--- /dev/null
+Using gcov with the Linux kernel
+================================
+
+gcov profiling kernel support enables the use of GCC's coverage testing
+tool gcov_ with the Linux kernel. Coverage data of a running kernel
+is exported in gcov-compatible format via the "gcov" debugfs directory.
+To get coverage data for a specific file, change to the kernel build
+directory and use gcov with the ``-o`` option as follows (requires root)::
+
+ # cd /tmp/linux-out
+ # gcov -o /sys/kernel/debug/gcov/tmp/linux-out/kernel spinlock.c
+
+This will create source code files annotated with execution counts
+in the current directory. In addition, graphical gcov front-ends such
+as lcov_ can be used to automate the process of collecting data
+for the entire kernel and provide coverage overviews in HTML format.
+
+Possible uses:
+
+* debugging (has this line been reached at all?)
+* test improvement (how do I change my test to cover these lines?)
+* minimizing kernel configurations (do I need this option if the
+ associated code is never run?)
+
+.. _gcov: http://gcc.gnu.org/onlinedocs/gcc/Gcov.html
+.. _lcov: http://ltp.sourceforge.net/coverage/lcov.php
+
+
+Preparation
+-----------
+
+Configure the kernel with::
+
+ CONFIG_DEBUG_FS=y
+ CONFIG_GCOV_KERNEL=y
+
+select the gcc's gcov format, default is autodetect based on gcc version::
+
+ CONFIG_GCOV_FORMAT_AUTODETECT=y
+
+and to get coverage data for the entire kernel::
+
+ CONFIG_GCOV_PROFILE_ALL=y
+
+Note that kernels compiled with profiling flags will be significantly
+larger and run slower. Also CONFIG_GCOV_PROFILE_ALL may not be supported
+on all architectures.
+
+Profiling data will only become accessible once debugfs has been
+mounted::
+
+ mount -t debugfs none /sys/kernel/debug
+
+
+Customization
+-------------
+
+To enable profiling for specific files or directories, add a line
+similar to the following to the respective kernel Makefile:
+
+- For a single file (e.g. main.o)::
+
+ GCOV_PROFILE_main.o := y
+
+- For all files in one directory::
+
+ GCOV_PROFILE := y
+
+To exclude files from being profiled even when CONFIG_GCOV_PROFILE_ALL
+is specified, use::
+
+ GCOV_PROFILE_main.o := n
+
+and::
+
+ GCOV_PROFILE := n
+
+Only files which are linked to the main kernel image or are compiled as
+kernel modules are supported by this mechanism.
+
+
+Files
+-----
+
+The gcov kernel support creates the following files in debugfs:
+
+``/sys/kernel/debug/gcov``
+ Parent directory for all gcov-related files.
+
+``/sys/kernel/debug/gcov/reset``
+ Global reset file: resets all coverage data to zero when
+ written to.
+
+``/sys/kernel/debug/gcov/path/to/compile/dir/file.gcda``
+ The actual gcov data file as understood by the gcov
+ tool. Resets file coverage data to zero when written to.
+
+``/sys/kernel/debug/gcov/path/to/compile/dir/file.gcno``
+ Symbolic link to a static data file required by the gcov
+ tool. This file is generated by gcc when compiling with
+ option ``-ftest-coverage``.
+
+
+Modules
+-------
+
+Kernel modules may contain cleanup code which is only run during
+module unload time. The gcov mechanism provides a means to collect
+coverage data for such code by keeping a copy of the data associated
+with the unloaded module. This data remains available through debugfs.
+Once the module is loaded again, the associated coverage counters are
+initialized with the data from its previous instantiation.
+
+This behavior can be deactivated by specifying the gcov_persist kernel
+parameter::
+
+ gcov_persist=0
+
+At run-time, a user can also choose to discard data for an unloaded
+module by writing to its data file or the global reset file.
+
+
+Separated build and test machines
+---------------------------------
+
+The gcov kernel profiling infrastructure is designed to work out-of-the
+box for setups where kernels are built and run on the same machine. In
+cases where the kernel runs on a separate machine, special preparations
+must be made, depending on where the gcov tool is used:
+
+a) gcov is run on the TEST machine
+
+ The gcov tool version on the test machine must be compatible with the
+ gcc version used for kernel build. Also the following files need to be
+ copied from build to test machine:
+
+ from the source tree:
+ - all C source files + headers
+
+ from the build tree:
+ - all C source files + headers
+ - all .gcda and .gcno files
+ - all links to directories
+
+ It is important to note that these files need to be placed into the
+ exact same file system location on the test machine as on the build
+ machine. If any of the path components is symbolic link, the actual
+ directory needs to be used instead (due to make's CURDIR handling).
+
+b) gcov is run on the BUILD machine
+
+ The following files need to be copied after each test case from test
+ to build machine:
+
+ from the gcov directory in sysfs:
+ - all .gcda files
+ - all links to .gcno files
+
+ These files can be copied to any location on the build machine. gcov
+ must then be called with the -o option pointing to that directory.
+
+ Example directory setup on the build machine::
+
+ /tmp/linux: kernel source tree
+ /tmp/out: kernel build directory as specified by make O=
+ /tmp/coverage: location of the files copied from the test machine
+
+ [user@build] cd /tmp/out
+ [user@build] gcov -o /tmp/coverage/tmp/out/init main.c
+
+
+Troubleshooting
+---------------
+
+Problem
+ Compilation aborts during linker step.
+
+Cause
+ Profiling flags are specified for source files which are not
+ linked to the main kernel or which are linked by a custom
+ linker procedure.
+
+Solution
+ Exclude affected source files from profiling by specifying
+ ``GCOV_PROFILE := n`` or ``GCOV_PROFILE_basename.o := n`` in the
+ corresponding Makefile.
+
+Problem
+ Files copied from sysfs appear empty or incomplete.
+
+Cause
+ Due to the way seq_file works, some tools such as cp or tar
+ may not correctly copy files from sysfs.
+
+Solution
+ Use ``cat``' to read ``.gcda`` files and ``cp -d`` to copy links.
+ Alternatively use the mechanism shown in Appendix B.
+
+
+Appendix A: gather_on_build.sh
+------------------------------
+
+Sample script to gather coverage meta files on the build machine
+(see 6a)::
+
+ #!/bin/bash
+
+ KSRC=$1
+ KOBJ=$2
+ DEST=$3
+
+ if [ -z "$KSRC" ] || [ -z "$KOBJ" ] || [ -z "$DEST" ]; then
+ echo "Usage: $0 <ksrc directory> <kobj directory> <output.tar.gz>" >&2
+ exit 1
+ fi
+
+ KSRC=$(cd $KSRC; printf "all:\n\t@echo \${CURDIR}\n" | make -f -)
+ KOBJ=$(cd $KOBJ; printf "all:\n\t@echo \${CURDIR}\n" | make -f -)
+
+ find $KSRC $KOBJ \( -name '*.gcno' -o -name '*.[ch]' -o -type l \) -a \
+ -perm /u+r,g+r | tar cfz $DEST -P -T -
+
+ if [ $? -eq 0 ] ; then
+ echo "$DEST successfully created, copy to test system and unpack with:"
+ echo " tar xfz $DEST -P"
+ else
+ echo "Could not create file $DEST"
+ fi
+
+
+Appendix B: gather_on_test.sh
+-----------------------------
+
+Sample script to gather coverage data files on the test machine
+(see 6b)::
+
+ #!/bin/bash -e
+
+ DEST=$1
+ GCDA=/sys/kernel/debug/gcov
+
+ if [ -z "$DEST" ] ; then
+ echo "Usage: $0 <output.tar.gz>" >&2
+ exit 1
+ fi
+
+ TEMPDIR=$(mktemp -d)
+ echo Collecting data..
+ find $GCDA -type d -exec mkdir -p $TEMPDIR/\{\} \;
+ find $GCDA -name '*.gcda' -exec sh -c 'cat < $0 > '$TEMPDIR'/$0' {} \;
+ find $GCDA -name '*.gcno' -exec sh -c 'cp -d $0 '$TEMPDIR'/$0' {} \;
+ tar czf $DEST -C $TEMPDIR sys
+ rm -rf $TEMPDIR
+
+ echo "$DEST successfully created, copy to build system and unpack with:"
+ echo " tar xfz $DEST"
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