[PATCH] drm: Simplify fb refcounting rules around ->update_plane

[Daniel Vetter]

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 their
  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.

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

Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/drm_crtc.c         | 12 +++++-------
 drivers/gpu/drm/drm_plane_helper.c | 15 ---------------
 2 files changed, 5 insertions(+), 22 deletions(-)

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index d8b7099abece..966b480ed543 100644
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/drivers/gpu/drm/drm_crtc.c
@@ -2187,24 +2187,24 @@ 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;
 	}
 	drm_modeset_unlock_all(dev);
 
 out:
-	if (fb)
-		drm_framebuffer_unreference(fb);
+	if (plane->fb)
+		drm_framebuffer_reference(old_fb);
 	if (old_fb)
 		drm_framebuffer_unreference(old_fb);
+	drm_framebuffer_unreference(fb);
 
 	return ret;
 }
@@ -2239,9 +2239,7 @@ 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) {
diff --git a/drivers/gpu/drm/drm_plane_helper.c b/drivers/gpu/drm/drm_plane_helper.c
index e768d35ff22e..8db129c44fea 100644
--- a/drivers/gpu/drm/drm_plane_helper.c
+++ b/drivers/gpu/drm/drm_plane_helper.c
@@ -175,22 +175,7 @@ int drm_primary_helper_update(struct drm_plane *plane, struct drm_crtc *crtc,
 	set.connectors = connector_list;
 	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
-	 * 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;
-- 
1.9.2