drm/syncobj: Add a callback mechanism for replace_fence (v3)

It is useful in certain circumstances to know when the fence is replaced
in a syncobj.  Specifically, it may be useful to know when the fence
goes from NULL to something valid.  This does make syncobj_replace_fence
a little more expensive because it has to take a lock but, in the common
case where there is no callback list, it spends a very short amount of
time inside the lock.

v2:
 - Don't lock in drm_syncobj_fence_get.  We only really need to lock
   around fence_replace to make the callback work.
v3:
 - Fix the cb_list comment to make kbuild happy

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/drm_syncobj.c

@@ -80,6 +80,46 @@ struct drm_syncobj *drm_syncobj_find(struct drm_file *file_private,
 }
 EXPORT_SYMBOL(drm_syncobj_find);
 
+static void drm_syncobj_add_callback_locked(struct drm_syncobj *syncobj,
+					    struct drm_syncobj_cb *cb,
+					    drm_syncobj_func_t func)
+{
+	cb->func = func;
+	list_add_tail(&cb->node, &syncobj->cb_list);
+}
+
+/**
+ * drm_syncobj_add_callback - adds a callback to syncobj::cb_list
+ * @syncobj: Sync object to which to add the callback
+ * @cb: Callback to add
+ * @func: Func to use when initializing the drm_syncobj_cb struct
+ *
+ * This adds a callback to be called next time the fence is replaced
+ */
+void drm_syncobj_add_callback(struct drm_syncobj *syncobj,
+			      struct drm_syncobj_cb *cb,
+			      drm_syncobj_func_t func)
+{
+	spin_lock(&syncobj->lock);
+	drm_syncobj_add_callback_locked(syncobj, cb, func);
+	spin_unlock(&syncobj->lock);
+}
+EXPORT_SYMBOL(drm_syncobj_add_callback);
+
+/**
+ * drm_syncobj_add_callback - removes a callback to syncobj::cb_list
+ * @syncobj: Sync object from which to remove the callback
+ * @cb: Callback to remove
+ */
+void drm_syncobj_remove_callback(struct drm_syncobj *syncobj,
+				 struct drm_syncobj_cb *cb)
+{
+	spin_lock(&syncobj->lock);
+	list_del_init(&cb->node);
+	spin_unlock(&syncobj->lock);
+}
+EXPORT_SYMBOL(drm_syncobj_remove_callback);
+
 /**
  * drm_syncobj_replace_fence - replace fence in a sync object.
  * @syncobj: Sync object to replace fence in
@@ -91,10 +131,24 @@ void drm_syncobj_replace_fence(struct drm_syncobj *syncobj,
 			       struct dma_fence *fence)
 {
 	struct dma_fence *old_fence;
+	struct drm_syncobj_cb *cur, *tmp;
 
 	if (fence)
 		dma_fence_get(fence);
-	old_fence = xchg(&syncobj->fence, fence);
+
+	spin_lock(&syncobj->lock);
+
+	old_fence = syncobj->fence;
+	syncobj->fence = fence;
+
+	if (fence != old_fence) {
+		list_for_each_entry_safe(cur, tmp, &syncobj->cb_list, node) {
+			list_del_init(&cur->node);
+			cur->func(syncobj, cur);
+		}
+	}
+
+	spin_unlock(&syncobj->lock);
 
 	dma_fence_put(old_fence);
 }
@@ -130,7 +184,7 @@ void drm_syncobj_free(struct kref *kref)
 	struct drm_syncobj *syncobj = container_of(kref,
 						   struct drm_syncobj,
 						   refcount);
-	dma_fence_put(syncobj->fence);
+	drm_syncobj_replace_fence(syncobj, NULL);
 	kfree(syncobj);
 }
 EXPORT_SYMBOL(drm_syncobj_free);
@@ -146,6 +200,8 @@ static int drm_syncobj_create(struct drm_file *file_private,
 		return -ENOMEM;
 
 	kref_init(&syncobj->refcount);
+	INIT_LIST_HEAD(&syncobj->cb_list);
+	spin_lock_init(&syncobj->lock);
 
 	idr_preload(GFP_KERNEL);
 	spin_lock(&file_private->syncobj_table_lock);

include/drm/drm_syncobj.h

@@ -28,6 +28,8 @@
 
 #include "linux/dma-fence.h"
 
+struct drm_syncobj_cb;
+
 /**
  * struct drm_syncobj - sync object.
  *
@@ -43,8 +45,21 @@ struct drm_syncobj {
 	/**
 	 * @fence:
 	 * NULL or a pointer to the fence bound to this object.
+	 *
+	 * This field should not be used directly.  Use drm_syncobj_fence_get
+	 * and drm_syncobj_replace_fence instead.
 	 */
 	struct dma_fence *fence;
+	/**
+	 * @cb_list:
+	 * List of callbacks to call when the fence gets replaced
+	 */
+	struct list_head cb_list;
+	/**
+	 * @lock:
+	 * locks cb_list and write-locks fence.
+	 */
+	spinlock_t lock;
 	/**
 	 * @file:
 	 * a file backing for this syncobj.
@@ -52,6 +67,25 @@ struct drm_syncobj {
 	struct file *file;
 };
 
+typedef void (*drm_syncobj_func_t)(struct drm_syncobj *syncobj,
+				   struct drm_syncobj_cb *cb);
+
+/**
+ * struct drm_syncobj_cb - callback for drm_syncobj_add_callback
+ * @node: used by drm_syncob_add_callback to append this struct to
+ *	  syncobj::cb_list
+ * @func: drm_syncobj_func_t to call
+ *
+ * This struct will be initialized by drm_syncobj_add_callback, additional
+ * data can be passed along by embedding drm_syncobj_cb in another struct.
+ * The callback will get called the next time drm_syncobj_replace_fence is
+ * called.
+ */
+struct drm_syncobj_cb {
+	struct list_head node;
+	drm_syncobj_func_t func;
+};
+
 void drm_syncobj_free(struct kref *kref);
 
 /**
@@ -91,6 +125,11 @@ drm_syncobj_fence_get(struct drm_syncobj *syncobj)
 
 struct drm_syncobj *drm_syncobj_find(struct drm_file *file_private,
 				     u32 handle);
+void drm_syncobj_add_callback(struct drm_syncobj *syncobj,
+			      struct drm_syncobj_cb *cb,
+			      drm_syncobj_func_t func);
+void drm_syncobj_remove_callback(struct drm_syncobj *syncobj,
+				 struct drm_syncobj_cb *cb);
 void drm_syncobj_replace_fence(struct drm_syncobj *syncobj,
 			       struct dma_fence *fence);
 int drm_syncobj_find_fence(struct drm_file *file_private,