of file svcsock.c
+		nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
+						<debugfs>/dynamic_debug/ddebug
 
-	  Disable a module:
+		// enable all the messages in file svcsock.c
+		nullarbor:~ # echo -n 'file svcsock.c +p' >
+						<debugfs>/dynamic_debug/ddebug
 
-	  	$echo "set enabled=0 <module_name>" > dynamic_printk/modules
+		// enable all the messages in the NFS server module
+		nullarbor:~ # echo -n 'module nfsd +p' >
+						<debugfs>/dynamic_debug/ddebug
 
-	  Enable all modules:
+		// enable all 12 messages in the function svc_process()
+		nullarbor:~ # echo -n 'func svc_process +p' >
+						<debugfs>/dynamic_debug/ddebug
 
-		$echo "set enabled=1 all" > dynamic_printk/modules
+		// disable all 12 messages in the function svc_process()
+		nullarbor:~ # echo -n 'func svc_process -p' >
+						<debugfs>/dynamic_debug/ddebug
 
-	  Disable all modules:
-
-		$echo "set enabled=0 all" > dynamic_printk/modules
-
-	  Finally, passing "dynamic_printk" at the command line enables
-	  debugging for all modules. This mode can be turned off via the above
-	  disable command.
+	  See Documentation/dynamic-debug-howto.txt for additional information.
 
 source "samples/Kconfig"