Re: [PATCH 1/2] libata: switch to using block layer tagging support

[Jeff Garzik <jeff@garzik.org>]

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