drm: atomic: Clarify documentation around drm_atomic_crtc_needs_modeset

Add some additional comments to more explicitly describe the meaning and
usage of the three CRTC modeset detection booleans: mode_changed,
connectors_changed and active_changed.

Suggested-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Brian Starkey <brian.starkey@arm.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1476352028-16701-1-git-send-email-brian.starkey@arm.com

include/drm/drm_atomic.h

@@ -365,8 +365,17 @@ int __must_check drm_atomic_nonblocking_commit(struct drm_atomic_state *state);
  *
  * To give drivers flexibility struct &drm_crtc_state has 3 booleans to track
  * whether the state CRTC changed enough to need a full modeset cycle:
- * connectors_changed, mode_changed and active_change. This helper simply
+ * connectors_changed, mode_changed and active_changed. This helper simply
  * combines these three to compute the overall need for a modeset for @state.
+ *
+ * The atomic helper code sets these booleans, but drivers can and should
+ * change them appropriately to accurately represent whether a modeset is
+ * really needed. In general, drivers should avoid full modesets whenever
+ * possible.
+ *
+ * For example if the CRTC mode has changed, and the hardware is able to enact
+ * the requested mode change without going through a full modeset, the driver
+ * should clear mode_changed during its ->atomic_check.
  */
 static inline bool
 drm_atomic_crtc_needs_modeset(struct drm_crtc_state *state)

include/drm/drm_crtc.h

@@ -116,6 +116,11 @@ struct drm_plane_helper_funcs;
  * never return in a failure from the ->atomic_check callback. Userspace assumes
  * that a DPMS On will always succeed. In other words: @enable controls resource
  * assignment, @active controls the actual hardware state.
+ *
+ * The three booleans active_changed, connectors_changed and mode_changed are
+ * intended to indicate whether a full modeset is needed, rather than strictly
+ * describing what has changed in a commit.
+ * See also: drm_atomic_crtc_needs_modeset()
  */
 struct drm_crtc_state {
 	struct drm_crtc *crtc;