iio: test: add basic tests for the iio-rescale driver

The iio-rescale driver supports various combinations of scale types and
offsets. These can often result in large integer multiplications. Make
sure these calculations are done right by adding a set of kunit test
cases that build on top of iio-test-format.

To run these tests, add the following to .kunitconfig
	$ cat .kunitconfig
	CONFIG_IIO=y
	CONFIG_IIO_RESCALE_KUNIT_TEST=y
	CONFIG_KUNIT=y

Then run:
	$ ./tools/testing/kunit/kunit.py run --kunitconfig .kunitconfig

Signed-off-by: Liam Beguin <liambeguin@gmail.com>
Reviewed-by: Peter Rosin <peda@axentia.se>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220213025739.2561834-7-liambeguin@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/test/Kconfig

@@ -4,6 +4,16 @@
 #
 
 # Keep in alphabetical order
+config IIO_RESCALE_KUNIT_TEST
+	bool "Test IIO rescale conversion functions"
+	depends on KUNIT=y && !IIO_RESCALE
+	default KUNIT_ALL_TESTS
+	help
+	  If you want to run tests on the iio-rescale code say Y here.
+
+	  This takes advantage of ARCH=um to run tests and should be used by
+	  developers to tests their changes to the rescaling logic.
+
 config IIO_TEST_FORMAT
         bool "Test IIO formatting functions"
         depends on KUNIT=y

drivers/iio/test/Makefile

@@ -4,5 +4,6 @@
 #
 
 # Keep in alphabetical order
+obj-$(CONFIG_IIO_RESCALE_KUNIT_TEST) += iio-test-rescale.o ../afe/iio-rescale.o
 obj-$(CONFIG_IIO_TEST_FORMAT) += iio-test-format.o
 CFLAGS_iio-test-format.o += $(DISABLE_STRUCTLEAK_PLUGIN)