regmap: Add some basic kunit tests

On the theory that it's better to make a start let's add some KUnit tests
for regmap. Currently this is a bit of a mess but it passes and hopefully
will at some point help catch problems. We provide very basic cover for
most of the core functionality that operates at the register level,
repeating each test for each cache type in order to exercise the caches.
There is no coverage of anything to do with the bulk operations at the bus
level or formatting for byte stream buses yet.

Each test creates it's own regmap since the cache structures are built
incrementally, meaning we gain coverage from the different access
patterns, and some of the tests cover different init scenarios.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20230324-regmap-kunit-v2-2-b208801dc2c8@kernel.org

drivers/base/regmap/Kconfig

@@ -9,6 +9,13 @@ config REGMAP
 	select MDIO_BUS if REGMAP_MDIO
 	bool
 
+config REGMAP_KUNIT
+	tristate "KUnit tests for regmap"
+	depends on KUNIT
+	default KUNIT_ALL_TESTS
+	select REGMAP
+	select REGMAP_RAM
+
 config REGMAP_AC97
 	tristate
 

drivers/base/regmap/Makefile

@@ -5,6 +5,7 @@ CFLAGS_regmap.o := -I$(src)
 obj-$(CONFIG_REGMAP) += regmap.o regcache.o
 obj-$(CONFIG_REGMAP) += regcache-rbtree.o regcache-flat.o
 obj-$(CONFIG_DEBUG_FS) += regmap-debugfs.o
+obj-$(CONFIG_REGMAP_KUNIT) += regmap-kunit.o
 obj-$(CONFIG_REGMAP_AC97) += regmap-ac97.o
 obj-$(CONFIG_REGMAP_I2C) += regmap-i2c.o
 obj-$(CONFIG_REGMAP_RAM) += regmap-ram.o