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>

diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
index d8b7099..f6633cb 100644 (file)
--- a/drivers/gpu/drm/drm_crtc.c
+++ b/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) {

diff --git a/drivers/gpu/drm/drm_plane_helper.c b/drivers/gpu/drm/drm_plane_helper.c
index 9540ff9..d966afa 100644 (file)
--- a/drivers/gpu/drm/drm_plane_helper.c
+++ b/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;