drm/dp: Add drm_dp_downstream_mode()

The downstream facing port caps in the DPCD can give us a hint
as to what kind of display mode the sink can use if it doesn't
have an EDID. Use that information to pick a suitable mode.

v2: Use Returns: for kdoc (Lyude)
    Add kdocs for drm_display_mode_from_cea_vic() (Lyude)

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200904115354.25336-14-ville.syrjala@linux.intel.com
Reviewed-by: Lyude Paul <lyude@redhat.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/drm_edid.c b/drivers/gpu/drm/drm_edid.c
index 6840f05..a82f37d 100644
--- a/drivers/gpu/drm/drm_edid.c
+++ b/drivers/gpu/drm/drm_edid.c
@@ -3738,6 +3738,34 @@ drm_add_cmdb_modes(struct drm_connector *connector, u8 svd)
 	bitmap_set(hdmi->y420_cmdb_modes, vic, 1);
 }
 
+/**
+ * drm_display_mode_from_cea_vic() - return a mode for CEA VIC
+ * @dev: DRM device
+ * @vic: CEA VIC of the mode
+ *
+ * Creates a new mode matching the specified CEA VIC.
+ *
+ * Returns: A new drm_display_mode on success or NULL on failure
+ */
+struct drm_display_mode *
+drm_display_mode_from_cea_vic(struct drm_device *dev,
+			      u8 video_code)
+{
+	const struct drm_display_mode *cea_mode;
+	struct drm_display_mode *newmode;
+
+	cea_mode = cea_mode_for_vic(video_code);
+	if (!cea_mode)
+		return NULL;
+
+	newmode = drm_mode_duplicate(dev, cea_mode);
+	if (!newmode)
+		return NULL;
+
+	return newmode;
+}
+EXPORT_SYMBOL(drm_display_mode_from_cea_vic);
+
 static int
 do_cea_modes(struct drm_connector *connector, const u8 *db, u8 len)
 {