@@ -77,6 +77,23 @@ static struct drm_display_mode *find_420_only_mode(struct drm_connector *connect
 	return NULL;
 }
 
+static struct drm_display_mode *find_420_also_mode(struct drm_connector *connector)
+{
+	struct drm_device *drm = connector->dev;
+	struct drm_display_mode *mode;
+
+	mutex_lock(&drm->mode_config.mutex);
+	list_for_each_entry(mode, &connector->modes, head) {
+		if (drm_mode_is_420_also(&connector->display_info, mode)) {
+			mutex_unlock(&drm->mode_config.mutex);
+			return mode;
+		}
+	}
+	mutex_unlock(&drm->mode_config.mutex);
+
+	return NULL;
+}
+
 static int set_connector_edid(struct kunit *test, struct drm_connector *connector,
 			      const void *edid, size_t edid_len)
 {

@@ -2700,11 +2717,103 @@ static void drm_test_check_mode_valid_reject_max_clock(struct kunit *test)
 	KUNIT_EXPECT_EQ(test, preferred->clock, 25200);
 }
 
+/*
+ * Test that drm_hdmi_connector_mode_valid() will accept modes that require a
+ * 4:2:0 chroma subsampling, even if said mode would violate maximum clock
+ * constraints if it used RGB 4:4:4.
+ */
+static void drm_test_check_mode_valid_yuv420_only_max_clock(struct kunit *test)
+{
+	struct drm_atomic_helper_connector_hdmi_priv *priv;
+	struct drm_display_mode *dank;
+	struct drm_connector *conn;
+
+	priv = drm_kunit_helper_connector_hdmi_init_with_edid_funcs(test,
+				BIT(HDMI_COLORSPACE_RGB) |
+				BIT(HDMI_COLORSPACE_YUV420),
+				8,
+				&dummy_connector_hdmi_funcs,
+				test_edid_hdmi_1080p_rgb_yuv_4k_yuv420_dc_max_200mhz);
+	KUNIT_ASSERT_NOT_NULL(test, priv);
+
+	conn = &priv->connector;
+	KUNIT_ASSERT_EQ(test, conn->display_info.max_tmds_clock, 200 * 1000);
+
+	dank = find_420_only_mode(conn);
+	KUNIT_ASSERT_NOT_NULL(test, dank);
+	KUNIT_EXPECT_EQ(test, dank->hdisplay, 3840);
+	KUNIT_EXPECT_EQ(test, dank->vdisplay, 2160);
+
+	/*
+	 * Note: The mode's "clock" here is not accurate to the actual TMDS
+	 * clock that HDMI will use for a subsampled mode. Hence, why the mode's
+	 * clock is above the .max_tmds_clock of 200MHz.
+	 */
+	KUNIT_EXPECT_EQ(test, dank->clock, 297000);
+}
+
+/*
+ * Test that drm_hdmi_connector_mode_valid() will reject modes that require
+ * 4:2:0 chroma subsampling, if the connector does not support 4:2:0.
+ */
+static void
+drm_test_check_mode_valid_reject_yuv420_only_connector(struct kunit *test)
+{
+	struct drm_atomic_helper_connector_hdmi_priv *priv;
+	struct drm_display_mode *dank;
+	struct drm_connector *conn;
+
+	priv = drm_kunit_helper_connector_hdmi_init_with_edid_funcs(test,
+				BIT(HDMI_COLORSPACE_RGB),
+				8,
+				&dummy_connector_hdmi_funcs,
+				test_edid_hdmi_1080p_rgb_yuv_4k_yuv420_dc_max_200mhz);
+	KUNIT_ASSERT_NOT_NULL(test, priv);
+
+	conn = &priv->connector;
+	KUNIT_ASSERT_EQ(test, conn->display_info.max_tmds_clock, 200 * 1000);
+
+	dank = find_420_only_mode(conn);
+	KUNIT_EXPECT_NULL(test, dank);
+}
+
+/*
+ * Test that drm_hdmi_connector_mode_valid() will accept modes that allow (among
+ * other color formats) 4:2:0 chroma subsampling, even if the connector does not
+ * support 4:2:0, but the mode's clock works for RGB 4:4:4.
+ */
+static void
+drm_test_check_mode_valid_accept_yuv420_also_connector_rgb(struct kunit *test)
+{
+	struct drm_atomic_helper_connector_hdmi_priv *priv;
+	struct drm_display_mode *mode;
+	struct drm_connector *conn;
+
+	priv = drm_kunit_helper_connector_hdmi_init_with_edid_funcs(test,
+				BIT(HDMI_COLORSPACE_RGB),
+				8,
+				&dummy_connector_hdmi_funcs,
+				test_edid_hdmi_4k_rgb_yuv420_dc_max_340mhz);
+	KUNIT_ASSERT_NOT_NULL(test, priv);
+
+	conn = &priv->connector;
+	KUNIT_ASSERT_EQ(test, conn->display_info.max_tmds_clock, 340 * 1000);
+
+	mode = find_420_also_mode(conn);
+	KUNIT_ASSERT_NOT_NULL(test, mode);
+	KUNIT_EXPECT_EQ(test, mode->hdisplay, 3840);
+	KUNIT_EXPECT_EQ(test, mode->vdisplay, 2160);
+	KUNIT_EXPECT_EQ(test, mode->clock, 297000);
+}
+
 static struct kunit_case drm_atomic_helper_connector_hdmi_mode_valid_tests[] = {
 	KUNIT_CASE(drm_test_check_mode_valid),
 	KUNIT_CASE(drm_test_check_mode_valid_reject),
 	KUNIT_CASE(drm_test_check_mode_valid_reject_rate),
 	KUNIT_CASE(drm_test_check_mode_valid_reject_max_clock),
+	KUNIT_CASE(drm_test_check_mode_valid_yuv420_only_max_clock),
+	KUNIT_CASE(drm_test_check_mode_valid_reject_yuv420_only_connector),
+	KUNIT_CASE(drm_test_check_mode_valid_accept_yuv420_also_connector_rgb),
 	{ }
 };