drm: require each CRTC to have a unique primary plane

User-space expects to be able to pick a primary plane for each CRTC
exposed by the driver. Make sure this assumption holds in
drm_mode_config_validate.

Use the legacy drm_crtc.primary field to check this, because it's
simpler and we require drivers to set it anyways. Accumulate a set of
primary planes which are already used for a CRTC in a bitmask. Error out
if a primary plane is re-used.

v2: new patch

v3:
- Use u64 instead of __u64 (Jani)
- Use `unsigned int` instead of `unsigned` (Jani)

v4:
- Use u32 instead of u64 for plane mask (Ville)
- Use drm_plane_mask instead of BIT (Ville)
- Fix typos (Ville)
Signed-off-by: Simon Ser <contact@emersion.fr>
Reviewed-by: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Ville Syrjala <ville.syrjala@linux.intel.com>
Cc: Pekka Paalanen <ppaalanen@gmail.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20201211184634.74534-4-contact@emersion.fr

drivers/gpu/drm/drm_mode_config.c

@@ -626,6 +626,9 @@ void drm_mode_config_validate(struct drm_device *dev)
 {
 	struct drm_encoder *encoder;
 	struct drm_crtc *crtc;
+	struct drm_plane *plane;
+	u32 primary_with_crtc = 0, cursor_with_crtc = 0;
+	unsigned int num_primary = 0;
 
 	if (!drm_core_check_feature(dev, DRIVER_MODESET))
 		return;
@@ -647,12 +650,29 @@ void drm_mode_config_validate(struct drm_device *dev)
 			     "Bogus primary plane possible_crtcs: [PLANE:%d:%s] must be compatible with [CRTC:%d:%s]\n",
 			     crtc->primary->base.id, crtc->primary->name,
 			     crtc->base.id, crtc->name);
+			WARN(primary_with_crtc & drm_plane_mask(crtc->primary),
+			     "Primary plane [PLANE:%d:%s] used for multiple CRTCs",
+			     crtc->primary->base.id, crtc->primary->name);
+			primary_with_crtc |= drm_plane_mask(crtc->primary);
 		}
 		if (crtc->cursor) {
 			WARN(!(crtc->cursor->possible_crtcs & drm_crtc_mask(crtc)),
 			     "Bogus cursor plane possible_crtcs: [PLANE:%d:%s] must be compatible with [CRTC:%d:%s]\n",
 			     crtc->cursor->base.id, crtc->cursor->name,
 			     crtc->base.id, crtc->name);
+			WARN(cursor_with_crtc & drm_plane_mask(crtc->cursor),
+			     "Cursor plane [PLANE:%d:%s] used for multiple CRTCs",
+			     crtc->cursor->base.id, crtc->cursor->name);
+			cursor_with_crtc |= drm_plane_mask(crtc->cursor);
 		}
 	}
+
+	drm_for_each_plane(plane, dev) {
+		if (plane->type == DRM_PLANE_TYPE_PRIMARY)
+			num_primary++;
+	}
+
+	WARN(num_primary != dev->mode_config.num_crtc,
+	     "Must have as many primary planes as there are CRTCs, but have %u primary planes and %u CRTCs",
+	     num_primary, dev->mode_config.num_crtc);
 }

drivers/gpu/drm/drm_plane.c

@@ -54,6 +54,12 @@
  * enum drm_plane_type). A plane can be compatible with multiple CRTCs, see
  * &drm_plane.possible_crtcs.
  *
+ * Each CRTC must have a unique primary plane userspace can attach to enable
+ * the CRTC. In other words, userspace must be able to attach a different
+ * primary plane to each CRTC at the same time. Primary planes can still be
+ * compatible with multiple CRTCs. There must be exactly as many primary planes
+ * as there are 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