The normal way for &struct buffer_heads to be passed to a device
driver is for them to be collected into requests on a request
queue, and then to allow the device driver to select requests
off that queue when it is ready. This works well for many block
devices. However some block devices (typically virtual devices
such as md or lvm) do not benefit from the processing on the
request queue, and are served best by having the requests passed
directly to them. This can be achieved by providing a function
to blk_queue_make_request.