drm: clarify adjusted_mode documentation for bridges

This patch clarifies the adjusted_mode documentation
for bridges.

Signed-off-by: Philippe Cornu <philippe.cornu@st.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Archit Taneja <architt@codeaurora.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180409152427.12449-1-philippe.cornu@st.com

include/drm/drm_bridge.h

@@ -178,6 +178,22 @@ struct drm_bridge_funcs {
 	 * then this would be &drm_encoder_helper_funcs.mode_set. The display
 	 * pipe (i.e.  clocks and timing signals) is off when this function is
 	 * called.
+	 *
+	 * The adjusted_mode parameter is the mode output by the CRTC for the
+	 * first bridge in the chain. It can be different from the mode
+	 * parameter that contains the desired mode for the connector at the end
+	 * of the bridges chain, for instance when the first bridge in the chain
+	 * performs scaling. The adjusted mode is mostly useful for the first
+	 * bridge in the chain and is likely irrelevant for the other bridges.
+	 *
+	 * For atomic drivers the adjusted_mode is the mode stored in
+	 * &drm_crtc_state.adjusted_mode.
+	 *
+	 * NOTE:
+	 *
+	 * If a need arises to store and access modes adjusted for other
+	 * locations than the connection between the CRTC and the first bridge,
+	 * the DRM framework will have to be extended with DRM bridge states.
 	 */
 	void (*mode_set)(struct drm_bridge *bridge,
 			 struct drm_display_mode *mode,

include/drm/drm_crtc.h

@@ -134,10 +134,13 @@ struct drm_crtc_state {
 	 *
 	 * Internal display timings which can be used by the driver to handle
 	 * differences between the mode requested by userspace in @mode and what
-	 * is actually programmed into the hardware. It is purely driver
-	 * implementation defined what exactly this adjusted mode means. Usually
-	 * it is used to store the hardware display timings used between the
-	 * CRTC and encoder blocks.
+	 * is actually programmed into the hardware.
+	 *
+	 * For drivers using drm_bridge, this stores hardware display timings
+	 * used between the CRTC and the first bridge. For other drivers, the
+	 * meaning of the adjusted_mode field is purely driver implementation
+	 * defined information, and will usually be used to store the hardware
+	 * display timings used between the CRTC and encoder blocks.
 	 */
 	struct drm_display_mode adjusted_mode;