269 lines
9.5 KiB
Plaintext
269 lines
9.5 KiB
Plaintext
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Introduction
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============
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This document describes how to use the dynamic debug (ddebug) feature.
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Dynamic debug is designed to allow you to dynamically enable/disable kernel
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code to obtain additional kernel information. Currently, if
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CONFIG_DYNAMIC_DEBUG is set, then all pr_debug()/dev_dbg() calls can be
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dynamically enabled per-callsite.
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Dynamic debug has even more useful features:
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* Simple query language allows turning on and off debugging statements by
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matching any combination of:
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- source filename
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- function name
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- line number (including ranges of line numbers)
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- module name
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- format string
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* Provides a debugfs control file: <debugfs>/dynamic_debug/control which can be
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read to display the complete list of known debug statements, to help guide you
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Controlling dynamic debug Behaviour
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===================================
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The behaviour of pr_debug()/dev_dbg()s are controlled via writing to a
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control file in the 'debugfs' filesystem. Thus, you must first mount the debugfs
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filesystem, in order to make use of this feature. Subsequently, we refer to the
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control file as: <debugfs>/dynamic_debug/control. For example, if you want to
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enable printing from source file 'svcsock.c', line 1603 you simply do:
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nullarbor:~ # echo 'file svcsock.c line 1603 +p' >
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<debugfs>/dynamic_debug/control
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If you make a mistake with the syntax, the write will fail thus:
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nullarbor:~ # echo 'file svcsock.c wtf 1 +p' >
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<debugfs>/dynamic_debug/control
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-bash: echo: write error: Invalid argument
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Viewing Dynamic Debug Behaviour
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===========================
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You can view the currently configured behaviour of all the debug statements
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via:
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nullarbor:~ # cat <debugfs>/dynamic_debug/control
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# filename:lineno [module]function flags format
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/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:323 [svcxprt_rdma]svc_rdma_cleanup - "SVCRDMA Module Removed, deregister RPC RDMA transport\012"
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/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:341 [svcxprt_rdma]svc_rdma_init - "\011max_inline : %d\012"
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/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:340 [svcxprt_rdma]svc_rdma_init - "\011sq_depth : %d\012"
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/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svc_rdma.c:338 [svcxprt_rdma]svc_rdma_init - "\011max_requests : %d\012"
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...
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You can also apply standard Unix text manipulation filters to this
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data, e.g.
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nullarbor:~ # grep -i rdma <debugfs>/dynamic_debug/control | wc -l
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62
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nullarbor:~ # grep -i tcp <debugfs>/dynamic_debug/control | wc -l
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42
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Note in particular that the third column shows the enabled behaviour
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flags for each debug statement callsite (see below for definitions of the
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flags). The default value, no extra behaviour enabled, is "-". So
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you can view all the debug statement callsites with any non-default flags:
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nullarbor:~ # awk '$3 != "-"' <debugfs>/dynamic_debug/control
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# filename:lineno [module]function flags format
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/usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c:1603 [sunrpc]svc_send p "svc_process: st_sendto returned %d\012"
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Command Language Reference
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==========================
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At the lexical level, a command comprises a sequence of words separated
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by whitespace characters. Note that newlines are treated as word
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separators and do *not* end a command or allow multiple commands to
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be done together. So these are all equivalent:
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nullarbor:~ # echo -c 'file svcsock.c line 1603 +p' >
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<debugfs>/dynamic_debug/control
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nullarbor:~ # echo -c ' file svcsock.c line 1603 +p ' >
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<debugfs>/dynamic_debug/control
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nullarbor:~ # echo -c 'file svcsock.c\nline 1603 +p' >
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<debugfs>/dynamic_debug/control
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nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
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<debugfs>/dynamic_debug/control
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Commands are bounded by a write() system call. If you want to do
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multiple commands you need to do a separate "echo" for each, like:
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nullarbor:~ # echo 'file svcsock.c line 1603 +p' > /proc/dprintk ;\
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> echo 'file svcsock.c line 1563 +p' > /proc/dprintk
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or even like:
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nullarbor:~ # (
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> echo 'file svcsock.c line 1603 +p' ;\
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> echo 'file svcsock.c line 1563 +p' ;\
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> ) > /proc/dprintk
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At the syntactical level, a command comprises a sequence of match
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specifications, followed by a flags change specification.
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command ::= match-spec* flags-spec
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The match-spec's are used to choose a subset of the known dprintk()
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callsites to which to apply the flags-spec. Think of them as a query
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with implicit ANDs between each pair. Note that an empty list of
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match-specs is possible, but is not very useful because it will not
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match any debug statement callsites.
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A match specification comprises a keyword, which controls the attribute
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of the callsite to be compared, and a value to compare against. Possible
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keywords are:
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match-spec ::= 'func' string |
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'file' string |
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'module' string |
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'format' string |
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'line' line-range
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line-range ::= lineno |
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'-'lineno |
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lineno'-' |
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lineno'-'lineno
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// Note: line-range cannot contain space, e.g.
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// "1-30" is valid range but "1 - 30" is not.
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lineno ::= unsigned-int
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The meanings of each keyword are:
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func
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The given string is compared against the function name
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of each callsite. Example:
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func svc_tcp_accept
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file
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The given string is compared against either the full
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pathname or the basename of the source file of each
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callsite. Examples:
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file svcsock.c
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file /usr/src/packages/BUILD/sgi-enhancednfs-1.4/default/net/sunrpc/svcsock.c
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module
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The given string is compared against the module name
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of each callsite. The module name is the string as
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seen in "lsmod", i.e. without the directory or the .ko
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suffix and with '-' changed to '_'. Examples:
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module sunrpc
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module nfsd
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format
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The given string is searched for in the dynamic debug format
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string. Note that the string does not need to match the
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entire format, only some part. Whitespace and other
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special characters can be escaped using C octal character
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escape \ooo notation, e.g. the space character is \040.
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Alternatively, the string can be enclosed in double quote
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characters (") or single quote characters (').
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Examples:
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format svcrdma: // many of the NFS/RDMA server dprintks
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format readahead // some dprintks in the readahead cache
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format nfsd:\040SETATTR // one way to match a format with whitespace
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format "nfsd: SETATTR" // a neater way to match a format with whitespace
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format 'nfsd: SETATTR' // yet another way to match a format with whitespace
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line
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The given line number or range of line numbers is compared
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against the line number of each dprintk() callsite. A single
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line number matches the callsite line number exactly. A
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range of line numbers matches any callsite between the first
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and last line number inclusive. An empty first number means
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the first line in the file, an empty line number means the
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last number in the file. Examples:
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line 1603 // exactly line 1603
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line 1600-1605 // the six lines from line 1600 to line 1605
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line -1605 // the 1605 lines from line 1 to line 1605
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line 1600- // all lines from line 1600 to the end of the file
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The flags specification comprises a change operation followed
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by one or more flag characters. The change operation is one
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of the characters:
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-
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remove the given flags
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+
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add the given flags
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=
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set the flags to the given flags
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The flags are:
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f
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Include the function name in the printed message
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l
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Include line number in the printed message
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m
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Include module name in the printed message
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p
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Causes a printk() message to be emitted to dmesg
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t
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Include thread ID in messages not generated from interrupt context
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Note the regexp ^[-+=][flmpt]+$ matches a flags specification.
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Note also that there is no convenient syntax to remove all
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the flags at once, you need to use "-flmpt".
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Debug messages during boot process
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==================================
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To be able to activate debug messages during the boot process,
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even before userspace and debugfs exists, use the boot parameter:
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ddebug_query="QUERY"
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QUERY follows the syntax described above, but must not exceed 1023
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characters. The enablement of debug messages is done as an arch_initcall.
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Thus you can enable debug messages in all code processed after this
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arch_initcall via this boot parameter.
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On an x86 system for example ACPI enablement is a subsys_initcall and
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ddebug_query="file ec.c +p"
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will show early Embedded Controller transactions during ACPI setup if
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your machine (typically a laptop) has an Embedded Controller.
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PCI (or other devices) initialization also is a hot candidate for using
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this boot parameter for debugging purposes.
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Examples
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========
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// enable the message at line 1603 of file svcsock.c
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nullarbor:~ # echo -n 'file svcsock.c line 1603 +p' >
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<debugfs>/dynamic_debug/control
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// enable all the messages in file svcsock.c
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nullarbor:~ # echo -n 'file svcsock.c +p' >
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<debugfs>/dynamic_debug/control
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// enable all the messages in the NFS server module
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nullarbor:~ # echo -n 'module nfsd +p' >
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<debugfs>/dynamic_debug/control
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// enable all 12 messages in the function svc_process()
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nullarbor:~ # echo -n 'func svc_process +p' >
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<debugfs>/dynamic_debug/control
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// disable all 12 messages in the function svc_process()
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nullarbor:~ # echo -n 'func svc_process -p' >
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<debugfs>/dynamic_debug/control
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// enable messages for NFS calls READ, READLINK, READDIR and READDIR+.
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nullarbor:~ # echo -n 'format "nfsd: READ" +p' >
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<debugfs>/dynamic_debug/control
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