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>

1 files changed, 39 insertions, 0 deletions

diff --git a/include/drm/drm_syncobj.h b/include/drm/drm_syncobj.h
index ce94d14c5087..c00fee539822 100644
--- a/include/drm/drm_syncobj.h
+++ b/include/drm/drm_syncobj.h
@@ -28,6 +28,8 @@
 
 #include "linux/dma-fence.h"
 
+struct drm_syncobj_cb;
+
 /**
  * struct drm_syncobj - sync object.
  *
@@ -43,15 +45,47 @@ 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.
          */
         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,