selftests: introduce gen_tar Makefile target

The gen_kselftest_tar.sh always packages *all* selftests and doesn't
pass along any variables to `make install` to influence what should be
built. This can result in an early error on the command line ("Unknown
tarball format TARGETS=XXX"), or unexpected test failures as the
tarball contains tests people wanted to skip on purpose.

Since the makefile already contains all the logic, we can add a target
for packaging. Keep the default .gz target the script uses, and actually
extend the supported formats by using tar's autodetection.

To not break current workflows, keep the gen_kselftest_tar.sh script as
it is, with an added suggestion to use the makefile target instead.

Signed-off-by: Veronika Kabatova <[e-mail]>
Reviewed-by: Stefano Brivio <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

Documentation/dev-tools/kselftest.rst

···
    $ cd kselftest
    $ ./run_kselftest.sh
 
+Packaging selftests
+===================
+
+In some cases packaging is desired, such as when tests need to run on a
+different system. To package selftests, run::
+
+   $ make -C tools/testing/selftests gen_tar
+
+This generates a tarball in the `INSTALL_PATH/kselftest-packages` directory. By
+default, `.gz` format is used. The tar format can be overridden by specifying
+a `FORMAT` make variable. Any value recognized by `tar's auto-compress`_ option
+is supported, such as::
+
+    $ make -C tools/testing/selftests gen_tar FORMAT=.xz
+
+`make gen_tar` invokes `make install` so you can use it to package a subset of
+tests by using variables specified in `Running a subset of selftests`_
+section::
+
+    $ make -C tools/testing/selftests gen_tar TARGETS="bpf" FORMAT=.xz
+
+.. _tar's auto-compress: https://www.gnu.org/software/tar/manual/html_node/gzip.html#auto_002dcompress
+
 Contributing new tests
 ======================
 

tools/testing/selftests/Makefile

···
 	$(error Error: set INSTALL_PATH to use install)
 endif
 
+FORMAT ?= .gz
+TAR_PATH = $(abspath ${INSTALL_PATH}/kselftest-packages/kselftest.tar${FORMAT})
+gen_tar: install
+	@mkdir -p ${INSTALL_PATH}/kselftest-packages/
+	@tar caf ${TAR_PATH} --exclude=kselftest-packages -C ${INSTALL_PATH} .
+	@echo "Created ${TAR_PATH}"
+
 clean:
 	@for TARGET in $(TARGETS); do \
 		BUILD_TARGET=$$BUILD/$$TARGET;	\
 		$(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET clean;\
 	done;
 
-.PHONY: khdr all run_tests hotplug run_hotplug clean_hotplug run_pstore_crash install clean
+.PHONY: khdr all run_tests hotplug run_hotplug clean_hotplug run_pstore_crash install clean gen_tar

tools/testing/selftests/gen_kselftest_tar.sh

···
 	# directory
 	./kselftest_install.sh "$install_dir"
 	(cd "$install_work"; tar $copts "$dest"/kselftest${ext} $install_name)
+
+	# Don't put the message at the actual end as people may be parsing the
+	# "archive created" line in their scripts.
+	echo -e "\nConsider using 'make gen_tar' instead of this script\n"
+
 	echo "Kselftest archive kselftest${ext} created!"
 
 	# clean up top-level install work directory