Re: WIP: ATA to CAM integration

    More on the port multiplier spec.  It turns out that the port-multiplier
    port selector is in the command table, so it is per command-tag.  There
    is confusion in the spec though:

    section 9.1:

    In this mode of operation, a communication path is opened between the
    HBA and a device through the Port Multiplier.  Since Port Multipliers are
    meant to be simple, the burden of making a connection is on the AHCI
    software, to ensure that multiple commands are not outstanding to
    different devices behind the Port Multiplier.

    section 9.1.2:

    "Since queued commands result in two different operations (command issue,
    clear of BSY, then data transfer), if commands were sent to different
    ports, the Port Multiplier may issue FISes back to the HBA in
    an interleaved manner from different ports.  This will break an HBA that
    only supports command-based switching.  Therefore, when executing native
    command queueing commands, system software must only add commands
    to the command list that target a single port behind the Port Multiplier,
    wait for the commands to finish (PxSACT bits all cleared), then add
    commands for a different port.  Additionally, the tags used
    must match the command slot entries."

    --

    It's unclear to me what this means.  Can we use NCQ to queue multiple
    commands to multiple ports behind a single port multiplier in parallel
    or can't we?  It's very confusing.

						-Matt