regmap: Specifically test writing 0 as a value to sparse caches

Since 0 can look a lot like a NULL pointer when used in a cache some clever
data structures might potentially introduce bugs specific to handling it.
Add some explicit testing of storing 0 as a value in a sparse cache, at the
minute there are no issues and this will stop any appearing in the future.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://patch.msgid.link/20240924-regcache-zero-value-v1-1-8a1224214b52@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: broonie

drivers/base/regmap/regmap-kunit.c

@@ -1499,6 +1499,48 @@ static void cache_present(struct kunit *test)
 		KUNIT_ASSERT_TRUE(test, regcache_reg_cached(map, param->from_reg + i));
 }
 
+static void cache_write_zero(struct kunit *test)
+{
+	const struct regmap_test_param *param = test->param_value;
+	struct regmap *map;
+	struct regmap_config config;
+	struct regmap_ram_data *data;
+	unsigned int val;
+	int i;
+
+	config = test_regmap_config;
+
+	map = gen_regmap(test, &config, &data);
+	KUNIT_ASSERT_FALSE(test, IS_ERR(map));
+	if (IS_ERR(map))
+		return;
+
+	for (i = 0; i < BLOCK_TEST_SIZE; i++)
+		data->read[param->from_reg + i] = false;
+
+	/* No defaults so no registers cached. */
+	for (i = 0; i < BLOCK_TEST_SIZE; i++)
+		KUNIT_ASSERT_FALSE(test, regcache_reg_cached(map, param->from_reg + i));
+
+	/* We didn't trigger any reads */
+	for (i = 0; i < BLOCK_TEST_SIZE; i++)
+		KUNIT_ASSERT_FALSE(test, data->read[param->from_reg + i]);
+
+	/* Write a zero value */
+	KUNIT_EXPECT_EQ(test, 0, regmap_write(map, 1, 0));
+
+	/* Read that zero value back */
+	KUNIT_EXPECT_EQ(test, 0, regmap_read(map, 1, &val));
+	KUNIT_EXPECT_EQ(test, 0, val);
+
+	/* From the cache? */
+	KUNIT_ASSERT_TRUE(test, regcache_reg_cached(map, 1));
+
+	/* Try to throw it away */
+	KUNIT_EXPECT_EQ(test, 0, regcache_drop_region(map, 1, 1));
+	KUNIT_ASSERT_FALSE(test, regcache_reg_cached(map, 1));
+}
+
 /* Check that caching the window register works with sync */
 static void cache_range_window_reg(struct kunit *test)
 {
@@ -2012,6 +2054,7 @@ static struct kunit_case regmap_test_cases[] = {
 	KUNIT_CASE_PARAM(cache_drop_all_and_sync_no_defaults, sparse_cache_types_gen_params),
 	KUNIT_CASE_PARAM(cache_drop_all_and_sync_has_defaults, sparse_cache_types_gen_params),
 	KUNIT_CASE_PARAM(cache_present, sparse_cache_types_gen_params),
+	KUNIT_CASE_PARAM(cache_write_zero, sparse_cache_types_gen_params),
 	KUNIT_CASE_PARAM(cache_range_window_reg, real_cache_types_only_gen_params),
 
 	KUNIT_CASE_PARAM(raw_read_defaults_single, raw_test_types_gen_params),