On Wed, Aug 04, 2021 at 11:41:39AM +0200, Christoph Hellwig wrote:

allows device mapper to use the normal scheme
of calling add_disk when it is ready to accept I/O.

For clarity, even after this patchset, the device is not ready to accept
I/O when add_disk is called.  It is ready to accept I/O later if a 
'resume' happens triggering the 'change' uevent that userspace reacts
to by setting up the /dev entries for it.
 
Alasdair