o expect the disk's start routine to return an int. If the
int is non-zero, we enqueue the request and try again
later.
o have a dk_start() routine which runs the request queue.
o have a dk_iodone() function which should be called by the
driver using the framwork from its iodone. dk_iodone will
retry the queue since presumably further progress may be
possible once a request is complete. It is required that
the underlying driver have the resources to keep at least
one transaction in flight at any time.
Modified cgd to:
o be able to keep one transaction in flight at any time
(almost) by keeping a buffer of size MAXPHYS in its softc
and use it.
We still need to make the cgd_cbufpool per device rather than global
and provide a low water mark for it.
Addresses PR: kern/24715
(at least according to the submitter.)
