dma-buf: clarify dma_fence_add_callback documentation

That the caller doesn't need to keep a reference is rather
risky and not defensive at all.

Especially dma_buf_poll got that horrible wrong, so better
remove that sentence and also clarify that the callback
might be called in atomic or interrupt context.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210901120240.7339-3-christian.koenig@amd.com

drivers/dma-buf/dma-fence.c

@@ -616,20 +616,17 @@ EXPORT_SYMBOL(dma_fence_enable_sw_signaling);
  * @cb: the callback to register
  * @func: the function to call
  *
+ * Add a software callback to the fence. The caller should keep a reference to
+ * the fence.
+ *
  * @cb will be initialized by dma_fence_add_callback(), no initialization
  * by the caller is required. Any number of callbacks can be registered
  * to a fence, but a callback can only be registered to one fence at a time.
  *
- * Note that the callback can be called from an atomic context.  If
- * fence is already signaled, this function will return -ENOENT (and
+ * If fence is already signaled, this function will return -ENOENT (and
  * *not* call the callback).
  *
- * Add a software callback to the fence. Same restrictions apply to
- * refcount as it does to dma_fence_wait(), however the caller doesn't need to
- * keep a refcount to fence afterward dma_fence_add_callback() has returned:
- * when software access is enabled, the creator of the fence is required to keep
- * the fence alive until after it signals with dma_fence_signal(). The callback
- * itself can be called from irq context.
+ * Note that the callback can be called from an atomic context or irq context.
  *
  * Returns 0 in case of success, -ENOENT if the fence is already signaled
  * and -EINVAL in case of error.