drm/vc4: Make sure vc4_bo_{inc,dec}_usecnt() calls are balanced

Commit b9f19259 ("drm/vc4: Add the DRM_IOCTL_VC4_GEM_MADVISE ioctl")
introduced a mechanism to mark some BOs as purgeable to allow the driver
to drop them under memory pressure. In order to implement this feature
we had to add a mechanism to mark BOs as currently used by a piece of
hardware which materialized through the ->usecnt counter.

Plane code is supposed to increment usecnt when it attaches a BO to a
plane and decrement it when it's done with this BO, which was done in
the ->prepare_fb() and ->cleanup_fb() hooks. The problem is, async page
flip logic does not go through the regular atomic update path, and
->prepare_fb() and ->cleanup_fb() are not called in this case.

Fix that by manually calling vc4_bo_{inc,dec}_usecnt() in the
async-page-flip path.

Note that all this should go away as soon as we get generic async page
flip support in the core, in the meantime, this fix should do the
trick.

Fixes: b9f19259 ("drm/vc4: Add the DRM_IOCTL_VC4_GEM_MADVISE ioctl")
Reported-by: Peter Robinson <pbrobinson@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Link: https://patchwork.freedesktop.org/patch/msgid/20180430133232.32457-1-boris.brezillon@bootlin.com
Link: https://patchwork.freedesktop.org/patch/msgid/20180430133232.32457-1-boris.brezillon@bootlin.com

drivers/gpu/drm/vc4/vc4_crtc.c

@@ -760,6 +760,7 @@ static irqreturn_t vc4_crtc_irq_handler(int irq, void *data)
 struct vc4_async_flip_state {
 	struct drm_crtc *crtc;
 	struct drm_framebuffer *fb;
+	struct drm_framebuffer *old_fb;
 	struct drm_pending_vblank_event *event;
 
 	struct vc4_seqno_cb cb;
@@ -789,6 +790,23 @@ vc4_async_page_flip_complete(struct vc4_seqno_cb *cb)
 
 	drm_crtc_vblank_put(crtc);
 	drm_framebuffer_put(flip_state->fb);
+
+	/* Decrement the BO usecnt in order to keep the inc/dec calls balanced
+	 * when the planes are updated through the async update path.
+	 * FIXME: we should move to generic async-page-flip when it's
+	 * available, so that we can get rid of this hand-made cleanup_fb()
+	 * logic.
+	 */
+	if (flip_state->old_fb) {
+		struct drm_gem_cma_object *cma_bo;
+		struct vc4_bo *bo;
+
+		cma_bo = drm_fb_cma_get_gem_obj(flip_state->old_fb, 0);
+		bo = to_vc4_bo(&cma_bo->base);
+		vc4_bo_dec_usecnt(bo);
+		drm_framebuffer_put(flip_state->old_fb);
+	}
+
 	kfree(flip_state);
 
 	up(&vc4->async_modeset);
@@ -813,9 +831,22 @@ static int vc4_async_page_flip(struct drm_crtc *crtc,
 	struct drm_gem_cma_object *cma_bo = drm_fb_cma_get_gem_obj(fb, 0);
 	struct vc4_bo *bo = to_vc4_bo(&cma_bo->base);
 
+	/* Increment the BO usecnt here, so that we never end up with an
+	 * unbalanced number of vc4_bo_{dec,inc}_usecnt() calls when the
+	 * plane is later updated through the non-async path.
+	 * FIXME: we should move to generic async-page-flip when it's
+	 * available, so that we can get rid of this hand-made prepare_fb()
+	 * logic.
+	 */
+	ret = vc4_bo_inc_usecnt(bo);
+	if (ret)
+		return ret;
+
 	flip_state = kzalloc(sizeof(*flip_state), GFP_KERNEL);
-	if (!flip_state)
+	if (!flip_state) {
+		vc4_bo_dec_usecnt(bo);
 		return -ENOMEM;
+	}
 
 	drm_framebuffer_get(fb);
 	flip_state->fb = fb;
@@ -826,10 +857,23 @@ static int vc4_async_page_flip(struct drm_crtc *crtc,
 	ret = down_interruptible(&vc4->async_modeset);
 	if (ret) {
 		drm_framebuffer_put(fb);
+		vc4_bo_dec_usecnt(bo);
 		kfree(flip_state);
 		return ret;
 	}
 
+	/* Save the current FB before it's replaced by the new one in
+	 * drm_atomic_set_fb_for_plane(). We'll need the old FB in
+	 * vc4_async_page_flip_complete() to decrement the BO usecnt and keep
+	 * it consistent.
+	 * FIXME: we should move to generic async-page-flip when it's
+	 * available, so that we can get rid of this hand-made cleanup_fb()
+	 * logic.
+	 */
+	flip_state->old_fb = plane->state->fb;
+	if (flip_state->old_fb)
+		drm_framebuffer_get(flip_state->old_fb);
+
 	WARN_ON(drm_crtc_vblank_get(crtc) != 0);
 
 	/* Immediately update the plane's legacy fb pointer, so that later