drm/atomic: Refcounting for plane_state->fb

So my original plan was that the drm core refcounts framebuffers like
with the legacy ioctls. But that doesn't work for a bunch of reasons:

- State objects might live longer than until the next fb change
  happens for a plane. For example delayed cleanup work only happens
  _after_ the pageflip ioctl has completed. So this definitely doesn't
  work without the plane state holding its own references.

- The other issue is transition from legacy to atomic implementations,
  where the driver works under a mix of both worlds. Which means
  legacy paths might not properly update the ->fb pointer under
  plane->state->fb. Which is a bit a problem when then someone comes
  around and _does_ try to clean it up when it's long gone.

The second issue is just a bit a transition bug, since drivers should
update plane->state->fb in all the paths that aren't converted yet.
But a bit more robustness for the transition can't hurt - we pull
similar tricks with cleaning up the old fb in the transitional helpers
already.

The pattern for drivers that transition is

	if (plane->state)
		drm_atomic_set_fb_for_plane(plane->state, plane->fb);

inserted after the fb update has logically completed at the end of
->set_config (or ->set_base/mode_set if using the crtc helpers),
->page_flip, ->update_plane or any other entry point which updates
plane->fb.

v2: Update kerneldoc - copypasta fail.

v3: Fix spelling in the commit message (Sean).

Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>

drivers/gpu/drm/drm_atomic.c

@@ -367,6 +367,34 @@ drm_atomic_set_crtc_for_plane(struct drm_plane_state *plane_state,
 }
 EXPORT_SYMBOL(drm_atomic_set_crtc_for_plane);
 
+/**
+ * drm_atomic_set_fb_for_plane - set crtc for plane
+ * @plane_state: atomic state object for the plane
+ * @fb: fb to use for the plane
+ *
+ * Changing the assigned framebuffer for a plane requires us to grab a reference
+ * to the new fb and drop the reference to the old fb, if there is one. This
+ * function takes care of all these details besides updating the pointer in the
+ * state object itself.
+ */
+void
+drm_atomic_set_fb_for_plane(struct drm_plane_state *plane_state,
+			    struct drm_framebuffer *fb)
+{
+	if (plane_state->fb)
+		drm_framebuffer_unreference(plane_state->fb);
+	if (fb)
+		drm_framebuffer_reference(fb);
+	plane_state->fb = fb;
+
+	if (fb)
+		DRM_DEBUG_KMS("Set [FB:%d] for plane state %p\n",
+			      fb->base.id, plane_state);
+	else
+		DRM_DEBUG_KMS("Set [NOFB] for plane state %p\n", plane_state);
+}
+EXPORT_SYMBOL(drm_atomic_set_fb_for_plane);
+
 /**
  * drm_atomic_set_crtc_for_connector - set crtc for connector
  * @conn_state: atomic state object for the connector

drivers/gpu/drm/drm_atomic_helper.c

@@ -1182,7 +1182,7 @@ int drm_atomic_helper_update_plane(struct drm_plane *plane,
 	ret = drm_atomic_set_crtc_for_plane(plane_state, crtc);
 	if (ret != 0)
 		goto fail;
-	plane_state->fb = fb;
+	drm_atomic_set_fb_for_plane(plane_state, fb);
 	plane_state->crtc_x = crtc_x;
 	plane_state->crtc_y = crtc_y;
 	plane_state->crtc_h = crtc_h;
@@ -1250,7 +1250,7 @@ int drm_atomic_helper_disable_plane(struct drm_plane *plane)
 	ret = drm_atomic_set_crtc_for_plane(plane_state, NULL);
 	if (ret != 0)
 		goto fail;
-	plane_state->fb = NULL;
+	drm_atomic_set_fb_for_plane(plane_state, NULL);
 	plane_state->crtc_x = 0;
 	plane_state->crtc_y = 0;
 	plane_state->crtc_h = 0;
@@ -1422,7 +1422,7 @@ int drm_atomic_helper_set_config(struct drm_mode_set *set)
 	ret = drm_atomic_set_crtc_for_plane(primary_state, crtc);
 	if (ret != 0)
 		goto fail;
-	primary_state->fb = set->fb;
+	drm_atomic_set_fb_for_plane(primary_state, set->fb);
 	primary_state->crtc_x = 0;
 	primary_state->crtc_y = 0;
 	primary_state->crtc_h = set->mode->vdisplay;
@@ -1694,7 +1694,7 @@ int drm_atomic_helper_page_flip(struct drm_crtc *crtc,
 	ret = drm_atomic_set_crtc_for_plane(plane_state, crtc);
 	if (ret != 0)
 		goto fail;
-	plane_state->fb = fb;
+	drm_atomic_set_fb_for_plane(plane_state, fb);
 
 	ret = drm_atomic_async_commit(state);
 	if (ret != 0)
@@ -1808,6 +1808,9 @@ EXPORT_SYMBOL(drm_atomic_helper_crtc_destroy_state);
  */
 void drm_atomic_helper_plane_reset(struct drm_plane *plane)
 {
+	if (plane->state && plane->state->fb)
+		drm_framebuffer_unreference(plane->state->fb);
+
 	kfree(plane->state);
 	plane->state = kzalloc(sizeof(*plane->state), GFP_KERNEL);
 }
@@ -1823,10 +1826,17 @@ EXPORT_SYMBOL(drm_atomic_helper_plane_reset);
 struct drm_plane_state *
 drm_atomic_helper_plane_duplicate_state(struct drm_plane *plane)
 {
+	struct drm_plane_state *state;
+
 	if (WARN_ON(!plane->state))
 		return NULL;
 
-	return kmemdup(plane->state, sizeof(*plane->state), GFP_KERNEL);
+	state = kmemdup(plane->state, sizeof(*plane->state), GFP_KERNEL);
+
+	if (state && state->fb)
+		drm_framebuffer_reference(state->fb);
+
+	return state;
 }
 EXPORT_SYMBOL(drm_atomic_helper_plane_duplicate_state);
 
@@ -1839,8 +1849,11 @@ EXPORT_SYMBOL(drm_atomic_helper_plane_duplicate_state);
  * subclassed plane state structure.
  */
 void drm_atomic_helper_plane_destroy_state(struct drm_plane *plane,
-					  struct drm_plane_state *state)
+					   struct drm_plane_state *state)
 {
+	if (state->fb)
+		drm_framebuffer_unreference(state->fb);
+
 	kfree(state);
 }
 EXPORT_SYMBOL(drm_atomic_helper_plane_destroy_state);

drivers/gpu/drm/drm_crtc_helper.c

@@ -34,11 +34,13 @@
 #include <linux/moduleparam.h>
 
 #include <drm/drmP.h>
+#include <drm/drm_atomic.h>
 #include <drm/drm_crtc.h>
 #include <drm/drm_fourcc.h>
 #include <drm/drm_crtc_helper.h>
 #include <drm/drm_fb_helper.h>
 #include <drm/drm_plane_helper.h>
+#include <drm/drm_atomic_helper.h>
 #include <drm/drm_edid.h>
 
 /**
@@ -998,15 +1000,14 @@ int drm_helper_crtc_mode_set_base(struct drm_crtc *crtc, int x, int y,
 	if (plane->funcs->atomic_duplicate_state)
 		plane_state = plane->funcs->atomic_duplicate_state(plane);
 	else if (plane->state)
-		plane_state = kmemdup(plane->state, sizeof(*plane_state),
-				      GFP_KERNEL);
+		plane_state = drm_atomic_helper_plane_duplicate_state(plane);
 	else
 		plane_state = kzalloc(sizeof(*plane_state), GFP_KERNEL);
 	if (!plane_state)
 		return -ENOMEM;
 
 	plane_state->crtc = crtc;
-	plane_state->fb = crtc->primary->fb;
+	drm_atomic_set_fb_for_plane(plane_state, crtc->primary->fb);
 	plane_state->crtc_x = 0;
 	plane_state->crtc_y = 0;
 	plane_state->crtc_h = crtc->mode.vdisplay;

drivers/gpu/drm/drm_plane_helper.c

@@ -27,7 +27,9 @@
 #include <drm/drmP.h>
 #include <drm/drm_plane_helper.h>
 #include <drm/drm_rect.h>
+#include <drm/drm_atomic.h>
 #include <drm/drm_crtc_helper.h>
+#include <drm/drm_atomic_helper.h>
 
 #define SUBPIXEL_MASK 0xffff
 
@@ -464,7 +466,7 @@ int drm_plane_helper_commit(struct drm_plane *plane,
 		if (plane->funcs->atomic_destroy_state)
 			plane->funcs->atomic_destroy_state(plane, plane_state);
 		else
-			kfree(plane_state);
+			drm_atomic_helper_plane_destroy_state(plane, plane_state);
 	}
 
 	return ret;
@@ -505,15 +507,14 @@ int drm_plane_helper_update(struct drm_plane *plane, struct drm_crtc *crtc,
 	if (plane->funcs->atomic_duplicate_state)
 		plane_state = plane->funcs->atomic_duplicate_state(plane);
 	else if (plane->state)
-		plane_state = kmemdup(plane->state, sizeof(*plane_state),
-				      GFP_KERNEL);
+		plane_state = drm_atomic_helper_plane_duplicate_state(plane);
 	else
 		plane_state = kzalloc(sizeof(*plane_state), GFP_KERNEL);
 	if (!plane_state)
 		return -ENOMEM;
 
 	plane_state->crtc = crtc;
-	plane_state->fb = fb;
+	drm_atomic_set_fb_for_plane(plane_state, fb);
 	plane_state->crtc_x = crtc_x;
 	plane_state->crtc_y = crtc_y;
 	plane_state->crtc_h = crtc_h;
@@ -552,15 +553,14 @@ int drm_plane_helper_disable(struct drm_plane *plane)
 	if (plane->funcs->atomic_duplicate_state)
 		plane_state = plane->funcs->atomic_duplicate_state(plane);
 	else if (plane->state)
-		plane_state = kmemdup(plane->state, sizeof(*plane_state),
-				      GFP_KERNEL);
+		plane_state = drm_atomic_helper_plane_duplicate_state(plane);
 	else
 		plane_state = kzalloc(sizeof(*plane_state), GFP_KERNEL);
 	if (!plane_state)
 		return -ENOMEM;
 
 	plane_state->crtc = NULL;
-	plane_state->fb = NULL;
+	drm_atomic_set_fb_for_plane(plane_state, NULL);
 
 	return drm_plane_helper_commit(plane, plane_state, plane->fb);
 }

include/drm/drm_atomic.h

@@ -46,6 +46,8 @@ drm_atomic_get_connector_state(struct drm_atomic_state *state,
 int __must_check
 drm_atomic_set_crtc_for_plane(struct drm_plane_state *plane_state,
 			      struct drm_crtc *crtc);
+void drm_atomic_set_fb_for_plane(struct drm_plane_state *plane_state,
+				 struct drm_framebuffer *fb);
 int __must_check
 drm_atomic_set_crtc_for_connector(struct drm_connector_state *conn_state,
 				  struct drm_crtc *crtc);