Gwendal Grignou wrote:
Form the ATA and SATA spec, NCQ is per device. It is possible to
assign the same tag on different port, the disks and PMP will not
care.
However, today, we assign tag on a port basis [see
__ata_qc_from_tag()], therefore only 32 commands can be inflight to
all devices behind a given port.
Being able to a do a mapping qc <---> {pmp port, tag} instead of just
qc <---> {tag} will provide a performance boost when disks supporting
NCQ are connected behind a PMP. Maybe it can be done, by moving qcmd
from ata_port to ata_link.
As Tejun said, the patch needs more work to be able to support same
tag used on 2 different links behind the same port.
Also, given we are changing the amount of commands we sent to the
controller, we would have to change can_queue from ATA_MAX_QUEUE to
ATA_MAX_QUEUE*n, where n is the number of ports supported by the
controller [max SATA_PMP_MAX_PORTS, but some controller, like SiI3132
only supports 5 devices, and other may only support n = 1]
When done, the patch will require a great amount of testing, given we
will exercise the controllers in a brand new way. A white list might
be necessary.
Well, I think a few things are getting confused, here.
Assigning 32 tags to each port is the best we can do, given existing
hardware, which provides command slots on a per-port basis.
can_queue is a largely a host-wide limit that does not really apply to
modern SATA, because we (and Jens patch) set queue depth properly.
can_queue is mainly used these days for master/slave arbitration, and we
mainly need to ensure that can_queue does not add additional, unneeded
limits on top of the more fine-grained limits being configured.
Jeff
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