drm: require a non_NULL drm_crtc.primary

If a CRTC is missing a legacy primary plane pointer, a lot of things
will be broken for user-space: fbdev stops working and the entire legacy
uAPI stops working.

Require all drivers to populate drm_crtc.primary to prevent these
issues. Warn if it's NULL.
Signed-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/20201211184634.74534-3-contact@emersion.fr

drivers/gpu/drm/drm_mode_config.c

@@ -639,6 +639,9 @@ void drm_mode_config_validate(struct drm_device *dev)
 	}
 
 	drm_for_each_crtc(crtc, dev) {
+		WARN(!crtc->primary, "Missing primary plane on [CRTC:%d:%s]\n",
+		     crtc->base.id, crtc->name);
+
 		if (crtc->primary) {
 			WARN(!(crtc->primary->possible_crtcs & drm_crtc_mask(crtc)),
 			     "Bogus primary plane possible_crtcs: [PLANE:%d:%s] must be compatible with [CRTC:%d:%s]\n",

drivers/gpu/drm/drm_plane.c

@@ -57,7 +57,7 @@
  * 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
+ * drivers must provide one primary plane per CRTC to avoid surprising legacy
  * userspace too much.
  */