Recent concurrency improvements in the AHCI driver and CAM need testing

[Matthew Dillon]

::this breaks USB (daX) disks for me. After all daX disks are initialized, it hangs after:
::
::da8 at umass-sim0 bus 0 target 0 lun 0
::da8: <WD Ext HDD 1021 2002> Fixed Direct Access SCSI-4 device 
::da8: Serial Number WMAVU2232228 
::da8: 40.000MB/s transfers
::da8: 1430797MB (2930272256 512 byte sectors: 255H 63S/T 182401C)
:
:    Ok, I'm on it.  It's a degenerate condition somewhere for the case
:    where the device is not NCQ.

    ... and it's fixed.  A calculation that was supported to always
    result in a value of 1 or more could sometimes result in a value
    of 0, in the case where max_tagged_dev_openings is set to 0.

    limit = (periph->sim->max_tagged_dev_openings * 2 + 1) / 3;

    to
     
    limit = periph->sim->max_tagged_dev_openings * 2 / 3 + 1;

					-Matt
					Matthew Dillon 
					<dillon at backplane.com>