drm: rework description of primary and cursor planes

The previous wording could be understood by user-space evelopers as "a
primary/cursor plane is only compatible with a single CRTC" [1].

Reword the planes description to make it clear the DRM-internal
drm_crtc.primary and drm_crtc.cursor planes are for legacy uAPI.

[1]: https://github.com/swaywm/wlroots/pull/2333#discussion_r456788057Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Acked-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201211184634.74534-1-contact@emersion.fr

drivers/gpu/drm/drm_crtc.c

@@ -256,6 +256,9 @@ struct dma_fence *drm_crtc_create_fence(struct drm_crtc *crtc)
  * planes). For really simple hardware which has only 1 plane look at
  * drm_simple_display_pipe_init() instead.
  *
+ * The @primary and @cursor planes are only relevant for legacy uAPI, see
+ * &drm_crtc.primary and &drm_crtc.cursor.
+ *
  * Returns:
  * Zero on success, error code on failure.
  */

drivers/gpu/drm/drm_plane.c

@@ -49,14 +49,16 @@
  * &struct drm_plane (possibly as part of a larger structure) and registers it
  * with a call to drm_universal_plane_init().
  *
- * Cursor and overlay planes are optional. All drivers should provide one
- * primary plane per CRTC to avoid surprising userspace too much. See enum
- * drm_plane_type for a more in-depth discussion of these special uapi-relevant
- * plane types. Special planes are associated with their CRTC by calling
- * drm_crtc_init_with_planes().
- *
  * The type of a plane is exposed in the immutable "type" enumeration property,
- * which has one of the following values: "Overlay", "Primary", "Cursor".
+ * which has one of the following values: "Overlay", "Primary", "Cursor" (see
+ * enum drm_plane_type). A plane can be compatible with multiple CRTCs, see
+ * &drm_plane.possible_crtcs.
+ *
+ * Legacy uAPI doesn't expose the primary and cursor planes directly. DRM core
+ * relies on the driver to set the primary and optionally the cursor plane used
+ * for legacy IOCTLs. This is done by calling drm_crtc_init_with_planes(). All
+ * drivers should provide one primary plane per CRTC to avoid surprising legacy
+ * userspace too much.
  */
 
 static unsigned int drm_num_planes(struct drm_device *dev)