drm/panel: Add display timing support

Many panel data sheets, additionally to typical values, list allowed
ranges for timings such as hsync/vsync lengths, porches, and the pixel
clock rate. These can be stored in a struct display_timing, to be used
by an encoder mode_fixup callback to clamp user provided timing values
or to validate workarounds for clock source limitations.

This patch adds a new drm_panel_funcs callback that returns the panel's
available display_timing entries.
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>

include/drm/drm_panel.h

@@ -29,6 +29,7 @@
 struct drm_connector;
 struct drm_device;
 struct drm_panel;
+struct display_timing;
 
 /**
  * struct drm_panel_funcs - perform operations on a given panel
@@ -38,6 +39,8 @@ struct drm_panel;
  * @enable: enable panel (turn on back light, etc.)
  * @get_modes: add modes to the connector that the panel is attached to and
  * return the number of modes added
+ * @get_timings: copy display timings into the provided array and return
+ * the number of display timings available
  *
  * The .prepare() function is typically called before the display controller
  * starts to transmit video data. Panel drivers can use this to turn the panel
@@ -68,6 +71,8 @@ struct drm_panel_funcs {
 	int (*prepare)(struct drm_panel *panel);
 	int (*enable)(struct drm_panel *panel);
 	int (*get_modes)(struct drm_panel *panel);
+	int (*get_timings)(struct drm_panel *panel, unsigned int num_timings,
+			   struct display_timing *timings);
 };
 
 struct drm_panel {