Correct a very rare case where command ordering could be compromised

Correct a very rare case where command ordering could be compromised
by a transaction performing a driver handled message sequence (an
scb with the MK_MESSAGE flag set).

SCBs that perform host managed messaging must always be
at the head of their per-target selection queue so that
the firmware knows to manually assert ATN if the current
negotiation agreement is packetized.  In the past we
guaranteed this by queuing these SCBs separarately in
the execution queue.  This exposes the system to potential
command reordering in two cases:

1) Another SCB for the same ITL nexus is queued that does
   not have the MK_MESSAGE flag set.  This SCB will be
   queued to the per-target list which can be serviced
   before the MK_MESSAGE scb that preceeded it.

2) If the target cannot accept all of the commands in the
   per-target selection queue in one selection, the remainder
   is queued to the tail of the selection queues so as to
   effect round-robin scheduling.  This could allow the
   MK_MESSAGE scb to be sent to the target before the
   requeued commands.

This commit changes the firmware policy to defer queuing
MK_MESSAGE SCBs into the selection queues until this can
be done without affecting order.  This means that the
target's selection queue is either empty, or the last
SCB on the execution queue is also a MK_MESSAGE SCB.
During any wait, the firmware halts the download of new
SCBs so only a single "holding location" is required.

Luckily, MK_MESSAGE SCBs are rare and typically occur only
during CAM's bus probe where only one command is outstanding
at a time.  However, during some recovery scenarios, the
reordering *could* occur.

aic79xx.c:
	Update ahd_search_qinfifo() and helper routines to
	search for pending MK_MESSAGE scbs and properly
	restitch the execution queue if either the MK_MESSAGE
	SCB is being aborted, or the MK_MESSAGE SCB can be
	queued due to the execution queue draining due to
	aborts.

	Enable LQOBUSFREE status to assert an interrupt.
	This should be redundant since a BUSFREE interrupt
	should always occur along with an LQOBUSFREE event,
	but on the Rev A, this doesn't seem to be guaranteed.

	When a PPR request is rejected when a previously
	existing packetized agreement is in place, assume
	that the target has been reset without our knowledge
	and revert to async/narrow transfers.  This corrects
	two issues: the stale ENATNO setting that was used
	to send the PPR is cleared so the firmware is not
	confused by a future packetized selection with
	ATN asserted but no MK_MESSAGE flag in the SCB and
	it speeds up recovery by aborting any pending
	packetized transactions that by definition are now
	dead.

	When re-queueing SCBs after a failed negotiation
	attempt, ensure command ordering by freezing the
	device queue first.

	Traverse the list of pending SCBs rather than the
	whole SCB array on the controller when pushing
	MK_MESSAGE flag changes out to the controller.
	The original code was optimized for the aic7xxx
	controllers where there are fewer controller slots
	then pending SCBs and the firmware picks SCB
	slots.  For the U320 controller, the hope is
	that we have fewer pending SCBs then the 512
	slots on the controller.

	Enhance some diagnostics.

	Factor out some common code.

aic79xx.h:
	Add prototype for new ahd_done_with_status() that is
	used to factor out some commone code.

aic79xx.reg:
	Add definisions for the pending MK_MESSAGE SCB.

aic79xx.seq:
	Defer MK_MESSAGE SCB queing to the execution queue
	so as to preserve command ordering.  Re-arrange some
	of the selection processing code so the above change
	had no performance impact on the common code path.

	Close a few critical section holes.

	When entering a non-packetized phase, manually enable
	busfree interrupts, since the controller hardware
	does not do this automatically.

aic79xx_inline.h:
	Enhance logging for queued SCBs.

aic79xx_osm.c:
	Add new a new DDB ahd command, ahd_dump, which
	invokes the ahd_dump_card_state() routine on the
	unit specified with the ahd_sunit DDB command.

aic79xx_pci.c:
	Turn on the BUSFREEREV bug for the Rev B. controller.
	This is required to close the busfree during non-packetized
	phase hole.

Obtained-from: FreeBSD