drm: Simplify fb refcounting rules around ->update_plane

The introduction of primary planes has apparently caused a bit of fb
refcounting fun for people. That makes it a good time to clean up the
arcane rules and slight differences between ->update_plane and
->set_config. The new rules are:

- The core holds a reference for both the new and the old fb (if
  they're non-NULL of course) while calling into the driver through
  either ->update_plane or ->set_config.

- Drivers may not clobber plane->fb if their callback fails. If they
  do that, they need to store a pointer to the old fb in it again.
  When calling into the driver plane->fb still points at the current
  (old) framebuffer.

- The core will update the plane->fb pointer on success. Drivers can
  do that themselves too, but aren't required to any more for the
  primary plane.

- The core will update fb refcounts for the plane->fb pointer,
  presuming the drivers hold up their end of the bargain.

v2: Remove now unused tmpfb (Thierry)

v3: Drop broken changes from drm_mode_setplane (Ville). Also polish
the commit message a bit.

v4: Also fix up the handling of ->disable_plane in
drm_plane_force_disable. The issue was that we didn't save plane->fb
over the ->disable_plane call. Just paranoia, nothing relies on this.

v5: Keep still useful comments about directly calling ->set_config,
which I should have done for v4 already. Requested by Matt.

Cc: Thierry Reding <treding@nvidia.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/drm_crtc.c

@@ -1145,16 +1145,17 @@ EXPORT_SYMBOL(drm_plane_cleanup);
  */
 void drm_plane_force_disable(struct drm_plane *plane)
 {
+	struct drm_framebuffer *old_fb = plane->fb;
 	int ret;
 
-	if (!plane->fb)
+	if (!old_fb)
 		return;
 
 	ret = plane->funcs->disable_plane(plane);
 	if (ret)
 		DRM_ERROR("failed to disable plane with busy fb\n");
 	/* disconnect the plane from the fb and crtc: */
-	__drm_framebuffer_unreference(plane->fb);
+	__drm_framebuffer_unreference(old_fb);
 	plane->fb = NULL;
 	plane->crtc = NULL;
 }
@@ -2187,16 +2188,18 @@ int drm_mode_setplane(struct drm_device *dev, void *data,
 	}
 
 	drm_modeset_lock_all(dev);
+	old_fb = plane->fb;
 	ret = plane->funcs->update_plane(plane, crtc, fb,
 					 plane_req->crtc_x, plane_req->crtc_y,
 					 plane_req->crtc_w, plane_req->crtc_h,
 					 plane_req->src_x, plane_req->src_y,
 					 plane_req->src_w, plane_req->src_h);
 	if (!ret) {
-		old_fb = plane->fb;
 		plane->crtc = crtc;
 		plane->fb = fb;
 		fb = NULL;
+	} else {
+		old_fb = NULL;
 	}
 	drm_modeset_unlock_all(dev);
 
@@ -2239,9 +2242,7 @@ int drm_mode_set_config_internal(struct drm_mode_set *set)
 	ret = crtc->funcs->set_config(set);
 	if (ret == 0) {
 		crtc->primary->crtc = crtc;
-
-		/* crtc->fb must be updated by ->set_config, enforces this. */
-		WARN_ON(fb != crtc->primary->fb);
+		crtc->primary->fb = fb;
 	}
 
 	list_for_each_entry(tmp, &crtc->dev->mode_config.crtc_list, head) {

drivers/gpu/drm/drm_plane_helper.c

@@ -124,7 +124,6 @@ int drm_primary_helper_update(struct drm_plane *plane, struct drm_crtc *crtc,
 		.y2 = crtc->mode.vdisplay,
 	};
 	struct drm_connector **connector_list;
-	struct drm_framebuffer *tmpfb;
 	int num_connectors, ret;
 
 	if (!crtc->enabled) {
@@ -178,21 +177,14 @@ int drm_primary_helper_update(struct drm_plane *plane, struct drm_crtc *crtc,
 	set.num_connectors = num_connectors;
 
 	/*
-	 * set_config() adjusts crtc->primary->fb; however the DRM setplane
-	 * code that called us expects to handle the framebuffer update and
-	 * reference counting; save and restore the current fb before
-	 * calling it.
-	 *
-	 * N.B., we call set_config() directly here rather than using
+	 * We call set_config() directly here rather than using
 	 * drm_mode_set_config_internal.  We're reprogramming the same
 	 * connectors that were already in use, so we shouldn't need the extra
 	 * cross-CRTC fb refcounting to accomodate stealing connectors.
 	 * drm_mode_setplane() already handles the basic refcounting for the
 	 * framebuffers involved in this operation.
 	 */
-	tmpfb = plane->fb;
 	ret = crtc->funcs->set_config(&set);
-	plane->fb = tmpfb;
 
 	kfree(connector_list);
 	return ret;