drm/doc: Clarify mode_fixup vs. atomic_check a bit more

Brought up by both Laurent and Andrzej when reviewing the new
->mode_valid hooks. Since mode_fixup is just a simpler version of the
much more generic atomic_check we can't really unify it with
mode_valid. Most drivers should probably switch their current
mode_fixup code to either the new mode_valid or the atomic_check
hooks, but e.g. that doesn't exist yet for bridges, and for CRTCs the
situation is a bit more complicated. Hence there's no clear
equivalence between mode_fixup and mode_valid, even if it looks like
that at first glance.

v2: Fix accidental double-dot (Adnrzej).

Cc: Andrzej Hajda <a.hajda@samsung.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: Jose Abreu <Jose.Abreu@synopsys.com>
Reviewed-by: Jose Abreu <joabreu@synopsys.com>
Reviewed-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170515091136.26307-2-daniel.vetter@ffwll.ch

include/drm/drm_modeset_helper_vtables.h

@@ -165,7 +165,11 @@ struct drm_crtc_helper_funcs {
 	 * allowed.
 	 *
 	 * Atomic drivers which need to inspect and adjust more state should
-	 * instead use the @atomic_check callback.
+	 * instead use the @atomic_check callback, but note that they're not
+	 * perfectly equivalent: @mode_valid is called from
+	 * drm_atomic_helper_check_modeset(), but @atomic_check is called from
+	 * drm_atomic_helper_check_planes(), because originally it was meant for
+	 * plane update checks only.
 	 *
 	 * Also beware that userspace can request its own custom modes, neither
 	 * core nor helpers filter modes to the list of probe modes reported by
@@ -547,7 +551,9 @@ struct drm_encoder_helper_funcs {
 	 * allowed.
 	 *
 	 * Atomic drivers which need to inspect and adjust more state should
-	 * instead use the @atomic_check callback.
+	 * instead use the @atomic_check callback. If @atomic_check is used,
+	 * this hook isn't called since @atomic_check allows a strict superset
+	 * of the functionality of @mode_fixup.
 	 *
 	 * Also beware that userspace can request its own custom modes, neither
 	 * core nor helpers filter modes to the list of probe modes reported by
@@ -734,6 +740,11 @@ struct drm_encoder_helper_funcs {
 	 * update the CRTC to match what the encoder needs for the requested
 	 * connector.
 	 *
+	 * Since this provides a strict superset of the functionality of
+	 * @mode_fixup (the requested and adjusted modes are both available
+	 * through the passed in &struct drm_crtc_state) @mode_fixup is not
+	 * called when @atomic_check is implemented.
+	 *
 	 * This function is used by the atomic helpers, but it is optional.
 	 *
 	 * NOTE: