drm/tests: Add Kunit Helpers

As the number of kunit tests in KMS grows further, we start to have
multiple test suites that, for example, need to register a mock DRM
driver to interact with the KMS function they are supposed to test.

Let's add a file meant to provide those kind of helpers to avoid
duplication.

Reviewed-by: Noralf Trønnes <noralf@tronnes.org>
Tested-by: Mateusz Kwiatkowski <kfyatek+publicgit@gmail.com>
Link: https://lore.kernel.org/r/20220728-rpi-analog-tv-properties-v9-2-24b168e5bcd5@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>

drivers/gpu/drm/tests/Makefile

@@ -8,6 +8,7 @@ obj-$(CONFIG_DRM_KUNIT_TEST) += \
 	drm_format_helper_test.o \
 	drm_format_test.o \
 	drm_framebuffer_test.o \
+	drm_kunit_helpers.o \
 	drm_mm_test.o \
 	drm_plane_helper_test.o \
 	drm_rect_test.o

drivers/gpu/drm/tests/drm_kunit_helpers.c

@@ -0,0 +1,64 @@
+#include <drm/drm_drv.h>
+#include <drm/drm_managed.h>
+
+#include <kunit/resource.h>
+
+#include <linux/device.h>
+
+struct kunit_dev {
+	struct drm_device base;
+};
+
+static const struct drm_mode_config_funcs drm_mode_config_funcs = {
+};
+
+static int dev_init(struct kunit_resource *res, void *ptr)
+{
+	char *name = ptr;
+	struct device *dev;
+
+	dev = root_device_register(name);
+	if (IS_ERR(dev))
+		return PTR_ERR(dev);
+
+	res->data = dev;
+	return 0;
+}
+
+static void dev_free(struct kunit_resource *res)
+{
+	struct device *dev = res->data;
+
+	root_device_unregister(dev);
+}
+
+struct drm_device *drm_kunit_device_init(struct kunit *test, u32 features, char *name)
+{
+	struct kunit_dev *kdev;
+	struct drm_device *drm;
+	struct drm_driver *driver;
+	struct device *dev;
+	int ret;
+
+	dev = kunit_alloc_resource(test, dev_init, dev_free, GFP_KERNEL, name);
+	if (!dev)
+		return ERR_PTR(-ENOMEM);
+
+	driver = kunit_kzalloc(test, sizeof(*driver), GFP_KERNEL);
+	if (!driver)
+		return ERR_PTR(-ENOMEM);
+
+	driver->driver_features = features;
+	kdev = devm_drm_dev_alloc(dev, driver, struct kunit_dev, base);
+	if (IS_ERR(kdev))
+		return ERR_CAST(kdev);
+
+	drm = &kdev->base;
+	drm->mode_config.funcs = &drm_mode_config_funcs;
+
+	ret = drmm_mode_config_init(drm);
+	if (ret)
+		return ERR_PTR(ret);
+
+	return drm;
+}

drivers/gpu/drm/tests/drm_kunit_helpers.h

@@ -0,0 +1,9 @@
+#ifndef DRM_KUNIT_HELPERS_H_
+#define DRM_KUNIT_HELPERS_H_
+
+struct drm_device;
+struct kunit;
+
+struct drm_device *drm_kunit_device_init(struct kunit *test, u32 features, char *name);
+
+#endif // DRM_KUNIT_HELPERS_H_