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+
+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