dma-resv: some doc polish for iterators

Hammer it a bit more in that iterators can be restarted and when that
matters, plus suggest to prefer the locked version whenver.

Also delete the two leftover kerneldoc for static functions plus
sprinkle some more links while at it.

v2: Keep some comments (Christian)

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: linux-media@vger.kernel.org
Cc: linaro-mm-sig@lists.linaro.org
Link: https://patchwork.freedesktop.org/patch/msgid/20211130152756.1388106-1-daniel.vetter@ffwll.ch

drivers/dma-buf/dma-resv.c

@@ -323,12 +323,8 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
 }
 EXPORT_SYMBOL(dma_resv_add_excl_fence);
 
-/**
- * dma_resv_iter_restart_unlocked - restart the unlocked iterator
- * @cursor: The dma_resv_iter object to restart
- *
- * Restart the unlocked iteration by initializing the cursor object.
- */
+/* Restart the iterator by initializing all the necessary fields, but not the
+ * relation to the dma_resv object. */
 static void dma_resv_iter_restart_unlocked(struct dma_resv_iter *cursor)
 {
 	cursor->seq = read_seqcount_begin(&cursor->obj->seq);
@@ -344,14 +340,7 @@ static void dma_resv_iter_restart_unlocked(struct dma_resv_iter *cursor)
 	cursor->is_restarted = true;
 }
 
-/**
- * dma_resv_iter_walk_unlocked - walk over fences in a dma_resv obj
- * @cursor: cursor to record the current position
- *
- * Return all the fences in the dma_resv object which are not yet signaled.
- * The returned fence has an extra local reference so will stay alive.
- * If a concurrent modify is detected the whole iteration is started over again.
- */
+/* Walk to the next not signaled fence and grab a reference to it */
 static void dma_resv_iter_walk_unlocked(struct dma_resv_iter *cursor)
 {
 	struct dma_resv *obj = cursor->obj;
@@ -387,6 +376,12 @@ static void dma_resv_iter_walk_unlocked(struct dma_resv_iter *cursor)
  * dma_resv_iter_first_unlocked - first fence in an unlocked dma_resv obj.
  * @cursor: the cursor with the current position
  *
+ * Subsequent fences are iterated with dma_resv_iter_next_unlocked().
+ *
+ * Beware that the iterator can be restarted.  Code which accumulates statistics
+ * or similar needs to check for this with dma_resv_iter_is_restarted(). For
+ * this reason prefer the locked dma_resv_iter_first() whenver possible.
+ *
  * Returns the first fence from an unlocked dma_resv obj.
  */
 struct dma_fence *dma_resv_iter_first_unlocked(struct dma_resv_iter *cursor)
@@ -406,6 +401,10 @@ EXPORT_SYMBOL(dma_resv_iter_first_unlocked);
  * dma_resv_iter_next_unlocked - next fence in an unlocked dma_resv obj.
  * @cursor: the cursor with the current position
  *
+ * Beware that the iterator can be restarted.  Code which accumulates statistics
+ * or similar needs to check for this with dma_resv_iter_is_restarted(). For
+ * this reason prefer the locked dma_resv_iter_next() whenver possible.
+ *
  * Returns the next fence from an unlocked dma_resv obj.
  */
 struct dma_fence *dma_resv_iter_next_unlocked(struct dma_resv_iter *cursor)
@@ -431,6 +430,8 @@ EXPORT_SYMBOL(dma_resv_iter_next_unlocked);
  * dma_resv_iter_first - first fence from a locked dma_resv object
  * @cursor: cursor to record the current position
  *
+ * Subsequent fences are iterated with dma_resv_iter_next_unlocked().
+ *
  * Return the first fence in the dma_resv object while holding the
  * &dma_resv.lock.
  */

include/linux/dma-resv.h

@@ -153,6 +153,13 @@ struct dma_resv {
  * struct dma_resv_iter - current position into the dma_resv fences
  *
  * Don't touch this directly in the driver, use the accessor function instead.
+ *
+ * IMPORTANT
+ *
+ * When using the lockless iterators like dma_resv_iter_next_unlocked() or
+ * dma_resv_for_each_fence_unlocked() beware that the iterator can be restarted.
+ * Code which accumulates statistics or similar needs to check for this with
+ * dma_resv_iter_is_restarted().
  */
 struct dma_resv_iter {
 	/** @obj: The dma_resv object we iterate over */
@@ -243,7 +250,11 @@ static inline bool dma_resv_iter_is_restarted(struct dma_resv_iter *cursor)
  * &dma_resv.lock and using RCU instead. The cursor needs to be initialized
  * with dma_resv_iter_begin() and cleaned up with dma_resv_iter_end(). Inside
  * the iterator a reference to the dma_fence is held and the RCU lock dropped.
- * When the dma_resv is modified the iteration starts over again.
+ *
+ * Beware that the iterator can be restarted when the struct dma_resv for
+ * @cursor is modified. Code which accumulates statistics or similar needs to
+ * check for this with dma_resv_iter_is_restarted(). For this reason prefer the
+ * lock iterator dma_resv_for_each_fence() whenever possible.
  */
 #define dma_resv_for_each_fence_unlocked(cursor, fence)			\
 	for (fence = dma_resv_iter_first_unlocked(cursor);		\