of: Add KUnit test to confirm DTB is loaded

Add a KUnit test that confirms a DTB has been loaded, i.e. there is a
root node, and that the of_have_populated_dt() API works properly. We
skip the test when CONFIG_OF_EARLY_FLATREE=n because in that case we
know architecture code hasn't called unflatten_(and_copy_)?device_tree()
which would populate some sort of root node.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Frank Rowand <frowand.list@gmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Cc: Brendan Higgins <brendan.higgins@linux.dev>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lore.kernel.org/r/20240217010557.2381548-8-sboyd@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>

drivers/of/.kunitconfig

@@ -0,0 +1,3 @@
+CONFIG_KUNIT=y
+CONFIG_OF=y
+CONFIG_OF_KUNIT_TEST=y

drivers/of/Kconfig

@@ -36,6 +36,15 @@ config OF_UNITTEST
 
 	  If unsure, say N here. This option is not safe to enable.
 
+config OF_KUNIT_TEST
+	tristate "Devicetree KUnit Test" if !KUNIT_ALL_TESTS
+	depends on KUNIT
+	default KUNIT_ALL_TESTS
+	help
+	  This option builds KUnit unit tests for device tree infrastructure.
+
+	  If unsure, say N here, but this option is safe to enable.
+
 config OF_ALL_DTBS
 	bool "Build all Device Tree Blobs"
 	depends on COMPILE_TEST

drivers/of/Makefile

@@ -19,4 +19,6 @@ obj-y	+= kexec.o
 endif
 endif
 
+obj-$(CONFIG_OF_KUNIT_TEST) += of_test.o
+
 obj-$(CONFIG_OF_UNITTEST) += unittest-data/

drivers/of/of_test.c

@@ -0,0 +1,57 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * KUnit tests for OF APIs
+ */
+#include <linux/module.h>
+#include <linux/of.h>
+
+#include <kunit/test.h>
+
+/*
+ * Test that the root node "/" can be found by path.
+ */
+static void of_dtb_root_node_found_by_path(struct kunit *test)
+{
+	struct device_node *np;
+
+	np = of_find_node_by_path("/");
+	KUNIT_EXPECT_NOT_ERR_OR_NULL(test, np);
+	of_node_put(np);
+}
+
+/*
+ * Test that the 'of_root' global variable is always populated when DT code is
+ * enabled. Remove this test once of_root is removed from global access.
+ */
+static void of_dtb_root_node_populates_of_root(struct kunit *test)
+{
+	KUNIT_EXPECT_NOT_ERR_OR_NULL(test, of_root);
+}
+
+static struct kunit_case of_dtb_test_cases[] = {
+	KUNIT_CASE(of_dtb_root_node_found_by_path),
+	KUNIT_CASE(of_dtb_root_node_populates_of_root),
+	{}
+};
+
+static int of_dtb_test_init(struct kunit *test)
+{
+	if (!IS_ENABLED(CONFIG_OF_EARLY_FLATTREE))
+		kunit_skip(test, "requires CONFIG_OF_EARLY_FLATTREE");
+
+	return 0;
+}
+
+/*
+ * Test suite to confirm a DTB is loaded.
+ */
+static struct kunit_suite of_dtb_suite = {
+	.name = "of_dtb",
+	.test_cases = of_dtb_test_cases,
+	.init = of_dtb_test_init,
+};
+
+kunit_test_suites(
+	&of_dtb_suite,
+);
+MODULE_LICENSE("GPL");