media: vb2: frame_vector.c: replace WARN_ONCE with a comment

The WARN_ONCE was issued also in cases that had nothing to do with VM_IO
(e.g. if the start address was just a random value and uaccess fails with
-EFAULT).

There are no reports of WARN_ONCE being issued for actual VM_IO cases, so
just drop it and instead add a note to the comment before the function.
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reported-by: Yikebaer Aizezi <yikebaer61@gmail.com>

drivers/media/common/videobuf2/frame_vector.c

@@ -31,6 +31,10 @@
  * different type underlying the specified range of virtual addresses.
  * When the function isn't able to map a single page, it returns error.
  *
+ * Note that get_vaddr_frames() cannot follow VM_IO mappings. It used
+ * to be able to do that, but that could (racily) return non-refcounted
+ * pfns.
+ *
  * This function takes care of grabbing mmap_lock as necessary.
  */
 int get_vaddr_frames(unsigned long start, unsigned int nr_frames, bool write,
@@ -59,8 +63,6 @@ int get_vaddr_frames(unsigned long start, unsigned int nr_frames, bool write,
 	if (likely(ret > 0))
 		return ret;
 
-	/* This used to (racily) return non-refcounted pfns. Let people know */
-	WARN_ONCE(1, "get_vaddr_frames() cannot follow VM_IO mapping");
 	vec->nr_frames = 0;
 	return ret ? ret : -EFAULT;
 }