kunit: allow kunit tests to be loaded as a module

As tests are added to kunit, it will become less feasible to execute
all built tests together.  By supporting modular tests we provide
a simple way to do selective execution on a running system; specifying

CONFIG_KUNIT=y
CONFIG_KUNIT_EXAMPLE_TEST=m

...means we can simply "insmod example-test.ko" to run the tests.

To achieve this we need to do the following:

o export the required symbols in kunit
o string-stream tests utilize non-exported symbols so for now we skip
  building them when CONFIG_KUNIT_TEST=m.
o drivers/base/power/qos-test.c contains a few unexported interface
  references, namely freq_qos_read_value() and freq_constraints_init().
  Both of these could be potentially defined as static inline functions
  in include/linux/pm_qos.h, but for now we simply avoid supporting
  module build for that test suite.
o support a new way of declaring test suites.  Because a module cannot
  do multiple late_initcall()s, we provide a kunit_test_suites() macro
  to declare multiple suites within the same module at once.
o some test module names would have been too general ("test-test"
  and "example-test" for kunit tests, "inode-test" for ext4 tests);
  rename these as appropriate ("kunit-test", "kunit-example-test"
  and "ext4-inode-test" respectively).

Also define kunit_test_suite() via kunit_test_suites()
as callers in other trees may need the old definition.

Co-developed-by: Knut Omang <knut.omang@oracle.com>
Signed-off-by: Knut Omang <knut.omang@oracle.com>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Acked-by: Theodore Ts'o <tytso@mit.edu> # for ext4 bits
Acked-by: David Gow <davidgow@google.com> # For list-test
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

include/kunit/test.h

@@ -12,6 +12,7 @@
 #include <kunit/assert.h>
 #include <kunit/try-catch.h>
 #include <linux/kernel.h>
+#include <linux/module.h>
 #include <linux/slab.h>
 #include <linux/types.h>
 
@@ -197,31 +198,47 @@ void kunit_init_test(struct kunit *test, const char *name);
 int kunit_run_tests(struct kunit_suite *suite);
 
 /**
- * kunit_test_suite() - used to register a &struct kunit_suite with KUnit.
+ * kunit_test_suites() - used to register one or more &struct kunit_suite
+ *			 with KUnit.
  *
- * @suite: a statically allocated &struct kunit_suite.
+ * @suites: a statically allocated list of &struct kunit_suite.
  *
- * Registers @suite with the test framework. See &struct kunit_suite for
+ * Registers @suites with the test framework. See &struct kunit_suite for
  * more information.
  *
- * NOTE: Currently KUnit tests are all run as late_initcalls; this means
+ * When builtin,  KUnit tests are all run as late_initcalls; this means
  * that they cannot test anything where tests must run at a different init
  * phase. One significant restriction resulting from this is that KUnit
  * cannot reliably test anything that is initialize in the late_init phase;
  * another is that KUnit is useless to test things that need to be run in
  * an earlier init phase.
  *
+ * An alternative is to build the tests as a module.  Because modules
+ * do not support multiple late_initcall()s, we need to initialize an
+ * array of suites for a module.
+ *
  * TODO(brendanhiggins@google.com): Don't run all KUnit tests as
  * late_initcalls.  I have some future work planned to dispatch all KUnit
  * tests from the same place, and at the very least to do so after
  * everything else is definitely initialized.
  */
-#define kunit_test_suite(suite)						       \
-	static int kunit_suite_init##suite(void)			       \
-	{								       \
-		return kunit_run_tests(&suite);				       \
-	}								       \
-	late_initcall(kunit_suite_init##suite)
+#define kunit_test_suites(...)						\
+	static struct kunit_suite *suites[] = { __VA_ARGS__, NULL};	\
+	static int kunit_test_suites_init(void)				\
+	{								\
+		unsigned int i;						\
+		for (i = 0; suites[i] != NULL; i++)			\
+			kunit_run_tests(suites[i]);			\
+		return 0;						\
+	}								\
+	late_initcall(kunit_test_suites_init);				\
+	static void __exit kunit_test_suites_exit(void)			\
+	{								\
+		return;							\
+	}								\
+	module_exit(kunit_test_suites_exit)
+
+#define kunit_test_suite(suite)	kunit_test_suites(&suite)
 
 /*
  * Like kunit_alloc_resource() below, but returns the struct kunit_resource