lib: util_macros_kunit: add kunit test for util_macros.h

A bug was found in the find_closest() (find_closest_descending() is also
affected after some testing), where for certain values with small
progressions of 1, 2 & 3, the rounding (done by averaging 2 values) causes
an incorrect index to be returned.

The bug is described in more detail in the commit which fixes the bug. 
This commit adds a kunit test to validate that the fix works correctly.

This kunit test adds some of the arrays (from the driver-sphere) that seem
to produce issues with the 'find_closest()' macro.  Specifically the one
from ad7606 driver (with which the bug was found) and from the ina2xx
drivers, which shows the quirk with 'find_closest()' with elements in a
array that have an interval of 3.

For the find_closest_descending() tests, the same arrays are used as for
the find_closest(), but in reverse; the idea is that
'find_closest_descending()' should return the sames indices as
'find_closest()' but in reverse.

For testing both macros, there are 4 special arrays created, one for
testing find_closest{_descending}() for arrays of progressions 1, 2, 3 and
4.  The idea is to show that (for progressions of 1, 2 & 3) the fix works
as expected.  When removing the fix, the issues should start to show up.

Then an extra array of negative and positive values is added.  There are
currently no such arrays within drivers, but one could expect that these
macros behave correctly even for such arrays.

To run this kunit:
  ./tools/testing/kunit/kunit.py run "*util_macros*"

Link: https://lkml.kernel.org/r/20241105145406.554365-2-aardelean@baylibre.com
Signed-off-by: Alexandru Ardelean <aardelean@baylibre.com>
Cc: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 2549b64b2280..d7dd38f14333 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2630,6 +2630,23 @@ config CHECKSUM_KUNIT
 
 	  If unsure, say N.
 
+config UTIL_MACROS_KUNIT
+	tristate "KUnit test util_macros.h functions at runtime" if !KUNIT_ALL_TESTS
+	depends on KUNIT
+	default KUNIT_ALL_TESTS
+	help
+	  Enable this option to test the util_macros.h function at boot.
+
+	  KUnit tests run during boot and output the results to the debug log
+	  in TAP format (http://testanything.org/). Only useful for kernel devs
+	  running the KUnit test harness, and not intended for inclusion into a
+	  production build.
+
+	  For more information on KUnit and unit tests in general please refer
+	  to the KUnit documentation in Documentation/dev-tools/kunit/.
+
+	  If unsure, say N.
+
 config HASH_KUNIT_TEST
 	tristate "KUnit Test for integer hash functions" if !KUNIT_ALL_TESTS
 	depends on KUNIT