drm: document that blobs are ref'counted

User-space doesn't need to keep track of blobs that might be in use by
the kernel. User-space can just destroy blobs as soon as they don't need
them anymore.

Signed-off-by: Simon Ser <contact@emersion.fr>
Signed-off-by: Daniel Stone <daniel@fooishbar.org>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/wgav99DTGfubfVPiurrydQEiyufYpxlJQZ0wJMWYBQ@cp7-web-042.plabs.ch

include/uapi/drm/drm_mode.h

@@ -924,6 +924,12 @@ struct drm_mode_create_blob {
  * struct drm_mode_destroy_blob - Destroy user blob
  * @blob_id: blob_id to destroy
  * Destroy a user-created blob property.
+ *
+ * User-space can release blobs as soon as they do not need to refer to them by
+ * their blob object ID.  For instance, if you are using a MODE_ID blob in an
+ * atomic commit and you will not make another commit re-using the same ID, you
+ * can destroy the blob as soon as the commit has been issued, without waiting
+ * for it to complete.
  */
 struct drm_mode_destroy_blob {
 	__u32 blob_id;